Burst-train generation for femtosecond laser filamentation-driven micromachining. Saeid Rezaei

Transcription

1 Burst-train generation for femtosecond laser filamentation-driven micromachining Saeid Rezaei A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Graduate Department of Electrical and Computer Engineering University of Toronto c Copyright 2011 Saeid Rezaei

2 Burst-train generator for femtosecond laser filamentation-driven micromachining Saeid Rezaei Submitted for the Degree of Master of Applied Science January 2011 Abstract Bursts of femtosecond laser pulses with a repetition rate of 38.5MHz were created by integrating a purpose-built burst resonator system with a Ti: Sapphire laser system. The timing control system in this work provided flexibility for generation of any desirable pulse train profile in the burst envelope. These laser pulses further have been used for high aspect ratio hole drilling inside glass materials. High repetition rate of the pulses produces heat accumulation effects during the laser machining interaction which provided the possibility of deeper etching, increased the aspect ratio, and mitigated cracks and damages effects of single pulses. During the process, time-resolved nanosecond scale side-view images have been recorded with a time-gated intensified-ccd camera enabling the observation of transient effects during the laser machining process. These side-view images further have been used to assess the physics of burst interaction and illustrate benefits and disadvantages of high repetition rate laser pulses. ii

3 Acknowledgements I would like to thank my supervisor, Professor Peter R. Herman for his kind support and guidance throughout this project. In addition, I wold like to thank Dr. Jianzhao Li and Dagmar Esser for their help and collaboration in this work. Special thanks also go to Dr. Abbas Hosseini, Dr. Takayuki TAMAKI and Ladan E. Abolghasemi for their kind helps in the lab. Further, I would like to thank all the Herman Group members for their assistance and friendship, and also my dearest friends Safa Akbarzade, Moez Haque, Matin Nabavi Niaki, Motah Nabavi Niaki, Anae Sobhani and Nima Zariean for their kind and generous help through the thesis. Finally, I like to thank my family for their constant support, inspiration and love for accomplishing my thesis abroad. iii

4 Contents 1 Introduction Thesis Objective Chapter-by-chapter Outline Background Ablation Mechanisms Ablation for different laser pulse durations Ablation for different energy levels Nonlinear phenomena Multiphoton photoionization Avalanche ionization Filamentation Heat accumulation effect Time-lapse images of microhole fabrication iv

5 3 Experimental method Ti: Sapphire laser system used for burst generation Burst resonator system Optical arrangement for burst generation PC timing control system Beam delivery method Time-Resolved imaging (ICCD) of laser interactions Results and Discussion Machining of high aspect ratio holes Focusing condition Pulse duration effect Significances of the burst effect Physical insight of high aspect ratio hole drilling Time-resolved ICCD imaging Filament brightness in pulse and burst mode Ablation plume and channel brightness Conclusion and Future Work 84 A Working with the burst generator system 86 v

6 A.1 Optical alignment A.2 Timing system Bibliography 104 vi

7 List of Figures 2.1 Scheme of pulse laser ablation. I(r), spatial intensity distribution; r, radius; τ, pulse duration [33] Scheme for 4 different ablation mechanism due to laser pulse energy for single and mutiple pulses in each case: (a) existence of microcracks in ablation for brittle materials, (b) inhomogeneous profile, (c) narrow V-shape hole can be reached by mutiple pulses, and (d) large aspect ration holes with recasting formation on the walls.[49] The schematics for nonlinear photoionization with different values for Keldysh parameter, γ [7] Schematic potential energy diagram for avalanche ionization (right) that follows ofter seed electrons are created by mutiphoton absorption (left) [7] Finite-difference model of glass temperature versus number of incident pulses, at a radius of 2µm from the focal point of the incident beam. The heat source due to each laser pulse was assumed to be a delta function in time which is valid for femtosecond range of pulse durations [9] Experimental setup for microhole drilling and in-situ imaging presented in [63]. 24 vii

8 2.7 Time-lapse images (one frame every 30 seconds, corresponding to impact of 150 laser pulses) of drilling of soda-lime silicate glass, for E p = 1.5mJ and I p = W/cm 2 [63] The hole depth versus the number of laser pulses in soda-lime silicate glass at 1kHz and 5Hz. For 1kHz: τ = 150fs; λ = 775nm ;E p = 0.85mJ/pulse; I p = W/cm 2. For 5Hz higher intensity: τ = 110fs; λ = 845nm; E p = 1.5mJ/pulse; I p = W/cm 2. For 5 Hz Lower Intensity: τ = 110fs; λ = 845nm ;E p = 1.0mJ/pulse; I p = W/cm 2 [63] An schematic of complete Spitfire Pro system, manufactured by Spectra-Physics A schematic of the burst generator integrated with the Spitfire laser system. The output parameters are shown. The repetition rate is decreased to 500Hz in order to match the timing system in Section Optical arrangement of the burst resonator cavity Intensity profile for the laser beam measured after the compressor for (a) single pulse mode and (b) 6 equal pulses in the burst train Pulse duration measurement using Autocorrelator system, showing (a) 90fs pulse duration for single pulse mode and (b) 93fs for 7 pulses in the burst train An schematic of time-delay method. By controlling the starting time for triggering of high voltage, different modulation levels can be introduced to the laser pulses. Vertical lines show the times laser pulses are passing the PC in each round trip An schematic of time-delay system used for the burst generator system viii

9 3.8 A few examples of different burst profiles generated by the burst generation system. This beam will detected using photodetectors and recorded on an oscilloscope.(a) single pulse, (b) three equal pulses, (c) six equal pulses, (d) five pulses with the decreasing profile, (e) five pulses with high-low profile and (f) four pulses with increasing pulse energies. The time between adjacent pulses is 26ns Beam delivery system for hole drilling in glass Photo taken in the lab. In this photo, burst resonator system is shown in addition to parts of the Spitfire and beam delivery system Scheme of the ICCD imaging setup. The intensified CCD-camera is triggered by a digital delay generator which also triggers the mechanical shutter for recording time-resolved images of the laser interaction zone. The shortpass filter λ=750nm avoids the imaging of the laser wavelength λ=800nm. A 20x microscope objective lens is used to capture the images of the laser interaction Side and top view images of holes created in BK7 with 6 equal pulses in a burst. The exposure time is 2 seconds for all cases, and the focusing position changes from 200µm inside the sample to 200µm above it, with 20µm steps. Pulse energy is 33.3µJ and pulse duration is 100fs at 800nm wavelength for all cases. The glass sample is on air on both top and bottom side, and the laser beam is coming from top of the image ix

10 4.2 Side and top view images of holes created in BK7 with 3 equal pulses in a burst. The exposure time is 2 seconds for all cases, and the focusing position changes from 200µm inside the sample to 200µm above it, with 20µm steps. Pulse energy is 33.3µJ and pulse duration is 100fs at 800nm wavelength for all cases. The glass sample is on air on both top and bottom side, and the laser beam is coming from top of the image Side and top view images of holes created in BK7 with single pulses. The exposure time is 2 seconds for all cases, and the focusing position changes from 200µm inside the sample to 200µm above it, with 20µm steps. Pulse energy is 33.3µJ and pulse duration is 100fs at 800nm wavelength for all cases. The glass sample is on air on both top and bottom side, and the laser beam is coming from top of the image The angle of the hole taper for three different burst modes versus the focusing position for data in Figure 4.1 to Hole depth versus the focusing position for three different burst profiles in BK7. This graph represent the results in Figure 4.1 to Hole depth versus the focusing position for three different burst profiles in Fused Silica Holes ablated inside BK7 with bursts of 5 pulses with 24µJ pulse energies. (a) 100fs, (b) 600fs, (c) 1ps and (d) 2ps. For the 5 pulse burst profile, 200, 400, 800, 1200 and 200ms exposures mean 500, 1000, 2000 and 3000 pulses were applied in each etched hole x

11 4.8 Holes created inside fused silica with bursts of 5 pulses with 24µJ pulse energies. (a) 100fs, (b) 600fs, (c) 1ps and (d) 2ps. For the 5 pulse burst profile, 200, 400, 800, 1200 and 200ms exposures mean 500, 1000, 2000 and 3000 pulses were applied in each etched hole Hole depth vs. number of pulses for experiments in Figures 4.7 and 4.8; (a) BK7 and (b) Fused Silica Side view images of holes created in BK7 with 20µJ single pulse for different number of pulses shown above in each hole Side view images of holes created in BK7 with 20µJ energy per pulse with 2 pulses in each burst for different number of pulses shown above in each hole Side view images of holes created in BK7 with 20µJ energy per pulse with 3 pulses in each burst for different number of pulses shown above in each hole Side view images of holes created in BK7 with 20µJ energy per pulse with 4 pulses in each burst for different number of pulses shown above in each hole Side view images of holes created in BK7 with 20µJ energy per pulse with 6 pulses in each burst for different number of pulses shown above in each hole Comparison of hole depth as a function of total number of pulses for 5 different burst condition. A log-scale is used to show the total number of pulses. Horizontal dotted lines represent the saturation points for each case. After this point, the holes will not grow anymore by increasing the number of pulses. 67 xi

12 4.16 Three different burst profiles used for hole drilling: (a) 6 pulses in a burst, (b) 3 pulses in burst with doubling the time (52ns) between each pulse, and (c) 3 normal pulses (26ns separation between each in a burst). Note that in (b), the high voltage over the Pockels Cell is zero for second and fourth round trip of the pulse, allowing the pulses to circulate with no ejection which increase the time separation to 52ns Holes created with the burst profile shown in Figure 4.16-(a) inside BK7 for 2 second exposure with different pulse energies Holes created with the burst profile shown in Figure 4.16-(b) inside BK7 for 2 second exposure with different pulse energies Holes created with the burst profile shown in Figure 4.16-(c) inside BK7 for 2 second exposure with different pulse energies Holes created with the single pulses inside BK7 for 2 second exposure with different pulse energies Graph of hole depth as a function of pulse energy for single pulse experiment and with bursts of 6 pulses, 3 pulses with 52ns spearation and 3 pulses with 26ns separation time as seen in Figure Time-resolved intensity images during the ablation process for the 1st and the 200th burst in the burst mode. The white line indicates the surface of the glass sample. The images on the far right side show the detected intensity values multiplied by a factor of 5. The beam propagation direction is top down. 76 xii

13 4.23 Intensity decay of the filament brightness during the first burst in the burst mode (black square) and the pulse mode (red triangle). A double exponential decay is fitted to the data. The red arrows indicate the times when four laser pulses strike the surface in the burst mode Brightness of the channel (black line) and of the ablation plume (red line) after the 1st, the 25th, the 50th and the 200th burst in the burst mode. The brightness is given in arbitrary values and the red arrows indicate the timing of four pulses in the burst A.1 The experimental implementation of the burst resonator cavity A.2 Software for running the BME P3 preamplifier A.3 Software for the Spectrum cards xiii

14 List of Tables 3.1 Characteristics of the lasers used in the Spitfire Pro system Focusing parameters of the Spitfire laser output using a 10X Objective lens (NA=0.16) Tests for repeatability using 6 pules/burst profiles. In each case, 20 separate exposures were are used in order to calculate the average and standard deviation xiv

15 Chapter 1 Introduction A few years following the invention of laser in 1960 by Charles Townes, DeMari et al. were first to demonstrate a femtosecond laser in 1966 by using a saturable absorber in the cavity [1, 2]. Since that time, many efforts have been made to overcome the difficulties in producing femtosecond pulses, such as using solid gain media, improving the stability and life-time of the systems and developing novel appliances that generate high peak powers without damage to optical components. The chirped pulse amplification method was first demonstrated by Strickland et al. in 1988 [3] making it possible to generate significantly higher peak intensities for ultrashort pulses. In 1991, Spence et al. introduced the first mode-locked Ti: Sapphire laser system which used the passive mode locking technique [4]. From that time, numerous companies began making commercial ultrafast laser systems with high output peak powers. For material processing with standard nanosecond laser systems, the absorbed energy of 1

16 1 Introduction 2 the laser pulse can be transferred through the substrate by thermal conduction which will result in lower spatial resolution and damage in the heat affected zone (HAZ). Further, formation of a plasma will reflect significant energy and prevent the efficient delivery of the total pulse energy to the substrate [5]. Unlike these processes, a femtosecond pulse can be absorbed only in the focal volume of the beam long before thermal transport can begin. One of the main advantages of the femtosecond laser systems is the small HAZ [6]; by focusing a beam inside transparent materials, the energy will be absorbed in the focal point due to multi-photon and often nonlinear absorption mechanisms [7]. This opens the possibility of fabricating sub-micron structures inside bulk materials. Hence, several applications have been demonstrated in the past two decades using ultrafast laser systems including the fabrication of optical waveguides in passive [8, 9] and active [10] materials, microfluidic channels in transparent materials [11, 12] and 3D photonic crystal structures inside photo-resist materials [13]. A principal application of ultrafast systems is material removal by high intensity laser interaction [5, 6, 7, 14, 15, 16, 17, 18]. Unlike the nanosecond pulses, surface damage or modification has a well-defined deterministic threshold for laser ablation [19] guaranteeing the reproducibility of the process which is attractive for many applications. Micro-channel drilling in various glasses has been a subject for intense study in various research and industrial labs for the past few years and several useful methods have already been demonstrated. Glasses are highly valued for their optical properties and can be used as optical waveguides, photonic crystals, grating as well as other applications in the

17 1 Introduction 3 field of nanometric chemical analysis and chemical reactions in medicine and biotechnology [20]. Among these fabrication methods are direct micromachining in air [16] or vacuum [5], chemical etching integrated with ultrafast laser interaction [21, 22, 23] and water immersion micromachining on the second surface of transparent materials [14]. Direct micromachining is a one step process in which the laser beam (can be continuous or pulsed system, from ultraviolet (UV) to infra-red (IR) spectrum) will be focused on the top surface of the glass, which can be in ambient air or in a vacuum environment. In chemical etching, tightly focused ultrafast pulses modify the glass sample either on the surface or in the volume. For the latter, modified regions can be removed in a second step by HF chemical processing through nanograting structures. The peak intensity power required for this process is less than thresholds for direct micromachining and smaller feature sizes of less than a micrometer can be achieved in this method. Laser machining with water immersion is also a one step process in which a focused laser beam (normally short pulses) will interact on the reverse side surface of transparent material which is in contact with water, which can reduce the crack formation during micromachining. Choosing an appropriate laser processing method is difficult and subject to the desired quality and feature size of machining, as well as the cost of the laser system and time of processing. Among all these methods, direct laser micromachining of glasses in air is the most simple approach and can result in good results [18]. Machining in vacuum requires costly systems for providing the vacuum condition which is not desirable or cost efficient for most applications. The vacuum removes the reconfigurability of the femtosecond machining approach, and is not

18 1 Introduction 4 convenient for loading several samples in a short time. In the chemical etching technique, two separate processes are required in which very small feature sizes can be fabricated; however, it is a more complex process and requires more time. The etching process itself requires a long time (few hours) for chemical (e.g. HF acid) to completely remove the exposed areas [24]. Reverse-side machining also requires more complex optical systems for high precision positioning of the sample, which is again inconvenient for loading multiple samples. In direct micromachining in air, burst pulses with high repetition rates can have a huge improvement in quality of the drilled holes [25]. In the normal mode of operation of conventional short pulse laser systems, ablation occurs by the interaction of individual laser pulses, and no cumulative effects survive from pulse to pulse. In the other words, each pulse will be introduced to brittle glass and interaction zone in the glass will fully cool down due to long time interval before the arrival of the next pulse. One effective way for ultrafast laser micromachining is to harness the heat accumulation effect [26] where brittle material becomes ductile with heating by the first pulse on each cycle interactions. Subsequent pulses can therefore perform a much more efficient machining on the material without etching surfaces. The burst train of high frequency pulses with sufficient peak intensities for machining the glass is desired to produce this cumulative heat accumulation effect. Commercial ultrafast laser systems can be divided into three categories: 1) mode-locked oscillators, 2) regenerative amplifier systems, and 3) fiber hybrid systems. Mode-locked oscillators normally operate at high repetition rates (around 75MHz), and usually are not able to produce high pulse intensities required for machining due to the low nano-joule pulse

19 1 Introduction 5 energy. This low energy is limited by the damage of the gain medium and often components in the oscillator. In regenerative amplifiers systems, seed laser pulses are provided with a mode-locked oscillator, and the chirped pulse amplification technique is used to stretch the pulse duration prior to amplification in regenerative resonator. However, these systems are incapable of operating at MHz pulse repetition rate, due to limitations for high speed switching as well as high pump sources. High pulse energy (few millijoules) are available from such systems with low repetition rates in the range of few khz. Hybrid systems, have an intermediate level for pulse energy and repetition rate of a few micro-joules and a few MHz, respectively. While the pulse energy of oscillator systems is below the required level for micromachining, the repetition rate of the regenerative amplifiers is too low for producing heat accumulation effect. In the case of hybrid short pulse laser systems, the pulse energy is sufficient for micromachining (100nJ to 10µJ) and the pulse repetition rate is also high enough ( 1MHz) for driving the heat accumulation effect; however, higher pulse energies are required for these systems to make them the best devices for deeper and more effective micromachining. For hole or via drilling in transparent glasses, amplified laser pulses with high peak intensity are required. To create high aspect ratio holes, one requires to increase the pulse energy which will result in more cracks inside glass and bigger HAZ. Crack formation is normally a result of the cooling cycle between individual pulses. In using unamplified laser systems, low energy pulses will be absorbed or scattered before reaching the deep depths of the glass hole and therefore cannot reach the ablation threshold at the bottom of the hole. The burst

20 1 Introduction 6 mode of operation is able to offer high aspect ratio crack-free holes drilling in glasses. Heat accumulation from pulse to pulse will increase the drilling efficiency for a fixed value of pulse energy, which will result in the formation of uniform holes without cracks and small HAZ. 1.1 Thesis Objective The presented work will cover the following areas: 1. Describe a new method of burst generation of femtosecond laser pulses using a commercial laser system integrated with a purpose built burst generator optical cavity. 2. Characterization of the the generated burst pulses in order to produce several different burst-train profiles in the burst with controlled pulse duration and beam shape quality. 3. Taking advantage of the high pulse energy and high repetition rate of the burst generator systems for microhole drilling inside glass materials. 4. Optimization of the laser and beam delivery conditions for achieving deeper hole depth with a fixed pulse energy and stationary focusing condition. 5. Nanosecond time-resolved ablation dynamic study of laser generated filaments, plasma and plume during high aspect ration hole drilling. The burst generation system was designed and built by Dr. Abbas Hosseini. The author s contribution in this research includes conducting all the experiments in this thesis, with the

21 1 Introduction 7 collaboration of Dagmar Esser and Dr. Jianzhao Li in the both hole-drilling and ablation dynamics study parts of the study. 1.2 Chapter-by-chapter Outline This thesis contains the following chapters: Chapter 2 Background : This chapter reviews the important physical phenomenon during ultrafast laser pulse interaction with transparent materials. Different ablation mechanisms are introduced in this chapter, focusing on nonlinear phenomena that underlie deep hole machining. A description of heat accumulation effects and introducing a similar work for machining and time-resolved imaging of physical reaction during drilling process will conclude this chapter. Chapter 3 Experimental Method : In this chapter, the experimental regenerative amplifier system is described. Then, the purpose-built burst generation system is presented by describing the optical cavity and the timing control system. Next, the beam delivery system is presented together with the time-gated intensified CCD (chargecoupled device) camera imaging system used for ablation dynamics study. Chapter 4 Results and Discussion : This chapter presents the experimental results of burst mode laser interaction with glass samples and studies different parameters affecting the micro-hole drilling process. Then the time gated images recorded with

22 1 Introduction 8 the intensified-ccd camera are presented and described to unravel the physical process of the ablation process during burst-train laser drilling. Chapter 5 Conclusion and Future works : In this chapter, significances of our method will be presented together with suggestions for advancing the method and directions for future work. Appendix 1 : A manual for operating with the burst generation cavity in addition to the alignment procedure for the cavity and a detailed description of the timing system will be presented.

23 Chapter 2 Background During the last two decades ultrashort laser pulses have been used to modify many kinds of transparent materials to create components for many applications in integrated optics and telecommunication devices. A localized region of modification in the focal volume can be created due to nonlinear absorption processes such as multiphoton absorption, which occur only at high intensities. Thus three-dimensional microstructures [27, 28, 29], microfluidic channels [12, 30], waveguides [31, 32] and other optical components are producible in transparent media like glasses and crystals. There are several advantages of using lasers for material structuring, including: The machining is contactless, which is the most important property of laser machining in comparison with mechanical machining. A high amount of energy can be delivered on the surface or to a very small volume 9

24 2 Background 10 inside the material. High resolution can be achieved by using short wavelength. Can provide clean 1 and rapid machining. Coherency permits holographic 2D or 3D structuring at resolutions smaller than λ/2, where λ is the working wavelength of the laser. Possibility of rapid prototyping which can be designed and controlled by computers. Can process difficult materials such as glass and ceramic (brittle materials). All these properties make laser fabrication a useful tool for creating plenty of miniaturized structures for different technologies. In order to achieve the desired structure, it is necessary to make sure the correct ablation will happen during the laser-matter interaction [33]. 2.1 Ablation Mechanisms For ablating materials using lasers, the laser pulse intensity needs to be larger than the material dependent energy density, ρ th [J/m 3 ] [34]. The ablation response of materials are related to laser beam properties, and both optical and thermal properties of the materials. Examples of these properties include reflectivity, R, linear, α, and nonlinear, β, absorption coefficients for optical, and heat diffusion coefficient, D, for thermal properties. 1 In terms of producing dusts and wastes, as well as the need for lubricating mechanism like mechanical machining

25 2 Background 11 The ablation mechanism for short pulse lasers normally follows this scenario: The laser energy will be first absorbed by electrons in the material. This absorption is different for opaque and transparent materials; i.e. in the opaque case, the linear absorption has the main contribution while in latter, the nonlinear absorption plays the most significant role to initiate ablations. In the latter case, mutiphoton photoionization creates electron and then avalanche ionization [19, 35, 36, 37] as will be described in sections and On longer time scales, the dominant part of the electron energy transfers to the lattice, reaching to melting or vaporizing temperatures, and results in a thermal equilibrium between the electron and lattice temperatures. This transfer time is known as electron-phonon relaxation time, τ e, and is typically on the order of picoseconds, or as small as 100fs in glasses [38] Ablation for different laser pulse durations The ablation process can be categorized for different ratios between τ e and laser pulse duration, τ. Laser interaction is separated into two cases defined by τ τ e and τ τ e [39]. When τ τ e, which is normally the case for laser pulses with pulse durations more than 10 picoseconds, thermal equilibrium can be modeled to be local, and the ablation threshold fluence, F th, varies with τ. A schematic for this case is shown in Fig A pulse with time duration of τ and spatial intensity profile of I(r) and an energy enough to ablate the sample hits the target. The plumes leave the sample at velocities on the order of few µm/ns from the surface of the sample [40]. This means the tail of the incoming laser pulse is influenced

26 2 Background 12 by the ablation plume, which decreases the energy deposition efficiency into the substrate surface [33]. Also, the heat affected zone (HAZ) extends a distance of 4Dτ, where D is heat diffusion coefficient, meaning that HAZ decreases with decreasing laser pulse duration [41]. Furthermore, a relatively thick layer of melt forms below the interaction zone, which is affected by recoil pressure of the expanding plume, and the liquid droplets are forced out of cavity [42]. In the case of having a sensitive sample (like biological tissues), a shock affected zone (SAZ) is also produced underneath the HAZ that leads to damage of the structure [43, 44]. For the cases with τ τ e, namely, the femtosecond range of pulse duration, the electrons are excited and thermalised directly on times in the range of 100fs [38], and reach very high temperatures ( 1000K) due to the low heat capacity of electrons. On femtosecond time scales, the lattice will only heat up after the time τ e. Thus, the modeling of this process for τ τ e is only valid if one considers two-temperature model for electrons and the lattice. Such a two-temperature model assumes thermal equilibrium for the electrons. Results from both experiments and theories show that some deviation from the τ law can be expanded for this case [45, 46, 47, 48] Ablation for different energy levels Following Körner et al., we describe four classes of ablation mechanisms according to laser pulse energy on the substrate [49]:

27 2 Background 13 Figure 2.1: Scheme of pulse laser ablation. I(r), spatial intensity distribution; r, radius; τ, pulse duration [33]. 1. Type A: Ablation by spalling. This mechanism occurs in brittle, highly transparent materials when the laser photon energy is much smaller than the binding energy or latent heat of melting. As a result, the laser beam will act like a volume source, resulting in high mechanical stress and cracks which are able to break down the whole piece of material without melting or vaporizing. Cracks in the sample are unavoidable in this mechanism. 2. Type B: Ablation by evaporation near the threshold. This mechanism is similar to

28 2 Background 14 section for nanosecond pulse duration, when a large portion of the pulse energy is used to melt the sample and only a small portion of it contributes to ablation removal of material. The topography that is generated from the interaction of the first pulse will alter the absorption of subsequent pulses which improve the beam and material inhomogeneities. The ablation itself will occur when the surface temperature (T ) reaches to the evaporation threshold: T = ρ 4D T t K π (2.1) where ρ is the laser power density, D T is the thermal diffusivity constant, t is the time and K is thermal conductivity. The thermal penetration depth then will be given by: d T = 4D T τ (2.2) where τ is pulse duration. 3. Type C: Ablation by stationary evaporation. In this mechanism, the fluence is high enough for ablation with evaporizing, but the vapor pressure is not high enough to ablate the molten phase. A deep and narrow hole will be formed as a result of mutiplereflection and the hole depth is limited to the amount of laser pulse intensity that can reach the bottom of the hole. 4. Type D: Ablation by stationary melt displacement and ejection. In this case, the fluence is large enough for ejecting the melted material. Large aspect ratio holes can be created

29 2 Background 15 with mutiple pulses, but a thick and porous layer will remain on the walls which is due to the recasting of the adherent melt on the walls during the ejection. Fig. 2.2 schematically describes these four classes of ablation. By increasing the incident laser beam power, a Type B mechanism will gradually change to a Type C and consequently to Type D process, while Type A is typically only a brittle materials phenomenon. Figure 2.2: Scheme for 4 different ablation mechanism due to laser pulse energy for single and mutiple pulses in each case: (a) existence of microcracks in ablation for brittle materials, (b) inhomogeneous profile, (c) narrow V-shape hole can be reached by mutiple pulses, and (d) large aspect ration holes with recasting formation on the walls.[49].

30 2.2 Nonlinear phenomena 2 Background 16 When electromagnetic waves travel through materials, the incident electric field will induce a polarization in the matter which varies linearly with the amplitude of the electric field, as well as parts which will vary nonlinearly. The polarization induced inside materials in this regime will read as: P = ɛ 0 [χ (1) E + χ (2) E E + χ (3) E E E +...] = P (1) + P (2) + P (3) +... (2.3) where ɛ 0 is the free space permittivity in F/m and χ (n) is the n th order nonlinear susceptibility for the material. In the case of non-isotropic materials, susceptibilities are non-diagonal tensors [50]. In materials with inversion symmetry, all the even order susceptibilities become zero and the polarization can be written with good approximation as: P = ɛ 0 [χ (1) χ(3) E 2 ] E (2.4) in which the effective susceptibility,χ eff, is given by: χ eff = χ (1) χ(3) E 2. (2.5) This means that the refractive index which is n 2 = 1 + χ (can have imaginary part in the case of lossy medium) will be a function of electric field intensity. As we are considering glass materials in this work which have inversion symmetry, this approximation is valid through

31 2 Background 17 this thesis. Examples of nonlinear phenomena that will taken place in glass materials are mutiphoton photoionization, avalanche ionization, and filamentation and self focusing. These are further discussed below Multiphoton photoionization Photoionization is a process in which electrons in atoms will be directly exited by the laser electric field. Since visible wavelength light does not have enough energy to excite electrons from the valence to the conduction band directly in transparent materials, mutiple photon processes such as multiphoton or tunnelling ionization are required for absorption [7]. The dominating process depends on the Keldysh parameter, given by [51]: γ = ω e [mcnɛ 0E g ] 1 2 (2.6) I where ω is the laser beam angular frequency, I is the laser intensity in the focal point, m and e are reduced mass and charge of electron, c is velocity of light, n is refractive index of the material at the angular frequency ω, ɛ 0 is free space permittivity and E g is the material band gap energy. Different domains of photoionization are described in Figure 2.3. In the case of strong laser intensity with low frequency, or with γ < 1.5, tunneling ionization dominates. As depicted in the left hand side in Figure 2.3, the electric field of the laser suppresses the Coulomb well that binds a valence electron to atom, where in the case of very strong electric field the Coulomb well can be suppressed enough so the bound electron will tunnel through

32 2 Background 18 the short barrier, and frees the electron. On the other hand, at high frequency with lower intensity, several photons will be absorbed simultaneously as shown in the right hand side in Figure 2.3. For this interaction, the number of photons absorbed by an atom should surpass the band-gap energy, or Nhν E g where N is the number of photons and hν is the photon energy. This case dominates for γ > 1.5 [7]. In the case of γ 1.5, the photoionization is intermediate with a combination of tunneling and mutiphoton ionization, described in the center scheme in Figure 2.3. Figure 2.3: The schematics for nonlinear photoionization with different values for Keldysh parameter, γ [7] Avalanche ionization Avalanche ionization will happen as follows: First, an electron in the conduction band will be ionized or promoted to the conduction band by any different processes, shown in Figure 2.4. This free electron then is accelerated by the electric field of the laser to high energy levels where the energetic electron can then collide and dramatically ionize other bound electrons as shown in Fig These two free electrons can further be accelerated and collisionally excite other electrons as long as laser beam is present. The growth rate of the electrons in

33 2 Background 19 the conduction band leads to avalanche ionization with population growth rate given by: dn dt = ηn (2.7) where η is avalanche ionization rate and N is the electron density. For avalanche ionization, the existence of excited electrons in the conduction band is an initial required condition. These initial electrons can be provided through multiphoton or tunneling ionization, ionized impurities or defect states, and thermally excited carriers [52, 7]. Figure 2.4: Schematic potential energy diagram for avalanche ionization (right) that follows ofter seed electrons are created by mutiphoton absorption (left) [7] Filamentation Filamentation occurs as a result of propagation of an intense laser pulse in transparent materials. When a collimated laser beam with a transverse Gaussian intensity distribution propagates through the medium, the center part of the beam will experience a higher

34 2 Background 20 refractive index due to the third order nonlinearity, expressed as: n = n 0 + n 2 I (2.8) where n 0 is the linear refractive index of medium, n 2 is second order nonlinear index and I is the laser beam intensity. Hence, the central part of the beam will travel at a lower speed, which will result in a concave phase front that then will focus the beam. This phenomenon is called self-focusing. Self-focusing is a threshold process that requires the peak power of the laser beam (in the case of CW Gaussian wave) to reach the critical power for the filamentation, P c, which is given by : P c = 3.77λ2 8πn 0 n 2 (2.9) where λ is the laser wavelength. The effect of filamentation can occur in all medias which are transparent and have a positive n 2. Filaments created by powerful ultrashort laser pulses in air can reach a length on the order of meters or even kilometers [53, 54]. Filaments in condensed matter are shorter because of the way free electrons are generated. Firstly, the central thin slice of the pulse is self-focused due to the its highest intensity, where the molecules undergo tunnel ionization. The electrons in the matter are excited from the valence band to the conduction band and receive energy through inverse Bremsstrahlung process [55]. Further excited electrons are generated due to partial cascade ionization. The electron density reaches N e cm 3 which is three orders of magnitude lower than the atomic

35 2 Background 21 density of condensed matter. A plasma is generated which then defocuses the central part of the pulse to cancel the self-focusing effect [56, 57, 58]. The energy of the slice is reduced due to the loss in ionization. Then, the next thin slice of the pulse self-focuses but at further positions in the propagation direction due to the lower peak power [55]. The process of Kerr lens self-focusing and defocussing in the plasma repeats itself. Thus an extended series of hot spots appear along the propagation axis (due to self-focusing and plasma defocusing), the so-called filament. Self-focusing describes the transverse dependence of the phase shift whereas self-phase modulation describes the temporal dependence which occurs due to the high intensity interaction in the self-focal volume. The refractive index of the material becomes time-dependent which is the reason for self-phase modulation. A consequence is the spectral broadening of the laser pulse. The pulse transforms itself into a chirped laser pulse whose white light is visible in the filament. This broad spectrum light generation is called supercontinuum [54]. 2.3 Heat accumulation effect Heat accumulation effects allow the formation of symmetric optical waveguides in glasses with a focused beam of ultrashort laser light [59, 60]. The high repetition rate (>200kHz) ultrashort laser pulses can cause heat accumulation effects that can decrease the defect-induced damages during laser matter interaction and avoid collateral damages such as microcracks formation [61, 25]. This process will happen when the time interval between laser pulses is

36 2 Background 22 less than the time required for the absorbed energy to diffuse out of the focal volume [9]. As a result, a more gentle heating over a period of several pulses is possible which will transform brittle materials such as glass to a ductile material. A numerical model of cumulative heating is shown in Figure 2.5 where a finite-difference thermal diffusion model is used to calculate the temperature at a radial position of 2µm from the center of the laser beam incident inside the glass sample for three different pulse repetition rates [9]. As shown, at 100kHz, the temperature relaxes to below the working point before next pulse arrival, while in the case of 500kHz and 1MHz, heat accumulation is strongly evident which will lead to higher temperatures in the melting volume by increasing the number of incident pulses. Figure 2.5: Finite-difference model of glass temperature versus number of incident pulses, at a radius of 2µm from the focal point of the incident beam. The heat source due to each laser pulse was assumed to be a delta function in time which is valid for femtosecond range of pulse durations [9]. The most common laser systems for fabricating the optical waveguides in glass are am-

37 2 Background 23 plified Ti: Sapphire lasers, operating at low repetition rates (1 to 200kHz). At these rates, pulse energies in the range of 1mJ are available which can produce high nonlinear absorption with weak focuses; however, waveguides created with such a focusing condition suffer from asymmetry in the structure. On the other hand, femtosecond laser oscillators operating at high ( 10MHz) repetition rates can produce more symmetric structures at such rates. Nonetheless, due to low pulse energies of these oscillators, high numerical aperture (NA) lenses are required which limits the working distance and depth for 3D fabrication [9]. For waveguide writing purposes, recently introduced fiber laser systems can provide ideal conditions for pulse energies and repetition rates suitable for low loss symmetric waveguide fabrications [62]. The pulse energies provided through this laser system are not sufficient enough for hole drilling purposes, where using high NA lenses reduces the aspect ratio of the drilled holes. 2.4 Time-lapse images of microhole fabrication Lawrence Shah et al. in [63] reported the micromachining in silicate glasses using femtosecond laser pulses. They used the experimental setup in Figure 2.6, in which the expanded beam of a femtosecond laser system producing pulses of τ=110fs and 1.6mJ energy at 845nm with 5Hz repetition rate is focused on a glass substrate with 20cm long focal length lens. In this setup, a small portion ( 1%) of the laser beam energy was frequency doubled to produce a synchronized visible illumination source for backlighting the target and capture

38 2 Background 24 the plasma spark during the laser interaction process. Figure 2.6: Experimental setup for microhole drilling and in-situ imaging presented in [63]. The images with 1/125 shutter speed in soda-lime silicate glass are shown in Figure 2.7 for 20 frames. The time difference between each of these frames is 30 seconds which is 150 pulses for the used repetition rate. From these frames, three different phases can be reported for femtosecond laser pulse deep hole drilling process. In the first phase, the laser plasma has a uniform shape inside the cylinder and an escaping plasma is also visible confirming the etching process (Frames 3-6 in Figure 2.7). In the second phase, which is the transition phase, the plasma distribution becomes nonuniform and the ablating tip is narrower as plasma cools before escaping the hole (Frames 7-10 in Figure 2.7). In the final phase and by frame 17 in Figure 2.7, the plasma is broken into two main sections with a bright area in the middle of the hole. At this phase, the plasma can not escape the hole and the etching process ends. A comparison of the penetration depth versus the number of laser pulses is presented in

39 2 Background 25 Figure 2.7: Time-lapse images (one frame every 30 seconds, corresponding to impact of 150 laser pulses) of drilling of soda-lime silicate glass, for E p = 1.5mJ and I p = W/cm 2 [63]. Figure 2.8, where these three phases can be identified. In this figure, a comparison between a 1kHz case and 5Hz is also shown which indicates that for the higher repetition rate, the

40 2 Background 26 Figure 2.8: The hole depth versus the number of laser pulses in soda-lime silicate glass at 1kHz and 5Hz. For 1kHz: τ = 150fs; λ = 775nm ;E p = 0.85mJ/pulse; I p = W/cm 2. For 5Hz higher intensity: τ = 110fs; λ = 845nm; E p = 1.5mJ/pulse; I p = W/cm 2. For 5 Hz Lower Intensity: τ = 110fs; λ = 845nm ;E p = 1.0mJ/pulse; I p = W/cm 2 [63]. penetration saturates at an earlier point compares to 5Hz case. Shah et al. reported that the nonlinear self-focusing phenomenon and the influence of the residual ablated materials within the laser-machined channel attributes to this early-saturation [63]. However, comparing the repetition rate to Figure 2.5, the process could not use the cumulative heat transform from pulse to pulse due to long (1ms) time slots between pulses. Furthermore, as discussed in Section 2.1.1, the ablated plume and plasma leave the substrate with µm/ns speeds, meaning that an imaging system with nanosecond time gates is required in order to capture the physical phenomena during microhole drilling, which is one of the subjects of study in this thesis.

41 Chapter 3 Experimental method In this chapter, different aspects of the experimental arrangement that we used to study the burst laser interaction are presented in details. In the first section, the standard laser system is described, and followed by the purpose-built burst resonator cavity in the second section that we integrated with the laser system. The third section describes the beam delivery system, and details about the ICCD imaging system is the last section of this chapter. 3.1 Ti: Sapphire laser system used for burst generation The commercial femtosecond laser system that has been used in this study is the Ti: Sapphire Chirped Pulse Amplifier (CPA), manufactured by Spectra-Physics (Mountain view, 27

42 3 Experimental method 28 CA, USA). This system provides pulses with Gaussian temporal profile at 800nm wavelength (near-ir) with low pulse repetition rates of up to 1kHz. It contains four different parts that are shown in Figure 3.1. The Millennia Pro, in Figure 3.1, is a diode-pumped Figure 3.1: An schematic of complete Spitfire Pro system, manufactured by Spectra-Physics. doubled frequency Nd:YVO 4 system that generates CW green beam of 4.4W power at visible wavelength of 532nm. This laser system serves as a pump laser for the Tsunami oscillator. The Tsunami system is a Ti: Sapphire passively mode locked laser that generates mid-ir pulses at 800nm wavelength with high repetition rate of 72.66MHz. The output pulses of this system have spectral bandwidth of 35nm and the pulse duration of 40fs when tuned properly. The pulse energies are very low and below 10nJ with the total output power of 500mW. The Tsunami pulses are used to seed the Spitfire Pro system. The Empower is a Q-switched Nd:YLF frequency doubled laser system, generating green light at 527nm wavelength with pulse repetition rate of 1kHz, which haa a 100ns pulse duration (FWHM) and the pulse energy up to 20mJ, for a total maximum output power of

43 3 Experimental method 29 20W. The Empower output beam serves to pump source for the Spitfire Pro laser system. The Spitfire Pro contains three internal parts:1) Stretcher, 2) Regenerative Amplifier and 3) Compressor, as shown in Figure 3.1. The Tsunami seed pulses enter the Spitfire, and pass through the grating stretcher, will be broaden to 400ps as a frequency chirped pulses that contain spectrum of the original Tsunami laser. The chirped pulses then enter the regenerative amplifier to be amplified in a Ti: Sapphire crystal that is pumped with the Empower beam. Since the repetition rate of seed pulses are so much higher than the one for the pump laser, two Pockels Cell (PC) are used in order to select individual pulses to be trapped inside amplifier at specific times (PC1), and also to be ejected from amplifier (PC2) when they reach the maximum power available thought the regenerative amplifier. The PC timing is very critical and it will effectively influence the amplification efficiency of the system. Finally, the amplified pulses with low repetition rate will go through the Compressor, which acts in the reverse way of the stretcher to compress the pulse duration down to 40fs. Table 3.1 summarizes the Spitfire Pro system typical operating parameters. Name Laser Type λ Repetition rate Pulse duration Typical output power Millennia Pro Nd:YVO 4 532nm CW - 4.4Watts Tsunami Ti:Sapphire Mode nm 72.66MHz <80fs 0.5Watts Locked Empower Nd:YLF 527nm 1kHz 100ns 13Watts Spitfire Pro Ti: Sapphire Regen. 800nm <1kHz <35fs 2.5Watts Amplifier Table 3.1: Characteristics of the lasers used in the Spitfire Pro system.

44 3.2 Burst resonator system 3 Experimental method 30 A key modification to the system to provide high repetition bursts and drive heat accumulation effects was the development of the burst resonator cavity. The overall modified laser system is shown in Figure 3.2 where the burst cavity is accessed between the regenerative amplifier and the compressor parts of the Spitfire. For this purpose, each 800nm pulse exits the Spitfire resonator, is broken to a burst train of laser pulses and is then retarded to the grating compressor. The reason for inserting the cavity in this part is, since the laser beam has long pulse duration between regenerative amplifier and the compressor, the intensity of such pulses will not exceed the damage threshold of the burst resonator s components. Figure 3.2: A schematic of the burst generator integrated with the Spitfire laser system. The output parameters are shown. The repetition rate is decreased to 500Hz in order to match the timing system in Section Due to the incapability of the timing system (Section 3.2.2) to operate at 1kHz repetition rate, 500Hz repetition rate has been chosen for the burst system.

45 3 Experimental method Optical arrangement for burst generation Figure 3.3 shows the optical arrangement inside the burst resonator cavity. The beam Figure 3.3: Optical arrangement of the burst resonator cavity. enters the burst optical system through the telescope to match the focusing conditions of the resonator which consist of several optics between two curved end mirrors of M4 and M5 (Figure 3.3). As M4 and M5 are similar mirrors, the stable focusing condition for the cavity happens when the Gaussian beam has its minimum waist in the middle of the cavity; thus, the entering telescope is used to satisfy this condition, and also to decrease the beam diameter to match the cavity spot size. The beam enters the resonator with horizontal polarization, through the polarizing beam

46 3 Experimental method 32 splitters (PBS) P1 and P2 and Faraday Rotator (FR) without any change in its polarization (FR is located in the opposite direction to the entering beam, meaning that the polarization would only be affected while the laser beam passes in opposite direction). Then, it passes the Pockels Cell (PC) which is a BBO crystal controlled by a high frequency operating driver (PCD-dpp - Bergmann Messgeräte Entwicklung KG), using M2 and M3 mirrors. The applied high voltage affects the beam polarization here on the PC, which is the controlling part of the system. The group velocity dispersion (GVD) compensation of the cavity is handled with a pair of prisms and end mirror M5, serving to cancel the GVD that is mainly caused by the BBO crystal inside the PC. The HWP is used to change the laser polarization to horizontal for matching the Brewster s angle of the prisms [64] and enforcing the reflection loss to be zero. Dispersion by the first prism spreads the spectrum angularly while the second prism collimates the beam. Mirror M5 returns the beam on the same path, leaning to different frequency elements traveling different path lengths on the neighbor path. Depending on the high voltage value, three different scenario are expected: 1. Zero voltage on Pockels Cell: For zero bias voltage, the beam passes the PC without any change in polarization. Then, the horizontal polarized beam will be transmitted through P2 and FR which will rotate its polarization to vertical. The vertical polarized beam will then be reflected by P1 and using M6, M7, M8 and a Half Wave Plate (HWP), will return back to the Spitfire. The HWP is used to bring the polarization back to horizontal, to match the input beam originally required by the Spitfire compressor system.

47 3 Experimental method Applied voltage V π on Pockels Cell :The required voltage for the electro-optic modulator to act as a Quarter Wave Plate (QWP) is V π =8.6kV [65]. In this case, the horizontal polarized beam completely converts to vertical polarization by double passing the PC in Figure 3.3. Then, the beam will be reflected by P2 toward the GVD compensation system and then returns by M5 end mirror and P2 to complete a cavity round trip. This beam is therefore trapped inside the cavity and resonates between two end mirrors (M4 and M5) as long as the PC voltage immediately returns to zero after the first pass. 3. Partial polarization shifting at Pockels Cell: In this case, the PC voltage is lower than V π, leading to an elliptical polarization after a double passing of the PC. Elliptical polarization separates into horizontal and vertical polarization beam components that physically will be separated at PC2: The horizontal part of the beam will eject the cavity through FR and P1, and the vertical part of the beam will remain trapped inside the cavity for another pass. In the burst operation, the first injected pulse is rotated to partial vertical polarization of up to 90% of its total amplitude, meaning that 90% of the initial amplitude stays in resonator for a round trip. Thus, a very high voltage is necessary for the creation of the first pulse. The trapped remained beam with vertical polarization then on the second trip undergoes a partial conversion to horizontal polarization with only a moderately low voltage to release the second pulse of the burst train. In an ideal operation for generating equal pulses in the burst, the first high voltage is close to V π, then it is the lowest for the second pulse and increases for releasing next pulses until V π is selected to release the final pulse (see

48 3 Experimental method 34 Figure 3.6). The maximum number of pulses that can be generated in this system depends on the loss of the cavity and the minimum required pulse energy for laser interactions. The transmission efficiency of the optics and the correct alignment of mirrors, prisms and PC are of siginificant importance to minimize the loss. In a typical operation of the system, the main source of the loss in the cavity is both on the BBO crystal and the prisms. Other major losses can occur in cases such as incorrect alignment of the beam splitters (polarizer beam splitters are very sensitive to the angle of the incident beam), and also due damage spots on the coating of the Faraday Rotator, beam splitters or mirrors. The cavity typical efficiency is 50% for generating 6-7 pulses in burst train. Figure 3.4: Intensity profile for the laser beam measured after the compressor for (a) single pulse mode and (b) 6 equal pulses in the burst train. With an ideal alignment for all the components, all the pulses in the burst train should have the same intensity profile. Figure 3.4 shows the intensity profile measurement in a case with 6 pulses in the burst train, comparing with the single pulse operation mode. This figure shows that all the five pulses after the first pulse in the burst train are going thought

49 3 Experimental method 35 the same path and they are having their maximum intensity at the same physical position. Also, Figure 3.5 demonstrates the autocorrelation measurement (APE Pulse Check) for the pulse duration. In the case shown in Figure 3.5, single pulse operation mode has 90fs pulse duration (in (a)), and integrated 7 pulses in the burst train are having 93fs pulse duration which means that the double prisms arrangement is successfully canceling all the GVD introduced to the laser pulses in each round trip. Note that for both Figures 3.4 and 3.5, bursts with equal pulses were used. Figure 3.5: Pulse duration measurement using Autocorrelator system, showing (a) 90fs pulse duration for single pulse mode and (b) 93fs for 7 pulses in the burst train PC timing control system To control the individual pulse amplitudes in the burst train, the voltage for the PC should be modulated with a modulation peak of up to 9kVolts in each round trip of the cavity (26 nanosecond). Due to the difficulties of the direct modulation, we decided to use the time-delays method which is described in Figure 3.6. In this method, the maximum value

50 3 Experimental method 36 for the high voltage is constant for all the iterations (equal to V π ), and only the starting time for triggering of the high voltage is delayed for different pulses. As shown in Figure 3.6, the τ A and τ C are delays for the ON signal of high voltage pulses and τ B and τ D are delays for the OFF signals. The vertical red lines show the times that the laser passes the PC, which is almost 2 nanosecond time difference between the initial and returned pulses reflected from M4. Figure 3.6: An schematic of time-delay method. By controlling the starting time for triggering of high voltage, different modulation levels can be introduced to the laser pulses. Vertical lines show the times laser pulses are passing the PC in each round trip. As illustrated, using the natural rising and falling parts of the high voltage signal, the effective voltage introduced to the crystal can be changed in each round trip controlled by the time delays τ A, τ B, τ C and τ D. So, the modulation of high voltage is done by controlling the time delays for each triggering pulse. The time triggering delay control system is shown in the Figure 3.7. In this system,

51 3 Experimental method 37 the laser oscillator clock signal is pre-amplified to feed the digital and analogue waveform generators (MI.7010 and MI.6111 Spectrum cards, respectively) with the 500Hz trigger signal from the laser amplifier. This pre-amplification is necessary because the small output signal of the oscillator can not be recognized with Spectrum cards. Figure 3.7: An schematic of time-delay system used for the burst generator system. These analogue and digital signals in 4 channels (A + C for two ONs and B + D for two OFFs) go to the time shifter board (BME-TS01 - Bergmann Messgeräte Entwicklung KG) in which the digital signals are delayed due to the set values of the analogue signals to feed the PC driver. The controlled delayed signals are generated every 2 millisecond (for the 500Hz laser repetition rate) and the number of the digital pulses usually varies between 1 to 5 pulses for each channel in typical operation mode, which can produce burst trains up to 10 pulses in the case of 5 pulses in the delay signals. There is no limitation on the maximum number of pulses that can be generated in the timing system, and the main limitation for

52 3 Experimental method 38 higher number of the pulses are those discussed in section Figure 3.8: A few examples of different burst profiles generated by the burst generation system. This beam will detected using photodetectors and recorded on an oscilloscope.(a) single pulse, (b) three equal pulses, (c) six equal pulses, (d) five pulses with the decreasing profile, (e) five pulses with high-low profile and (f) four pulses with increasing pulse energies. The time between adjacent pulses is 26ns. The time-delay method provides flexibility in controlling the burst pulse profile shape as well as the number of pulses in each burst. For instance, some of the burst profiles created with our system are shown in Figure 3.8 (a) to (f); as shown, both the total number of pulses and the amplitude of each individual pulse can be controlled in this arrangement. It should be noted that the direct modulation of the high voltage at such a high speed is not commercially available for using in this system, and our time-delay method is an alternative approach for producing these voltages. However, compared to the direct modulation method, the fluctuation over produced burst pulses is higher in time-delays technique and is subjected

53 3 Experimental method 39 to the stability of the delays, as well as rising and falling times of the high voltage. While using the time-delays method, it is necessary to measure the fluctuation over each pulse in the burst before starting each experiment to make sure that these fluctuations are small enough to have any affect on the results. 3.3 Beam delivery method The beam delivery system is also shown in Figure 3.9 schematically. The compressed burst pulses pass a set of HWP (placed in a rotational linear stage) and a polarizer, which controls the power of the laser beam. The beam is then focused using an objective lens above the glass sample. The focusing lens is also fixed on a linear stage (Aerotech ALS130) which can be vertically adjusted for different pre-defined focus positions. The glass sample is mounted on a two-axis air-bearing stage (Aerotech ABL1000, 2nm resolution and 50nm repeatability) and moved perpendicularly to the direction of the incident beam. All the moving stages (for glass sample, focusing beam and also the HWP) are controlled with NView Automation 3200 which is a computer software programmable with G-code in order to provide any desirable movement pattern. Using a weak transmitted beam thought the M1 mirror in Figure 3.9, the burst profile can be detected using a sensitive photodetector. The signal is always displayed on an oscilloscope in order to monitor the stability of the produced burst profile. However, since the sensitive surface of the photodetector is smaller than the output beam size, there is a possibility for

54 3 Experimental method 40 Figure 3.9: Beam delivery system for hole drilling in glass. detection of an incorrect intensity profile using direct beam. Hence, the best way to check the burst intensity profile is to use a reflection of the whole beam from a poor reflectance surface such as a white paper. A CCD camera in Figure 3.9 is also used to find the top surface of the sample using a transmitted beam over the mirror M5. Figure 3.10 also shows some parts of the experimental setup including the burst resonator system in the lab. During this study, we always used a 10X objective lens with NA=0.16 for the 5mm diameter laser beam. Assuming the collimated beam covers the entire lens, the beam radius at

55 3 Experimental method 41 Figure 3.10: Photo taken in the lab. In this photo, burst resonator system is shown in addition to parts of the Spitfire and beam delivery system. the focus (beam waist), w, can be calculated as [66]: w = λ M 2 π NA (3.1) where λ is the wavelength, NA is the numerical aperture of the focusing lens and M 2 is the beam profile quality factor, and according to ISO [67] defines as the beam parameter

56 3 Experimental method 42 product (product of beam radius measured at the beam waist and the beam divergence halfangle measured in the far field) divided by λ/π. This M 2 parameter limits the degree on which the beam can be focused for a given beam divergence angle [68]. The Rayleigh length, defined for a Gaussian beam as a distance from the beam waist to the position that the beam radius is increased by a factor of 2 of its minimum value in Equation 3.1, can be calculated as follow for a diffraction-limited beam: z R = π w2 n λ (3.2) where n is a refractive index of a material in which beam is propagating. Depth of the focus, is simply defined as 2z R, which is the distance in which the beam waist is smaller than 2w. This parameter defines the length in which the beam can be considered as a tightly focused beam. For the laser system in this study, the M factor is very close to one. Thus, Table 3.2 shows the beam waist and depth of the focus. Laser System Beam waist at the focus Depth of the focus Spitfire Pro 1.59µm 19.89µm Table 3.2: Focusing parameters of the Spitfire laser output using a 10X Objective lens (NA=0.16).

57 3 Experimental method Time-Resolved imaging (ICCD) of laser interactions An imaging system has been used in this study in order to record time-resolved images of the laser matter interaction in glass. Details of this system is shown in Figure Figure 3.11: Scheme of the ICCD imaging setup. The intensified CCD-camera is triggered by a digital delay generator which also triggers the mechanical shutter for recording timeresolved images of the laser interaction zone. The shortpass filter λ=750nm avoids the imaging of the laser wavelength λ=800nm. A 20x microscope objective lens is used to capture the images of the laser interaction.

58 3 Experimental method 44 A 20x microscope objective lens with the numerical aperture NA=0.4 was used to record the emitted radiations of the interaction process onto a chip of the time-gated intensified CCD-camera (Andor istar DH734). Time-resolved intensity images were captured during or after the ablation process to record various processes of laser interaction physics and relaxation mechanisms. A shortpass filter with λ=750nm was placed before the ICCD to avoid the detection of the laser light at a wavelength of λ=800nm. The time gate width for imaging was chosen to be t=2ns to best capture all the physics with the best accuracy. Lower time gate widths resulted in invalid data since the ICCD could not open and close exactly at these low time gates. Only one image was recorded at one position of the sample. After taking the image, the sample was moved perpendicular to the beam propagation direction in the plane of the imaging with a two-axis air-bearing motion stage (Aerotech ABL1000). Simultaneously, the delay time of the ICCD was changed in steps of t=2ns to read the next image on a fresh target spot. The camera was activated by a trigger signal from a digital delay generator (DDG) which itself was triggered by a trigger counting box. The DDG also triggered the mechanical shutter which mechanically opened for a defined time. The opening time of the shutter remains the same for one series of images with different time delays. The initial delay applied with the DDG to the ICCD camera to capture the first pulse on the target was 0.776µs. The maximum area from the sample that could be seen on the ICCD camera is 675µm 675µm; however, all the images that were captured with ICCD camera in this work are for the reduced area of 264µm 675µm. For find the times corresponding to the incoming of the individual laser pulses the following

59 3 Experimental method 45 procedure was used: The shortpass filter in Figure 3.11 was removed and a high Neutral Density (ND) filter was placed before the laser objective lens in order to decrease the laser pulse intensity. Then, the laser pulses focused on the top surface of the glass sample, and images with long time gates were taken with ICCD. While using long time gates, detection of the laser pulses on the glass surface is easy, and there is almost no photoluminescence emission due to the low intensity and all the detections were corresponding to actual laser pulses. The time gate was then reduced and an appropriate delay was applied each time to keep the laser pulses in the recorded images of the ICCD. By repeating the same procedure and adding appropriate delays, the time gate width could reduced from 10ms at the beginning to 2ns time width which was used for taking the time-resolved images. In the burst mode of operation, by adding or subtracting 26ns with DDG in Figure 3.11 different pulses in burst can be selected and 2ms delays will switch over different number of burst iterations.

60 Chapter 4 Results and Discussion In this chapter, the results for burst-train high aspect ratio hole drilling in glass and timeresolved imaging of ICCD camera will be presented. These results obtained by the purposebuilt burst generator system integrated with a Ti: Sapphire laser system, as discussed in Chapter 3. In all of the results in this chapter, the focusing lens was a 10x objective lens, with NA=0.16. Using higher NA lenses would result in a faster geometric expansion for the Gaussian beam. The ablation type here is Type D as described in Section Therefore, The hole depth was highly dependent on the amount of the laser pulse energy that could reach the bottom of the hole and as result, high numerical aperture lenses were not appropriate for deep hole drilling purposes because of losses through transparent sidewalls. The fast geometric spreading of the beam that lowers the intensity in deeper holes was another factor 46

61 4 Results and Discussion 47 that further favored low NA lenses. The first part of this chapter presents the experimental results of high aspect ratio hole drilling with examining different parameters and a discussion for each case, and the second part discusses the ICCD images to provide a physical insight for the burst pulse interaction with glass. 4.1 Machining of high aspect ratio holes Focusing condition BK7 glass sample with a 2mm thickness had been used to find the best focusing condition for the hole drilling. A focusing position range of 400µm had been explored in order to find the effect of the focal waist position relative to glass surface on the drilled holes. 20µm steps were chosen to match the depth of the focus for the lens as indicated in Table 3.2. The explored range exists over a range from -200µm to +200µm for the z position (lens position), with z=0 being the position where the beam is focused at the top surface of the sample. To find the z=0 position (beam focal waist on glass surface), a corrected reflection of the beam from the sample surface was detected by a CCD camera as in Figure 3.9. This z=0 corresponds to the place where the laser image detected on the CCD camera has the smallest diameter. This position was further corrected for the non-collimated diverging laser beam that is focused slightly farther than the focal plane. To find the correction offset,

62 4 Results and Discussion 48 an experiment with near ablation threshold pulse energy was made for different focusing position to narrow the range for focusing in which ablation would just happen. In this case, by decreasing the pulse energy, the range with laser intensity high enough for ablation reduced to a smaller area that converged to the depth of the focus. The offset for our system was 45µm. Three sets of experiment were conducted for this part, for three different burst profiles. For all of these cases, the pulse energy was 33.3µJ, meaning that at 500Hz repetition rate for bursts of 38MHz pulses, 6 pulses in a burst profile had 100mW of power, and power for three pulses and a single pulse case were 50mW and 16.6mW, respectively. The pulse duration was constant for all pulses at 100fs, detected on a Autocorrelation system. The side and top images of the created holes taken with an optical microscope are shown in Figures 4.1, 4.2 and 4.3. For all these cases, the exposure time was chosen to be long enough to drill the deepest hole with a sufficiently high number of burst trains. The term long enough will be discussed in Section Discussion As shown in Figures 4.1 to 4.3, when the beam is focused inside the glass substrate (z< 100µm), the taper of the hole has a wide V shape with θ 6 = 36.5 o, θ 3 = 46.6 o and θ 1 = 80 o at z= 200µm for 6 pulse burst, 3 pulse burst and single pulse, respectively. Underneath the holes, a lighter area corresponding to positive refractive index modified region is visible

63 4 Results and Discussion 49 Figure 4.1: Side and top view images of holes created in BK7 with 6 equal pulses in a burst. The exposure time is 2 seconds for all cases, and the focusing position changes from 200µm inside the sample to 200µm above it, with 20µm steps. Pulse energy is 33.3µJ and pulse duration is 100fs at 800nm wavelength for all cases. The glass sample is on air on both top and bottom side, and the laser beam is coming from top of the image.

64 4 Results and Discussion 50 Figure 4.2: Side and top view images of holes created in BK7 with 3 equal pulses in a burst. The exposure time is 2 seconds for all cases, and the focusing position changes from 200µm inside the sample to 200µm above it, with 20µm steps. Pulse energy is 33.3µJ and pulse duration is 100fs at 800nm wavelength for all cases. The glass sample is on air on both top and bottom side, and the laser beam is coming from top of the image.

65 4 Results and Discussion 51 Figure 4.3: Side and top view images of holes created in BK7 with single pulses. The exposure time is 2 seconds for all cases, and the focusing position changes from 200µm inside the sample to 200µm above it, with 20µm steps. Pulse energy is 33.3µJ and pulse duration is 100fs at 800nm wavelength for all cases. The glass sample is on air on both top and bottom side, and the laser beam is coming from top of the image.

66 4 Results and Discussion 52 for the case of 3 pulse bursts. The positive index modification does not exist for the 6 pulse bursts and the single pulse results. The reason is, in 6 pulses/burst case, the hole is already big enough to drill past such modified regions and removes them; as for z= 200µm and z= 180µm focal positions, these positive index volume areas are visible. It should be mentioned that z= 200µm, the beam is at the position of z= = 302µm inside the sample, where 1.51 is the refractive index for BK7 at λ =800nm. On the other hand, in the single pulse case the outcome is different. Positive refractive index can not be made inside BK7 with a single pulse, and heat accumulation effect (Section 2.3) appears necessary for this positive index change. Also, the filament length, which is a bright line under the hole, is smaller in single pulse mode with the shorter life time, which will be shown in Section By moving the focusing position toward the sample surface on the top, the center part of the hole will grow deeper in the substrate, and the tapered V shape of the hole also moves up to reduce the size of ablated entrance hole. Further, the refractive index modified zone will become substantially smaller in size. With the appropriate focusing position, the taper will be minimized and the hole shape will become uniform in diameter ( 20µm for 6 pulse burst and 10µm for single pulse). A similar scenario happens when the beam is focused far above the sample surface in the air: The beam size on sample surface will enlarge which will result in a wider V-groove for large z> 100µm lens displacement. In this case, because the intensity of the beam is less than the case in which beam is focused inside the substrate, the hole depth is smaller. The beam also can loose energy in air when it is tightly focused

67 4 Results and Discussion 53 which is a dominant source of loss for later case. Figure 4.4 shows the angle of the hole taper for this experiment. Figure 4.4: The angle of the hole taper for three different burst modes versus the focusing position for data in Figure 4.1 to 4.3. Figure 4.5 shows the measured hole depths as determined in Figure 4.1 to 4.3. One of the interesting features of this graph is that by increasing the number of pulses in the burst (1 3 6), the best position for focusing (i.e. where the deepest hole is produced) will shift inside the BK7 substrate. To further confirm this point, the same experiment with the same parameters (i.e. 3 different burst profile, 33.3µJ pulse energy, NA=0.16 focusing lens and 100fs pulse duration) was performed in 1mm thick fused silica samples. Graph of results is in Figure 4.6. By comparing these two graphs, we can see that for the 6 pulse burst case, the peak depth of 512µm happened at z= 100µm (z corrected = 151µm for n=1.51) for BK7 and at z= 60µm (z corrected = 87µm for n=1.453) for fused silica where a hole

68 4 Results and Discussion 54 Figure 4.5: Hole depth versus the focusing position for three different burst profiles in BK7. This graph represent the results in Figure 4.1 to 4.3. with 325µm depth was created. For 3 pulse case, peak depth was 305µm at z= 80µm (z corrected = 121µm) in BK7 and 197µm at z= 60µm (z corrected = 87µm) in fused silica. Finally in the single pulse mode, the deepest hole with 169µm depth was created at z= 20µm (z corrected = 30µm) in BK7 and at z= 20µm (z corrected = 29µm) in fused silica with the almost same depth of 119µm. These results show that, while fused silica is a harder material in comparison with BK7 to drill holes in, it also shows less sensitivity to the focusing position. This can be due to the higher bandgap energy of the fused silica compared to the BK7, which will lead to a weaker laser interaction (E g 9eV for fused silica and 4.7eV for BK7 [17]). Also, comparing Figures 4.5 and 4.6 shows that fused silica has a smoother response to the focusing position compared to BK7. In both cases, higher number of pulses

69 4 Results and Discussion 55 in burst train requires deeper focusing for the best results, and the best position for focusing in single pulse mode is close to sample surface. This focus shift phenomenon can be described as the following: The ablation mechanism for our condition is Type D, in which the fluence is enough for melting and ejecting material, as it can be identified with the recasted materials on the sidewalls of the hole [49]. The more fluence that can reach the bottom of the hole, the more materials can be removed and the deeper hole can be drilled. When the beam is focused inside the sample, more energy can reach the bottom of the hole which will result in a deeper hole. On the other hand, laser pulses lose energy as they propagate from sample surface to the bottom of the hole. But, by increasing the number of pulses in burst, heat accumulation effect the heat from pulse to pulse which provide more energy for drilling.more pulses in the burst train will allow one to take advantage of deeper focusing and get deeper holes by transporting more heat to the bottom of the hole through this heat accumulation effect Pulse duration effect As discussed in Chapter 2, reducing the laser pulse duration can change the ablation mechanism and decrease the HAZ in two ways: 1) Smaller heat transfers to the lattice due to very short pulse duration, and 2) Stronger nonlinear absorption which only occurs in the focal point of the beam due to higher intensity of ultrashort pulses. In this section, the effect of the pulse duration on hole drilling is discussed. Bursts with 5 pulses and 24µJ energy in each pulse were used (equal pulses). Holes created with several different exposure conditions

70 4 Results and Discussion 56 Figure 4.6: Hole depth versus the focusing position for three different burst profiles in Fused Silica. were made inside the glass samples (BK7 and fused silica) where the beam was focused on an optimum position of deep holes (z= 45µm inside sample in all cases) with a NA= 0.16 Objective lens. This experiment was repeated for four different pulse durations of τ =100fs, τ =600fs, τ =1ps and τ =2ps, and the side images of the holes were recorded and are shown in Figure 4.7 for BK7 sample and Figure 4.8 for fused silica. Graphs in Figure 4.9 show the measured hole depth obtained from these experiments. The pulse duration has almost no effect on hole drilling in fused silica, while it has a small effect on hole drilling in BK7. Although we expected to see a significant difference in hole depth while pulse duration changes of 100fs 2ps, data in Figure 4.7 indicates only negligible dependency. As discussed in Section 2.1.1, processes with pulse duration less than 10ps are in the same category, and within this category they all act almost similarly.

71 4 Results and Discussion 57 Figure 4.7: Holes ablated inside BK7 with bursts of 5 pulses with 24µJ pulse energies. (a) 100fs, (b) 600fs, (c) 1ps and (d) 2ps. For the 5 pulse burst profile, 200, 400, 800, 1200 and 200ms exposures mean 500, 1000, 2000 and 3000 pulses were applied in each etched hole.

72 4 Results and Discussion 58 Figure 4.8: Holes created inside fused silica with bursts of 5 pulses with 24µJ pulse energies. (a) 100fs, (b) 600fs, (c) 1ps and (d) 2ps. For the 5 pulse burst profile, 200, 400, 800, 1200 and 200ms exposures mean 500, 1000, 2000 and 3000 pulses were applied in each etched hole.

73 4 Results and Discussion 59 Holes in Figure 4.7-(b) to (d) and Figure 4.8-(a) to (c) do not have a straight shape as they slant at the bottom. As holes grow in the substrate, subsequent pulses have to travel through the already created channel to reach the bottom of the hole. These pulses will go through several reflections from sidewalls of the hole, and in some cases these reflections can change the direction in which the hole is growing. Several parameters such as beam quality and polarization of the beam may affect this curvature, which requires more study for a clear description and a control method Significances of the burst effect In order to further study the significance of burst train pulses on hole drilling, the influence of the number of pulses in each burst on the hole depth is examined. By keeping all other experimental parameters constant, holes inside BK7 were created with: 1) different number of pulses in a burst train, and 2) different exposure time. Pulses with 500Hz repetition rate containing bursts of 38MHz repetition rate at 800nm wavelength were focused with a NA=0.16 objective lens z= 45µm deep inside BK7 samples. Pulse energies are the same for all cases and equal to 20µJ in each pulse with 100fs pulse duration. The results are presented in Figures 4.10, 4.11, 4.12, 4.13 and Unless otherwise specified, all the burst profile have equal-pulse profiles, i.e. similar to Figure 3.8 pictures in part (b) and (c).

74 4 Results and Discussion 60 Figure 4.9: Hole depth vs. number of pulses for experiments in Figures 4.7 and 4.8; (a) BK7 and (b) Fused Silica.

75 4 Results and Discussion 61 Figure 4.10: Side view images of holes created in BK7 with 20µJ single pulse for different number of pulses shown above in each hole.

76 4 Results and Discussion 62 Figure 4.11: Side view images of holes created in BK7 with 20µJ energy per pulse with 2 pulses in each burst for different number of pulses shown above in each hole.

77 4 Results and Discussion 63 Figure 4.12: Side view images of holes created in BK7 with 20µJ energy per pulse with 3 pulses in each burst for different number of pulses shown above in each hole.

78 4 Results and Discussion 64 Figure 4.13: Side view images of holes created in BK7 with 20µJ energy per pulse with 4 pulses in each burst for different number of pulses shown above in each hole.

79 4 Results and Discussion 65 Figure 4.14: Side view images of holes created in BK7 with 20µJ energy per pulse with 6 pulses in each burst for different number of pulses shown above in each hole.

80 4 Results and Discussion 66 Discussion To have a deeper view of the results and provide a better comparison among these cases, Figure 4.15 shows the hole depth versus the number of pulses hitting the BK7 sample for all of the cases. Note that the total number of pulses is given by the number of burst trains multiplied by the number of pulses per burst. Each of the data sets in Figure 4.15 for different burst profiles can be divided into two regions: an increasing domain in which the holes are growing by more pulses hitting the sample, and a constant region in which the hole s depth will not change by increasing the total number of pulses. A dotted vertical lines identify these boundaries for each case. As a result, if the exposure time is longer than these threshold lines in each case, a maximum number of useful pulses has been applied. This time, depends on the number of pulses in each burst, the pulse energy and also the substrate. As an example, bursts with 6 pulses (Figure 4.14) will create the deepest hole in 600ms while single pulses (Figure 4.10) saturate sooner in 350ms. Also, by comparing Figures 4.7 and 4.8 one can say that for those parameters, the fused silica sample will saturate faster than BK7 1. Physical insight into the saturation process is given in Section By comparing the hole depths with different bursts in Figure 4.15, one can see that before the saturation process, fewer number of pulses in the burst will initially result in a deeper hole. So, at the beginning of the process single pulses yield to the highest depth. Two pulse 1 Saturation happens for fused silica at 400ms, which is 400ms 500Hz 5 = 1000 pulses, and for BK7 at 1200ms, 1200ms 500Hz 5 = 3000 pulses, all for 24µJ energy per pulse and bursts with 5 pulses.

81 4 Results and Discussion 67 Figure 4.15: Comparison of hole depth as a function of total number of pulses for 5 different burst condition. A log-scale is used to show the total number of pulses. Horizontal dotted lines represent the saturation points for each case. After this point, the holes will not grow anymore by increasing the number of pulses.

82 4 Results and Discussion 68 bursts will then overtake this position at N=240 pulses. At the end, the highest depth of 320µm was seen for 6 pulse bursts at N=3000 pulses. The interaction of laser pulses and the sample will generate plume and plasma above the interaction region, which can last for several nanoseconds. Since the time difference between pulses in the burst is 26 nanosecond, the plume and plasma region above sample will survive to absorb part of the energy of subsequent laser pulses. This means that the energy of later pulses in the burst train cannot reach the glass substrate due to plasma shielding. Another factor in Figure 4.15 is that bursts with higher number of pulses will have hole depths that saturate slower while making deeper holes. This is due to the strong heat accumulation effect in bursts with 6 pulses. Heat generated from each pulse at the bottom of the hole can be accumulated with subsequent pulses and as a result, the threshold temperature for melting and ablation can be achieved. After reaching this threshold, ejection of the melted area occurs which increases the hole s depth. By comparing Figures 4.10 to 4.14 in 4.15, it is shown that with the same pulse energy, the burst mode will drill deeper into the glass. In order to find out the pulse energy dependency, holes with four different burst profiles were fabricated with different pulse energy from 3.3µJ to 21.67µJ. These four profiles include the single pulse mode and three burst modes that are shown in Figure (a) to (c). In this experiment, pulse duration was 100fs and a NA=0.16 objective lens was used. The beam was focused 45µm inside the BK7 glass and the exposure time in each case was long enough to drill to the maximum depth, i.e. in 2 seconds. Results are shown in Figures 4.17, 4.18, 4.19 and 4.20, for 6 pulse bursts, 3 pulse

83 4 Results and Discussion 69 bursts with 52ns repetition rate, 3 pulse bursts with 26ns repetition rate and single pulse, respectively. Figure 4.16: Three different burst profiles used for hole drilling: (a) 6 pulses in a burst, (b) 3 pulses in burst with doubling the time (52ns) between each pulse, and (c) 3 normal pulses (26ns separation between each in a burst). Note that in (b), the high voltage over the Pockels Cell is zero for second and fourth round trip of the pulse, allowing the pulses to circulate with no ejection which increase the time separation to 52ns. The measured hole depths in Figures 4.17, 4.18, 4.19 and 4.20 are arranged in Figure 4.21 based on different pulse energies. This graph shows that the hole depth has an almost linear dependency on the pulse energy, and the slope will increase by increasing the number of pulses in the burst. As an example, with 6 pules in the burst and 11.6µJ pulse energy, 150µm hole is created which is 3 times deeper than 50µm hole for the same pulse energy in single pulse case. For 21.67µJ pulse energy, 330µm hole is created with 6 pulses and a 100µm in single pulse case, which once again has a 3 times improvement factor. An important feature in the graph of Figure 4.21 is the comparison of two different 3- pulse bursts with 26ns (Figure 4.16-(c)) and 52ns (Figure 4.16-(b)) time intervals. As it is shown, deeper holes were created for the 26ns case. This observation can also be described by the heat accumulation effect. In the case of 52ns pulse separation (Figure 4.16-(b)) the

84 4 Results and Discussion 70 Figure 4.17: Holes created with the burst profile shown in Figure 4.16-(a) inside BK7 for 2 second exposure with different pulse energies.

85 4 Results and Discussion 71 Figure 4.18: Holes created with the burst profile shown in Figure 4.16-(b) inside BK7 for 2 second exposure with different pulse energies.

86 4 Results and Discussion 72 Figure 4.19: Holes created with the burst profile shown in Figure 4.16-(c) inside BK7 for 2 second exposure with different pulse energies.

87 4 Results and Discussion 73 Figure 4.20: Holes created with the single pulses inside BK7 for 2 second exposure with different pulse energies. repetition rate is 19.2MHz while in 26ns pulse separation (Figure 4.16-(c)) the repetition rate is 38.5MHz; the lower repetition rate case means lower heat accumulation effect, according to simulations in Figure 2.5 and the results in Figure 4.21 are due to the lower etch depth. Comparing these results with Figure?? for holes before saturation, we can conclude the in these processes the heat accumulation effect is stronger than the plume shielding effect that prevent the deep hole drilling before saturation. To examine the reproducibility of our method, we repeated a single experiment twenty times and each time the hole depth was recorded. Bursts with 6 pulses and 17.7µJ pulse energy were used in two cases: before the saturation of holes for exposures of 1200 pulses, and after the saturation for 6000 pulses. The average and the standard deviation of the

88 4 Results and Discussion 74 Figure 4.21: Graph of hole depth as a function of pulse energy for single pulse experiment and with bursts of 6 pulses, 3 pulses with 52ns spearation and 3 pulses with 26ns separation time as seen in Figure 4.16.

89 4 Results and Discussion 75 holes depth are reported in Table 4.1 which highlights that the standard deviation is very low. The low standard deviation proves that the data for hole drilling are reproducible. Mode Average Standard deviation 1200 pulses - before saturation 222µm 2µm (1%) 6000 pulses - after saturation 279µm 11µm (4%) Table 4.1: Tests for repeatability using 6 pules/burst profiles. In each case, 20 separate exposures were are used in order to calculate the average and standard deviation. 4.2 Physical insight of high aspect ratio hole drilling In this section, the method described in Section 3.4 is used to provide physical insight of the microhole machining, nonlinear light interaction, and the plume physics Time-resolved ICCD imaging Time-resolved optical images were taken with the ICCD camera during laser machining with a burst consisting of four pulses with equal pulse energies and 26ns pulse separation. Images in Figure 4.22 from t=0ns up to t=178ns show the transient processes looking after the 1st burst and after the 200th burst, respectively. For the experiment with the first burst the energy of each pulse in the burst is E p = 48µJ; for the 200th burst the energy is E p = 43µJ. The dashed white lines show the surfaces of the glass sample (Figure 4.22). On the left column, a side-view of the structural damages is shown. A filament effect is clearly visible below the structural damages in the first burst microscope image which is a positive refractive index modified region in the shape of a line.

90 4 Results and Discussion 76 Figure 4.22: Time-resolved intensity images during the ablation process for the 1st and the 200th burst in the burst mode. The white line indicates the surface of the glass sample. The images on the far right side show the detected intensity values multiplied by a factor of 5. The beam propagation direction is top down. For the first burst data (Figure 4.22, top row) the first pulse of the first burst strikes the surface of the sample at t=0ns. Photoluminescence of the ablated material is observable in an ablation plume which departs from the surface demonstrated on the next few images. The expansion is impaired by atmospheric pressure. At t=26ns the second pulse of the 1st burst hits the sample. By now photoluminescence is seen from a thin filament with a width of δ 6µm in the volume of the sample. The photoluminescence survives tens of nanoseconds whereas its length remains nearly the same (l = 75µm). The third and the fourth pulses of the burst hit the sample at t=52ns and t=78ns, respectively. At t=78ns to 178ns the intensity of the ablation plume gets weaker. In order to make the ablation plume

91 4 Results and Discussion 77 still observable the intensity values for the image at t 178ns are multiplied by a factor of 5 (right column in Figure 4.22). The pit created after the 1st burst has δ = 26µm diameter and d= 10µm depth; material can easily be removed from the surface with the first pulse, and subsequent pulses in the burst effectively remove more material. For the 200th burst experiment the first pulse hits the bottom of the existing hole. There is no ablation plume above the sample surface during the next tens of nanoseconds. The detected intensity values in the hole show that ablated material is confined inside the high aspect ratio hole which strongly lights up over the whole observation time. The depth of the hole after the 200th burst is near the saturation depth, and only a small amount of ablated material escaping from the hole makes it deeper and saturates the depth. The hole created after the 200th burst shows a smooth and narrow shape with δ = 32µm diameter and d = 187µm depth. The aspect ratio is approximately 6:1. Here, the hole depth exceeds the length, l = 75µm of the filament observed in the 1st burst. For this reason, filamentation cannot be seen after the 200th burst; however, the formation of the hole is guided by the filament. Furthermore, microcracks are not observable in the cross section images. It is assumed that ablated material from the bottom and the walls of the hole which cannot depart is redeposited inside the hole. Thus deposited material smoothens the walls of the hole and help it to maintain a uniform diameter.

92 4 Results and Discussion Filament brightness in pulse and burst mode Samples for the filament photoluminescence are taken from a region of 3 3µm 2 around the brightest spot in the forming filament below the interaction areas in Figure 4.22, first row. The brighter values are added and divided by the number of pixels in order to get a normalized value for the filament brightness in the burst mode. The same method is applied to the appropriate ICCD images of the single pulse case which are taken with the same laser and focusing conditions (not shown here). Figure 4.23 shows the values for both the burst and the pulse mode where a strong dependency of the filament brightness with time is visible. The red arrows in the diagram indicate the timing of the four laser pulses in the burst mode. In the single pulse case, only first arrow corresponds to the laser pulse striking time. Figure 4.23: Intensity decay of the filament brightness during the first burst in the burst mode (black square) and the pulse mode (red triangle). A double exponential decay is fitted to the data. The red arrows indicate the times when four laser pulses strike the surface in the burst mode.

93 4 Results and Discussion 79 Concerning with the burst and the single pulse modes, an increase in the filament brightness is observable after the first pulse hits the sample. A local maximum is seen in both investigated modes at t=10ns after the first pulse hit. It is remarkable that the observed photoluminescence from the filament tracks is delayed. According to the arrival time of the pulse at t=0ns the maximum filament brightness is delayed by 10ns, the physics of which needs to be further investigated. After the maximum filament brightness is seen for the pulse mode, the intensity falloff can be described by a double exponential decay. Fitting the data results in a long lifetime of t p1 =(13.8 ±3.2)ns and a short lifetime of t p2 =(4.1±2.9)ns for the filament photoluminescence. In the burst mode the local maximum of the filament photoluminescence diagram, shown in Figure 4.23, arises with increasing the number of pulses in the burst train, and stays nearly constant for the third and the fourth pulse. Between the pulses the filament zone brightness remains at a relatively constants values. This effect is due to heat accumulation which takes place at high repetition rates (here f=38.5mhz) [69]. Compared to the single pulse mode, a delayed photoluminescence from the filament is also observable after each pulse. The brightness decreases by a double exponential response after the fourth pulse. The long and the short lifetime for the burst mode are t b1 =(80.7±5.9)ns and t b2 =(4.0±0.6)ns, respectively. The observed difference in the long lifetimes for the pulse mode t p1 and for the burst mode t b1 can be explained by thermally activated photoluminescence. Due to heat accumulation effects in the burst mode the filament brightness decreases over a longer time

94 4 Results and Discussion 80 period than in the pulse mode. This figure can be used as a experimental illustration of the heat accumulation effect reported by Eaton et al. in [9] which is shown in Figure Ablation plume and channel brightness The brightness of the ablation plume above the glass surface and the brightness of the plume inside the channel which forms the hole are studied. The integrated ablation plume brightness data are taken in a region of 13 13µm 2, at a position 65µm above the surface. Data for the channel brightness are taken in a rectangle nailed to inside the hole. For the 1st, the 25th, the 50th and the 200th burst, brightness data for the channel and the ablation plume are captured every t=2ns over a time period of t=200ns and plotted in Figure Corresponding to the arrival time of the four pulses in the burst the channel and plume brightness exhibit four local maxima in the 1st burst, with a time delay which is also observed here due to the time that removed materials need to move up from the sample surface. The channel brightness is increasing with each pulse due to heat accumulation effects. The ablation plume brightness slightly increases with each pulse and shows the smallest local maximum for the fourth pulse. Material is removed from the surface of the glass and expands into the air as can be seen in the brightness of the ablation plume. Comparing the ablation plume of the third pulse with the fourth pulse in the burst, a shielding effect can be concluded; plumes from third pulse prevents the total amount of fourth pulse energy to be delivered to the interaction zone, reducing the etching efficiency of the later pulses in the burst train. Similar effect also reported in Section for explanation of Figure 4.15.

95 4 Results and Discussion 81 Figure 4.24: Brightness of the channel (black line) and of the ablation plume (red line) after the 1st, the 25th, the 50th and the 200th burst in the burst mode. The brightness is given in arbitrary values and the red arrows indicate the timing of four pulses in the burst. At the 25th burst the channel lights up and material is ablated. It is remarkable that the maximum of the ablation plume brightness is delayed and does not start until the second pulse hit the sample. Material is still removed but it is more difficult to escape from the already existing hole since it needs more energy to move longer distances, and simultaneously laser pule will lose its energy to reach the bottom of the hole. Two pulses are needed to

96 4 Results and Discussion 82 remove material out of the hole in this case. The data for the 50th burst shows a similar behavior as previous burst. However, three pulses are now needed to ablate the material and escape from the inside of the hole. The etch rate slows down even more as pulses get weaker at the bottom of the hole, and also the longer distance that removed material needs to travel in order to scape. The channel plume brightness shows a strong emission which results from the excitation of the solid walls in the hole and also emit plume photoluminescence excitation trapped in hole. This is the moment in which the hole depth is saturated; without subsequent pulses in the burst, no material can be removed and escape from the hole. After the 200th burst very little material can be removed from the hole. The etch rate is almost zero and it is close to the saturation level. The channel photoluminescence brightness is weak and has a statistical noise which depends on the reflection and excitation from the walls of the hole. If the data of the plume brightness are compared with the etch rate of the drilled hole, a strong correlation can be noticed. By reaching the saturation depth for the hole, no more material can be removed any more and the plume brightness decreases to near zero levels. This plume emission serves as an in-situ monitor to control the hole drilling in transparent materials. A similar set of data for the single pulse mode with the same pulse energy was recorded which shows that no effective machining is possible compared to burst mode. At the first burst (with only one pulse), the ablation plume brightness reaches only a third compared with the four pulse burst mode data (not shown here). The subsequent pulses generate no

97 significant emission. 4 Results and Discussion 83

98 Chapter 5 Conclusion and Future Work In this work, burst laser interactions of femtosecond laser pulses has proven to offer significant improvement in high aspect ratio hole drilling in glass substrates. A direct single step machining method in air demonstrated deep ( 1mm), high quality hole etching with lower pulse energies ( 45µJ) compared to those required in other direct machining methods, described in Chapter 1. The novel burst generation system was presented here for the first time. This resonator is capable of producing high repetition rate burst trains of femtosecond pulses with high pulse energy. A high voltage system using a method of time-delays operating at 38.5MHz was chosen to control the burst train profile that can produce any desirable pulse amplitude profile. For the first time to our knowledge, time-resolved optical images were taken with an 84

99 5 Conclusion and Future Work 85 intensified CCD-camera during burst laser machining of deep holes in BK7 glass. Transient effects like photoluminescence, formation of a filament in the volume and expansion of the ablation plume are observed with 2ns time resolution. The time dynamics revealed from these images offer a deeper physical insight in burst laser processing of transparent materials. The correlation seen between the ablation plume brightness and the etch rate of holes in BK7 suggests that in-situ process monitoring may permit real-time control of hole drilling. A comparison between the burst train and single pulse exposure was conducted, which shows that the burst train drove an accumulated heat effect during laser-matter interaction that results in longer effective drilling period for generating deeper holes. The time-resolved ICCD images were used to physically identify this effect. For future work, an improvement in the reliability and stability of the burst generator system is necessary. An industrial version of this system would be integrated with more robust laser systems. A high efficiency mode of operation for burst generation can also be achieved in the stable industrial version. Different applications of burst-matter interaction are of great interest for future work. We only presented the hole drilling significances in this work, while the burst trains can also be used for cutting, welding and scribing. Another promising research direction in this area is the fabrication of optical waveguides, where strong heat accumulation effects of the burst operation mode can be used to induce gentle positive index changes to guide light. Optical sensing and telecommunication are two examples of industries where optical waveguides are needed.

100 Appendix A Working with the burst generator system Appendix A will present a manual for working with the burst resonator system explained in section 3.2. This manual will contain both the alignment tips for the optical system and the details of working with the high voltage control system. A.1 Optical alignment Fig. A.1 shows the experimental implementation of the burst resonator system presented in Fig In this figure shows the optical arrangements are shown in the exact way which the experiment was run. First important issue for the system is the total power that is entering the burst cavity at the entrance from Spitfire laser. Since the optics that are used have 86

101 A Working with the burst generator system 87 a damage threshold, Neutral Density (ND) filters can be used at the entrance to avoid the damage on the optics. The filter that we used in all the experiments had a ND=0.3 reflection 1, allowing a 1W of laser power entering the cavity at 500Hz repetition rate. Figure A.1: The experimental implementation of the burst resonator cavity. A telescope at the entrance in Figure A.1 is used to decrease the beam diameter from 1cm to 2.5mm in the cavity to fit all the optics. To have a stable resonator, the Gaussian beam has to have its minimum waist in the middle of the cavity, i.e. between end mirrors M4 and M5. To reach that, the original beam should first reach M4, and then focuses in the center of the cavity. In our setup, the distance between concave and convex lenses should be set in order to have the minimum spot of the original beam right after P2. While working with whole power and changing the telescope condition, beam spot can reach either P2 or FR resulting in damage spots in their coatings; therefore, a higher ND filter should be used 1 Reflection for the ND filters is R = 10 ND, which is in this case.

102 A Working with the burst generator system 88 for this alignment. The first mirror in the cavity, M1, should be align in this way for best operation: M1, P1, FR, P2 and M2 should be in one straight line. Having said that, the beam should pass both P1 and P2 and also FR without getting blocked. The aperture on the FR is very small and has almost the same diameter as the beam in the cavity which can be used as a reference point. For setting the correct angles for P1 and P2, best way is to put a power meter between P2 and M2, and then play with P1 and P2 in order to maximize the power. The correct angle will pass the maximum amount of horizontal polarization, which laser beam originally has when it enters the burst cavity. M2 and M3 can be used to center the beam into the Pockels Cell. In the case of the alignment, the beam should reach M2 and M3, and the PC in the center. There is a small circular aperture for the PC that can be used as a reference point. M4 should be aligned in the way to reflect the beam in the exact same position. Best way to check it is to put an aperture in between P2 and M2 close to P2, and then use M4 to have the reflected beam on the same position. At the beginning of the alignment procedure, it is better to have the PC completely perpendicular to the beam. To check the perpendicularity, the same aperture of the PC can be used on the rear facet of the PC. In the best alignment condition, no red beam should be seen neither on the aperture nor on the metallic case of the PC. Next thing to align is GVD compensation system, i.e. the prisms. First part of the system is the HWP that is being used to convert the vertical polarization to the horizontal in order to match Brewster s angle on the prisms. The beam should pass through the center, and to

103 A Working with the burst generator system 89 find the best angle for it one can either put a pair of horizontal polarizer and a power meter for exact measurement and maximize the power (or minimize in case of having a vertical polarizer). Another easy method for checking the correct angle is to look at the reflection of the beam from the prism (in the presence of prism ) and rotate the HWP in order to minimize reflection. Alternatively, when burst system is working, one can also turn this angle in order to maximize the amplitude of the pulses in the burst profile. It is clear that this angle can not change the amplitude of the first pulse. Next to align is the first prism. This prism is mounted on the linear moving stage that has the calculated Brewster s angle with the incoming beam. The best alignment should have all of these properties simultaneously: 1. The angle of the prism should be in a way to minimize the reflected beam from the first facet (Brewster s angle condition). 2. Two reflections can be recognized from this prism, from entering and exiting facets; these two reflections should be at the same height, meaning that prism is parallel to the surface of the laser beam. 3. Beam should reach the other prism, and the shape of the beam should look like a long linear ellipse, not a circle. This means that different frequency components of the laser pulse are separated, which is necessary for dispersion compensation. 4. By moving the prism on the linear stage, the position of the beam on the second prism should remain constant.

104 A Working with the burst generator system 90 Once the first prism is aligned, the second prism can be aligned according to the first one. For the second prism, the beam should pass the prism in such a way that the beam size stays constant; this means that the laser beam has the same angel (Brewster s angle) with the second prism and it is collected completely with the second prism. Another point for aligning the second prism is, similar to the first one, two reflection of the prism should be in the same height. The last mirror, M5, should be aligned to guide the reflected beam of the same path. A combination of end mirrors M4 and M5 can be used to correct the beam profile of the burst train. When the system is producing few number of pulses in the burst, one can use a beam profiler to see the intensity profile of the outgoing beam after the compressor. By comparing the single pulse mode 2 profile to the burst profile and comparing the physical center intensities of the beams, one can correct the beam shape. In an ideal case, all the pulses in the burst should have their centers at the same position. A.2 Timing system The timing system illustrated in the Figure 3.7. The preamplifier in this system, is BME P3 PCI card which takes the trigger signal of the Tsunami oscillator and convert the signal to meaningful pulses for the Spectrum cards. This board, and also the Spectrum cards, MI.7010 and MI.6111, are all in the same computer system and are controlled with the software. A running window of the software for BME P3 is shown in Figure A.2. To run this, it only 2 By blocking the prism at any time, system will operate at a single pulse mode.

105 A Working with the burst generator system 91 needs to be activated through the start label. Figure A.2: Software for running the BME P3 preamplifier. The Spectrum cards needs two input signals to work with; clock and trigger. The clock signal, is the preamplified signal from the BME P3 which has pulses with 13 nanosecond separation. The trigger signal, is the synchronization signal from Spitfire system. This signal has 500Hz repetition rate and is the CH3 output of the Spitfire. A delay in the range of the nanosecond can be added to this signal with the Spitfire controller system. To run the Spectrum cards, a software capable of running them both at the same time was used. Figure A.3 shows a window of this software while running. This software should be run in this way: First, the Period count should be set; the value for period count depends on the maximum number o the pulses that user wants to

106 A Working with the burst generator system 92 Figure A.3: Software for the Spectrum cards. generate, but a value of 5 or 6 is a typical choice. Second, the card should be initialized using the Initial button. Then, the maximum voltage of the analogue signal can be changed; this value will determine the maximum delay that time-delay system can apply to the PC. Maximum value for this voltage is 3000 mv, which is a typical choice. After setting the voltage, the program can start working. Each of the four analogue channels of the Spectrum card can be controlled with this software. In each period, 4 values from 0 to 127 can be set for each channel, corresponding to the zero voltage and the 3 voltage (in the typical setting of 3000 mv) in the output