NONLINEAR ABSORPTION INITIATED LASER-INDUCED DAMAGE AND CUBIC ZIRCONIA DISSERTATION. Presented to the Graduate Council of the

Transcription

1 377 NONLINEAR ABSORPTION INITIATED LASER-INDUCED DAMAGE IN r-irradiated FUSED SILICA, FLUOROZIRCONATE GLASS AND CUBIC ZIRCONIA DISSERTATION Presented to the Graduate Council of the University of North Texas in Partial Fulfillment of the Requirements For the Degree of DOCTOR OF PHILOSOPHY By Nastaran Mansour, B.S.,M.S, Denton, Texas August, 1988

2 1989 NASTARAN MANSOUR All Rights Reserved

3 Mansour, Nastaran, Nonlinear Absorption Initiated Laser- Induced Damage in r-irradiated Fused Silica, Fluorozirconate Glass and Cubic Zirconia. Doctor of Philosophy (Physics), August, 1988, 237 pp., 31 tables, 82 illustrations, bibliography, 121 titles. The contributions of nonlinear absorption processes to laser-induced damage of three selected groups of transparent dielectrics were investigated. The studied materials were 7-irradiated and non-irradiated fused silica, doped and undoped fluorozirconate glass and cubic zirconia stabilized with yttria. The laser-induced damage thresholds, prebreakdown transmission, and nonlinear absorption processes were studied for several specimens of each group. Experimental measurements were performed at wavelengths of 1064 nm and 532 nm using nanosecond and picosecond Nd:YAG laser pulses. In the 7-irradiated fused silica and fluorozirconate glasses, we found that there is a correlation between the damage thresholds at wavelength X and the linear absorption of the studied specimens at A/2. In other words, the laser-induced breakdown is related to the probability of all possible twophoton transitions. The results are found to be in excellent agreement with a proposed two-photon-initiated electron avalanche breakdown model. In this model, the initial "seed" electrons for the formation of an avalanche are produced by

4 two-photon excitations of E' centers and metallic impurity levels which are located within the bandgaps of 7-irradiated Si0 2 and fluorozirconate glasses, respectively. Once the initial electrons are liberated in the conduction band, a highly absorbing plasma is formed by avalanche impact ionization. The resultant heating causes optical damage. In cubic zirconia,we present direct experimental evidence that significant energy is deposited in the samples at wavelength 532 nm prior to electron avalanche formation. The mechanism is found to be due to formation of color centers (F + or F centers) by the two-photon absorption process. The presence of these centers was directly shown by transmission measurements. The two-photon absorption (2PA) process was independently investigated and 2PA coefficients obtained. The accumulated effects of the induced centers on the nonlinear absorption measurements were also considered and the 2PA coefficients were measured using short pulses where this effect is negligible. At room temperature, the color centers slowly diffuse out of the irradiated region. The density of these centers was monitored as a function of time. The initial distribution of the centers was assumed to have a Gaussian profile. For this model the diffusion equation was solved exactly and the diffusion constant obtained.

5 TABLE OF CONTENTS Page LIST OF TABLES LIST OF ILLUSTRATIONS vii X Chapter I. INTRODUCTION 1 Background II. EXPERIMENTALAPPARATUS 21 The Nanosecond Nd:YAG Laser System The Picosecond Nd:YAG Laser System The Optical Components Detectors and Data Acquisition Systems Calibration and Characterization of Laser Beams III. RESULTS OF DAMAGEMEASUREMENTS 48 Test Specimens Measurement Procedure The ^-Irradiated Si0 2 Results The Fluorozirconate Glass Results The Cubic Zirconia Results IV. ROLE OF SELF-FOCUSING 108 Critical Power for Self-Focusing Electrostriction Effects Electronic Kerr Effect,Polarization Dependent Self-Focusing V.PHYSICALMECHANISMS OF LASER-INDUCED BREAKDOWN IN THE STUDIED DIELECTRICS. 134 Model for Two-Photon Initiated Electron Avalanche Breakdown in^-irradiated Si0 2 and Fluorozirconate Glass Direct Observation of Two-Photon Absorption Prior to Avalanche Ionization Breakdown in Zr0 2 at 532 nm Diffusion of Color Center Generated by Two- Photon Absorption at 532 nm in Zr0 2 v

6 Page VI. SUMMARY 206 Suggestions for Future Work APPENDIX A: APPENDIX B: APPENDIX C: THE ENERGY ABSORPTION RATE BY PLASMA IN AN ELECTROMAGNETIC FIELD 214 THE ANALYTICAL SOLUTION OF THE DIFFERENTIAL ATTENUATION RATE OF CHAPTER V..219 THE ANALYTICAL SOLUTION OF THE DIFFUSION EQUATION OF CHAPTER V 225 BIBLIOGRAPHY 231 VI

7 LIST OF TABLES Table Page I. Composition of Fluorozirconate Glass 56 II. Laser-Induced Damage Data for Non- Irradiated and ^-Irradiated Si0 2 Specimens (Spectrasil A, B, and WF) at 532 nm 73 III. Laser-Induced Breakdown Threshold Data for Non-Irradiated and 7-Irradiated Spectrasil A, B, and WF at 1064 nm 74 IV. Breakdown Threshold Data for Si0 2 Samples at 532 nm 78 V. Breakdown Threshold Data for Si0 2 Samples at 1064 nm 79 VI. Laser-Induced Damage Threshold Data for Undoped and Doped Fluorozirconate Glass at 1064 nm Using Linearly Polarized Light 82 VII. Laser-Induced Damage Threshold Data for Undoped and Doped Fluorozirconate Glass at 1064 nm Using Circularly Polarized Light 83 VIII. Laser-Induced Damage Threshold Data for Undoped and Doped Fluorozirconate Glass Samples at 532 nm Polarized Light 86 IX. Laser-Induced Damage Threshold Data for Undoped and Doped Fluorozirconate Glass Samples at 1064 nm Polarized Light 87 X. Laser-Induced Damage Data for Zr0 2 Stabilized with Y at 1064 nm Using Linearly Polarized Light 92 XI. Laser-Induced Damage Data for Zr0 2 Stabilized with Y at 1064 nm Using Circularly Polarized Light 93 vii

8 Table Page XII. Laser-Induced Damage Data for Y Stabilized Zr0 2 at 532 nm 97 XIII. Laser-Induced Damage Data for Y Stabilized Zr0 2 at 1064 nm 98 XIV. Multiple Shot Laser-Induced Damage Data for Y Stabilized Cubic Zirconia at 532 nm. 101 XV. Multiple Shot Laser-Induced Damage Data for Y Stabilized Cubic Zirconia at 1064 nm 102 XVI. Values of Spot Size, Speed of Sound, Electro-Response Time, and Laser Pulsewidth 116 XVII. Values of Linear Index of Refraction, Density, and Speed of Sound 118 XVIII. Calculated Values of n 2, Calculated Power for Electrostriction Self-Focusing and Measured Breakdown Power Data for Fused Silica for 15 nsec Pulses at 532 nm 120 XIX. Calculated Values of n 2, Calculated Power for Electrostriction Self-Focusing and Measured Breakdown Power Data for Fused Silica for 18 nsec Pulses at 1064 nm 121 XX. Calculated Values of n 2, Calculated Power for Electrostriction Self-Focusing and Measured Breakdown Power Data for Fluorozirconate Glass Samples for 15 nsec Pulses at 532 nm 122 XXI. Calculated Values of n 2, Calculated Power for Electrostriction Self-Focusing and Measured Breakdown Power Data for Fluorozirconate Glass Samples for 18 nsec Pulses at 1064 nm 123 XXII. Calculated Values of n 2, Calculated Power for Electrostriction Self-Focusing and Measured Breakdown Power Data for Yttria Stabilized Cubic Zirconia for 15 nsec Pulses at 532 nm 124 Vlll

9 Table Page XXIII. Calculated Values of n 2, Calculated Power for Electrostriction Self-Focusing and Measured Breakdown Power Data for Yttria Stabilized Cubic Zirconia for 18 nsec Pulses at 1064 rati 125 XXIV. Wavelength Dependence of the Laser- Induced Breakdown Thresholds in Non-Irradiated and k-irradiated Spectrasil A, B, and WF SiO, 144 XXV. Wavelength Dependence of the Laser- Induced Breakdown Thresholds in Spectrasil A, B, and WF Si XXVI. Wavelength Dependence of the Laser- Induced Breakdown Thresholds in Undoped and Doped Fluorozirconate Glass 146 XXVII. Calculated Values of Focal Volume for the Tested Dielectric Materials 148 XXVIII.Extracted Values for Slope of Inverse Transmission Curves for Broad Range of Pulsewidths 162 XXIX. Two-Photon Absorption Coefficients 174 XXX. Wavelength Dependence of the 1-on-l Laser-Induced Breakdown Thresholds in Zr0 2 Stabilized with Y XXXI. Wavelength Dependence of the N-on-1 Laser-Induced Breakdown Thresholds in Zr0 2 Stabilized with Y IX

10 LIST OF ILLUSTRATIONS Figure Page 1.1 Distribution of Laser Damage Thresholds at 1064 nm for NaCl Breakdown Field in KC1 at Wavelengths of 1064 nm and 694 nm as Function of F-Centers Oval Experimental Schematic of Laser-Induced Breakdown The Nanosecond Nd:YAG Laser Round Trip Schematic in the Case of Closed Cavity Round Trip Schematic in the Case of Opened Cavity The Picosecond Nd:YAG Laser The Block Diagram of the Data Acquisition The Autocorrelation Scan for the Picosecond Laser Pulses Temporal Profile of Nanosecond Laser Pulses at 532 nm Temporal Profile of Nanosecond Laser Pulses at 1064 nm The Focused Spatial Profile of the Nanosecond Laser Pulses at 532 nm The Focused Spatial Profile of the Nanosecond Laser Pulses at 532 nm The Horizontal Spatial Profile of the Picosecond Laser Pulses The Vertical Spatial Profile of the Picosecond Laser Pulses Linear Transmission Spectra of Non- Irradiated Spectrasil A, B, and WF. 50 x

11 Figure Page 3.2 Linear Transmission Spectra of Spectrasil A, B, and WF Exposed to 10 8 rads of f-radiation Infrared Linear Transmission Spectra of Spectrasil A Infrared Linear Transmission Spectra of Spectrasil Infrared Linear Transmission Spectra of Spectrasil WF Linear Transmission Spectra for Fluorozirconate Glass Samples A and B Infrared Transmission Spectra for Fluorozirconate Glass Infrared Transmission Spectra for Fluorozirconate Glass Samples C and D Infrared Transmission Spectra for Fluorozirconate Glass Samples E and F Linear Transmission Spectra for Cubic Zirconia Infrared Linear Transmission Spectra for Zr Experimental Setup for Nanosecond Laser Pulses Experimental Setup for Picosecond Laser Pulses Histograph of the Single Shot Laser-Induced Damage Threshold Laser-Induced Damage Results of Fused Silica for 18 nsec 1064 nm Pulses Laser-Induced Damage Results of Fused Silica at 532 nm 3.17 Breakdown Electric Field for Non-Irradiated Spectrasil A, B, and WF for 15 nsec Laser Pulses at 532 nm 80 XI

12 Figure Page 3.18 Breakdown Electric Field for Non-Irradiated Spectrasil A, B, and WF for 18 nsec Laser Pulses at 1064 nm Experimentally Determined Breakdown Field for Doped and Undoped Fluorozirconate Glass Samples at 1064 nm for Picosecond Pulses Experimentally Determined Breakdown Field for Doped and Undoped Fluorozirconate Glass Samples for Nanosecond Pulses at 532 nm Experimentally Determined Breakdown Field for Doped and Undoped Fluorozirconate Glass Samples for 18 nsec Laser Pulses at 1064 nm Experimentally Determined Ratio of the Breakdown Field for Doped Fluorozirconate Glass Samples with Respect to Undoped Breakdown Electric Field for Linearly Polarized Light for Cubic Zirconia Breakdown Electric Field for Circularly Polarized Light for Cubic Zirconia Breakdown Electric Field for Linearly Polarized Light for Cubic Zirconia Breakdown Electric Field for Linearly Polarized Light for Cubic Zirconia Experimentally Determined Ratio of Single Shot Breakdown Field with Respect to the Multiple Shot Results for 15 nsec Laser Pulses at 532 nm Experimentally Determined Ratio of Single Shot Breakdown Field with Respect to the Multiple Shot Results for 18 nsec Laser Pulses at 1064 nm Schematic Representation of Self-Focusing 110 xxi

13 Figure Page 4.2 Experimentally Determined Ratio of Breakdown Electric Field for Circularly and Linearly Polarized Light for Undoped and Doped Fluorozirconate Glass Experimentally Determined Ratio of Breakdown Electric Field for Circularly and Linearly Polarized Light for Cubic Zirconia Schematic Representation of Electron Avalanche Ionization Process Wavelength Dependence of the Breakdown Field for r-irradiated and Non-Irradiated Spectrasil A, B, and WF 5.3 Wavelength Dependence of the Breakdown Field for Spectrasil A, B, and WF Wavelength Dependence of the Breakdown for Doped and Undoped Fluorozirconate Glass Square of Breakdown Electric Field at 532 nm Versus the Negative Natural Logarithm of the Linear Absorption at 266 nm for Spectrasil A Square of Breakdown Electric Field at 532 nm Versus the Negative Natural Logarithm of the Linear Absorption at 266 nm for Spectrasil B Square of Breakdown Electric Field at 532 nm Versus the Negative Natural Logarithm of the Linear Absorption at 266 nm for Spectrasil WF Square of Breakdown Electric Field at 1064 nm Versus the Negative Logarithm of the Linear Absorption at 532 nm for Fluorozirconate Glasses Using Picosecond Pulses Square of Breakdown Electric Field at 1064 nm Versus the Negative Logarithm of the Linear Absorption at 532 nm for Fluorozirconate Glasses Using Nanosecond Pulses 156 x m

14 Figure Page 5.10 Square of Breakdown Electric Field at 532 nm Versus the Negative Logarithm of the Linear Absorption at 266 nm for Fluorozirconate Glasses Using Nanosecond Pulses Inverse Transmission as a Function of Incident Irradiance for Nanosecond Pulses (a) Inverse Transmission as a Function of Incident Irradiance for Picosecond Pulses (b)Repeated Inverse Transmission Measurements at Same Position, Monitoring the Change in Transmission as Function of Number of Shots Inverse Transmission as a Function of Incident Irradiance for Zr0 2 Stabilized with 9-4% Y Inverse Transmission as a Function of Incident Irradiance for Zr0 2 Stabilized with 12% Y 0, [ Inverse Transmission as a Function of Incident Irradiance for ZrO, Stabilized with 15 % Y i7i 5.17 Inverse Transmission as a Function of Incident Irradiance for ZrO Stabilized with 18 % Y Inverse Transmission as a Function of Incident Irradiance for Zr0 2 Stabilized with 21% Y Schematic of the Unit Cell for Zr Transmission as a Function of Incident Irradiance in Zr0 2 Stabilized with 9.4% Y for 15 nsec Pulses at 532 nm Transmission as a Function of Incident Irradiance in Zr0 2 Stabilized with 12% Y for 15 nsec Pulses at 532 nm.178 xxv

15 Figure Page 5.22 Transmission as a Function of Incident Irradiance in Zr0 2 Stabilized with 15% Y for 15 nsec Pulses at 532 nm Transmission as a Function of Incident Irradiance inzr0 2 Stabilized with 18% Y for 15nsec Pulses at 532 nm Transmission as a Function of Incident Irradiance inzro,stabilized with 21% 2 Y for 15 nsec Pulses at 532 nm Transmission as a Function of Incident Irradiance in Zr0 2 Stabilized with 9.4% Y for 18 nsec Pulses at 1064 nm Transmission as a Function of Incident Irradiance in Zr0 2 Stabilizedwith 12% Y for 18 nsec Pulses at 1064 nm Transmission as afunction of Incident Irradiance inzr0 2 Stabilized with 15% Y for 18nsec Pulses at 1064 nm Transmission as a Function of Incident Irradiance in Zr0 2 Stabilized with 18% Y for 18 nsec Pulses at 1064 nm Transmission as a Function of Incident Irradiance inzr0 2 Stabilized with 21% Y for 18nsec Pulses at 1064 nm Wavelength Dependence of the 1-on-lBreakdown Field for Zr Wavelength Dependence of the N-on-1 Breakdown Field for Zr Distribution of Color Centers in Irradiated Region inzr (a) Monitoring the transmission of Zr0 2 After t=l min, t=2 min (b) Monitoring the transmission of Zr0 2 After t=16 min,t=32 min The Ratio of Induced Color Centers as a Function of Time 201 xv

16 CHAPTER I INTRODUCTION The subject of fundamental mechanisms of laser-induced damage in transparent optical materials has been under extensive investigation for more than 20 years. Vast amounts of effort have been expended on both the experimental and theoretical aspects of phenomena of optical breakdown in wide bandgap insulators. 1-4 In spite of these many years of research, the mechanism of laser-induced breakdown in solid transparent dielectrics still remains a complicated and debatable problem in the physics of the interaction between high power electromagnetic radiation and matter. Controversy exists as to whether the damage is caused by intrinsic or extrinsic processes. Some researchers 2 *5-11 conclude that the responsible mechanism for laser-induced damage in transparent dielectrics is electron avalanche ionization. The avalanche ionization is an intrinsic process. However, we find along with others 12 " 16 that the damage thresholds vary drastically on the same material under similar experimental conditions. This can only be true if the damage is initiated by extrinsic processes. An example of such a large variation in the results of damage resistance is given in Fig. l.l. This figure represents a distribution function of

17 1 M ft I '# to» i NO llrttmd, GW/m 1 Figure 1.1 Distribution of laser damage thresholds at 1064 nm for NaCl samples (after A.A.Manenkov 15 ).

18 damage thresholds for more than 100 samples of NaCl crystals (grown by different suppliers) at the NdrYAG laser wavelength of 1064 nm. In some specimens, the measured damage thresholds differ by about two orders of magnitude. These large variations in damage test results from sample-to-sample are manifestations of effects of extrinsic factors such as impurities, color centers, and defects in the materials. However, researchers have had little success in correlating the optical damage with specific impurities and/or defectsinitiated processes in the good quality optical materials. In recent work,i 6 " 19 the observation of frequency and spot size dependence of the damage lead to speculation that the damage is due to avalanche ionization initiated by multiphoton ionization of defects within the bandgap. However, a detailed quantitative description of the generation of the initial free carriers and the dynamics of the process is not yet available. The main objective of this work was to examine the role of multiphoton ionization in the process of laser-induced damage of wide bandgap insulators which had high densities of foreign elements such as color centers, impurities, and defects. This effort was provided by performing a detailed experimental and theoretical investigation of laser-induced breakdown in 7-irradiated and non-irradiated fused silica, doped and undoped fluorozirconate glasses, and cubic zirconia stabilized with different yttria concentrations. Measurements were made at near-infrared (1064 nm) and visible (532 nm) wavelengths using

19 nanosecond and picosecond laser pulses. From the results of these measurements, the decisive role of multiphoton absorption in initiating the avalanche ionization breakdown of the studied wide gap dielectrics has been established. The experimental arrangement used for our measurements is given in Chapter II. Each main part of the lasers, optics, and data acquisition system is described. A major portion of Chapter II is devoted to the characterization of the high power pulses from the Nd:YAG lasers operating in a single transverse and longitudinal TEMqo mode. The results of laser-induced damage threshold measurements of the selected wide bandgap dielectrics are reported in Chapter III. These measurements include the damage resistance of 7-irradiated and non-irradiated Spectrasil A, B, and WF (water free) Si0 2. The thresholds of the irradiated Spectrasil with lo«f 10*, and 10«rads of 7-rays are calculated, and comparisons with damage results of non irradiated samples are made. Investigations of laser-induced damage to fluorozirconate glasses doped with metallic impurities of Cr, Mn, Ni, and Nd are tabulated and compared to damage results of undoped specimens. The damage resistance levels of cubic zirconia (Zr0 2 ) stabilized with yttria (Y ) in concentrations of 9.4%, 12%,15%,18%,and 21% are listed. The relatively high damage threshold of the new synthetically-grown crystal of Zr0 2 indicates that it is an excellent candidate for laser materials in the near infrared.

20 The role of self-focusing in the measurements of laserinduced damage thresholds of the tested materials is discussed in Chapter IV. This analysis was accomplished by comparing the calculated values of the critical power for self-focusing to the laser-induced breakdown powers of studied specimens. The self-induced effect was kept small by restricting damaging powers to well below the calculated critical powers. The field intensities necessary to cause optical damage were obtained by focusing strongly with external optics (using short focal length lens). The exposure of fused silica (Si0 2 ) to the 7-radiation induces high densities of E' centers 20 in the materials. These centers are located 2 to 4 ev below the conduction band of Si0 2. The presence of these absorbing defects is directly shown by conventional linear absorption spectroscopy.in Chapter III. These defect levels can provide channels for initiating electrons for the avalanche ionization process. Photons from an intense laser pulse at 532 nm can librate an electron by twophoton ionization from these trap levels within the gap. To justify the contributions of these electrons in initiating the damage process, in Chapter V a theoretical model is developed that includes the generation of starting free carriers by twophoton ionization from the trap levels. The prediction of the model was found to be in excellent agreement with the experimental results of damage measurements for the Si0 2 samples.

21 The validity of this model was examined by applying it to the damage measurement results for doped fluorozirconate glasses. Doping with metallic impurities generates absorption bands within the bandgap of these materials. The presence of the trap levels (absorption bands) could be a source of the starting electrons for the avalanche. The experimental results of fluorozirconate glasses were found to be in good agreement with the prediction of the two-photon assisted electron avalanche model. The better correlation that was obtained with this model for picosecond pulses will be discussed in Chapter V. The contribution of the nonlinear absorption (multiphoton absorption) process was directly observed in cubic zirconia (Zr0 2 ) stabilized with yttria (Y ). Stabilizing the Zr0 2 with Y 2 0 3, induces high densities of oxygen ion vacancies 21 in this material. The valence band electrons can be captured by multiphoton absorption and form the color centers (F + or F ). The pre-breakdown studies in Zr0 2 at 532 nm, presented in Chapter V, provide detailed information on the formation of these centers by two-photon absorption prior to optical breakdown. The theory of the two-photon absorption (2PA) process for the case of pulse irradiance with Gaussian spatial and temporal profiles is reviewed in the same chapter. This theoretical work provides us with an expression for the inverse transmission in terms of linear and two photon absorption coefficients. The experimental results of inverse transmission for all studied Zr0 2 samples are presented and 2PA coefficients are extracted.

22 Chapter V also investigates the diffusion of multiphotongenerated color centers in Zr0 2 at room temperature. In the laser-irradiated region, the initial distribution of color centers was assumed to have a Gaussian profile in the radial direction. For this model, the diffusion equation was exactly solved and diffusion constants for thismaterial were obtained. Finally, the results of laser-induced damage studies of the tested dielectric materials are summarized in Chapter VI. Background The phenomenon of laser-induced damage in optical materials was first observed in the early 1960s 22 " 26 shortly after the discovery of pulsed lasers. Optical damage isdefined as an irreversible, catastrophic alteration of materials' properties by an intense light beam. It isusually generated by localized melting in the bulk or on the surfaces of the solid materials or possibly vaporization and cracking or shock wave shattering. Researchers soon realized that damage in optical components of lasers was one of the major obstacles for restricting the ultimate power output. Ever since, interest in this subject has been stimulated because of the practical needs of laser technology. There have been extensive studies in different classes of materials such as dielectric crystals, glasses,polymers, semiconductors, etc. In the presence of intense laser light,the optical failure of transparent optics is generally signified by a visible flash

23 8 or spark in the medium, resulting from the formation of a luminous plasma. In some cases, the spark is seen to extend over a long distance. A record spark length of 60 meters was reported by Parfenov et al. 27 using a 160 J pulsed Nd:Glass laser beam focused in air. In transparent solids, the plasma quickly grows to a density sufficient to absorb the laser light strongly. The consequent heating of the lattice by the plasma causes irreversible physical damage. The entire process may occur in a time duration of about 1psec (10~ 12 sec). Anthes and Bass, 28 using psec pulses at 532 nm, made streak camera recordings of the transmitted damaging pulse in fused quartz. They found that the plasma formed in a time shorter than the 6 psec streak camera resolution. A relatively large number of theoretical models have been proposed, each usually emphasizing only one or some aspects of the most nonlinear process (or processes) of laser-induced damage. Mainly, the discussion in the literature has covered phenomena such as collisional (electron avalanche) ionization multiphoton ionization, thermal ionization of highly absorbing inclusions and defects, and phonon-assisted processes. Among those theories, electron avalanche ionization explains qualitatively much of the process of laser-induced damage in transparent materials. Electron avalanche ionization (production of free electrons) in transparent solids was studied in the presence of high electrostatic fields (dc-fields) in the early 1930s. In

24 1934, Haworth and Bozorth 29 measured exceedingly noisy currents prior to dc breakdown in glass materials. Similar observations were reported by Von Hippel 30 within the alkali halides. He observed that the pre-breakdown currents quickly grow inmagnitude as the dc-field is increased. These important experimental evidences indicate that electron production occurs at fields lower than that required for breakdown and that rapid formation (avalanche type) of carriers is related to the phenomena of dc breakdown. Thus, the avalanche electron multiplication process was introduced as the dominant mechanism for dc breakdown in insulating solids. The theoretical studies of this process were extensively developed by Von Hippel, Zener, 33 Houston, 34 Frchlich, 35 Seitz, 36 and Callen. 37 In practice, from the utilization of avalanche electron processes in semiconductors, a series of solid-state devices such as transistors and diodes have been built. 38 In 1964, Bruma 2^ suggested that electron avalanche breakdown was the mechanism for laser-induced damage in sapphire, glass, and diamond materials. Zverev et al. 39 introduced an impact ionization model which qualitatively explained their experimental results of laser-induced damage in ruby and leucosapphire crystals at wavelengths of 1064 and 694 nm. In 1968, Yasojima et al. 40 reported experimental results of the breakdown in ionic crystals of NaCl, KC1, KBr, and KI caused by a Q-switched ruby laser (694 nm). They found the magnitude of r.m.s. fields for laser-induced breakdown were

25 10 nearly equal to those for dc breakdown obtained by Cooper et al. 41 In 1971, Yablonovitch 5 studied laser-induced breakdown in ten samples of alkali-halide for 100 nsec pulses at C0 2 wavelengths (10640 nm). He found that the laser-induced breakdown field thresholds have the same systematic variation with the bandgap of the tested materials as in the dc breakdown results reported by Von Hippie. 31 Also, the dc and laser breakdown fields for the studied alkali halide samples were found to be the same within experimental uncertainties. 5 Similar measurements were extended to shorter wavelengths using the Nd:YAG laser of 1064 nm and the ruby laser of 694 nm by Fradin and Bass. 6 It was found that the results for the breakdown field were comparable to those of dc and C0 2 laser field strengths. 2 / 6 # 7 / 42 Electron avalanche due to impact ionization was well understood as the operative mechanism in the dc breakdown. This mechanism received increasing consideration as the laser-induced damage process for transparent optical materials. Further experimental support for the avalanche process was obtained based on characteristic statistical properties 8 ' 9 of the damage process, and on the pulselength dependence of the optical breakdown field strength. 10 ' 11 The direct evidence for avalanche ionization (charge multiplication)was reported by Yasojima et al. 43 in LiF and KC1 using nsec pulses of 1064 nm and 694 nm laser light. They added a dc bias to the samples in order to collect the charge

26 11 induced by the laser radiation prior to optical breakdown. Their results show that the collected charge in LiF is nearly dependent on the third power of the laser intensity. In single crystal Kcl at liquid nitrogen temperature (77 K) the increase of electrons prior to breakdown was observed to be exponential. Another interesting experimental work on laser-induced optical breakdown in dielectric materials was performed by Gorshkov et al. 44 They measured laser-induced damage to the fused quartz by irradiating the sample simultaneously with 266 nm and 1064 nm pulses. They also monitored the photoconductivity and, thus, the number of free electrons in the irradiated region prior to breakdown. A significant decrease in the damage thresholds at one of the wavelengths in the presence of the other wavelength was measured. Their results confirmed that the damage was due to the same process at both wavelengths and that the librated electrons start the avalanche mechanism. The electron avalanche ionization process has been identified as one of the most probable laser-induced damage mechanisms in transparent solids. 1-4 Theoretical aspects of this mechanism have now been developed, 45 " 48 and the predictions of the theory have been satisfied for specific experimental studies invarieties of dielectric materials which exhibit high resistance to laser-induced breakdown f 49 For example, Manenkov 15 reported laser-induced damage measurements on a large number of dielectric crystal samples. Experiments were carried out for nsec pulses at nm,

27 nm, 694 nm, and 532 nm. He found that only those samples with high damage thresholds exhibited consistent results with the theory of avalanche ionization. Oness 12 performed the measurements of laser-induced damage thresholds for forty samples of NaCl crystals which were prepared by different suppliers. She found as much as a factor of four improvement in damage thresholds for those NaCl samples grown with special care. Allen et al. 13 and Wang et al. 18 reported ten times larger laser-induced breakdown thresholds for dielectric crystals grown in a reactive atmosphere than those of conventionally grown crystals. Similarly, Soileau 16 measured substantially higher breakdown thresholds for reactive atmosphere processed alkali halide materials than for commercially available specimens of the same materials. The large variation in the damage thresholds indicates that extrinsic factors such as impurities, color centers, and, in general, various kinds of defects strongly influence the process of the intrinsic electron avalanche ionization breakdown. However, the quantitative role of these extrinsic factors in the avalanche ionization process still remains unclear. In the literature, only in a few cases 16 ' 18 / 19 / 43 are the important contributions of the extrinsic initiated processes analyzed. For example, Yasojima et al. 43 studied laser-induced damage in KC1 crystals containing F-centers of the order of

28 cm -3. Measurements were performed with nsec pulses at Nd:Glass wavelength (1064 nm) and ruby wavelength (694 nm). They found that the optical breakdown field decreases with increasing F-center concentrations. However, at a wavelength of 1060 nm, the decrease was more remarkable than at the wavelength of 694 nm, as shown in Fig The results of uncolored crystals indicated that the measured threshold fields at 1064 nm were much higher than at 694 nm. These results were explained very convincingly by multiphoton ionization from the F-centers initiating the avalanche breakdown. Soileau 16 reported the wavelength and focal volume dependence of the laser-induced breakdown for dielectric materials of NaCl, KC1, and KBr. The damage measurements were performed for nanosecond pulses at nm, 3800 nm, 2700 nm, and 1064 nm over a 10 4 range in focal volumes. The results indicated that at small focal volumes (3 to 5xl0~ 9 cm 3 ) the damage threshold decreased with decreasing laser wavelengths, while at the limit of large focal volume (greater than 10~ 6 cm 3 ) the damage thresholds increased with decreasing laser wavelengths. The wavelength dependence at large volumes was consistent with the prediction of electron avalanche ionization. 3 However, the wavelength dependence at small volumes in the avalanche process remains controversial. Van Stryland et al.18 investigated the focal volume dependence of the laser-induced breakdown for picosecond pulses at 1064 nm in

29 14 KCI(Marshaw) o! 1.06p :0.6943p Density offcenler(lo,b crn 3 ) Figure 1.2 Breakdown field in KC1 wavelength of 1064ratiand 694 nm as function of F-centers (after Y. Yasoiima et al. 43 ).

30 15 single crystal NaCl and fused Si0 2. The laser pulsewidth was varied from 40 psec to 200 psec and the focal volume was varied by over two orders of magnitude. Their results were fitted to a linear dependence of breakdown electric field on the product of tp 1 / 4 v~ 1, where t p is the laser pulsewidth and v is the focal volume. The pulsewidth dependence was in agreement with the prediction of avalanche breakdown theory. 50 However, the volume dependence could not be explained by the prediction of avalanche process. 50 The wavelength and volume dependence led these authors to speculate that for small focal volumes the seed electrons in the avalanche process must be produced by multiphoton excitation of defects or impurities level within the bandgap of studied samples. As the laser wavelength becomes shorter, multiphoton initiating electron channels are expected to be more probable. However, this process may depend on several factors such as irradiance level of the breakdown event (thus, the laser pulsewidth), the photon energy of the laser, the energy gap, structures of extrinsic elements of the samples and irradiated volumes of the sample. The contributions of multiphoton ionization in the laser-induced damage process have been studied to a lesser extent. There are only a few studies 17 ' in which this question is analyzed and the importance of this process is addressed. The motivation of this study was to examine the role of multiphoton absorption in the mechanism of laser-induced

31 16 breakdown of transparent dielectrics which had high densities of defects. To provide this effort, three groups of dielectric materials were selectively studied. The laser-induced damage thresholds and pre-breakdown transmission measurements were made on several specimens of each of the three classes of materials at two laser wavelengths of 1064 nm and 532 nm. From the results of these measurements, the dominant role of multiphoton absorption in initiating laser-induced breakdown of the studied wide gap insulators were shown.

32 CHAPTER BIBLIOGRAPHY 1. The National Bureau of Standard Series of Publications on Laser-Induced Damage in Optical Materials, edited by A. J. Glass and A. H. Guenther from 1970 to 1978, publication numbers 341, 356, 372, 387, 414, 435, 462, 509, and 541. The special publication numbers 568 and 620 are edited by H. E. Benett, A. J. Glass, A. H. Guenther and B. Newnam for 1979 and The publication numbers 638, 669, 688, and 727 are edited by H. E. Bennett, A. H. Guenther, D. Milam, and B. E. Newnam for 1981 to N. Bloembergen, IEEE J. Quantum Electron, QE-10, 375 (1974). 3. W. L. Smith, Opt. Eng. 17, 489 (1978). 4 A. A. Manenkov and A. M. Prokhorov, Sov. Phys. Usp. 29, 104 (1986). 5. E. Yablonovitch, Appl. Phys. Lett. 19, 495 (1971). 6. D. W. Fradin andm. Bass, Appl. Phys. Lett. 22^, 206 (1973). 7. D. W. Fradin, E. Yablonovitch, and M. Bass, Appl. Opt. 12, 700 (1973). 8. M. Bass and H. H. Barrett, IEEE J. Quantum Electron, QE-8, 338 (1972) M. Bass and H. H. Barrett, Appl. Opt. 12, 690 (1973). D. W. Fradin, N. Bloembergen, and T. P. Lettelier, Appl. Phys. Lett. 22^, 635 (1973). 17

33 E. Yablonovitch and N. Bloembergen, Phys. Rev. Lett. 29, 907 (1972). 12. D. Oness, Appl. Phys. Lett. 8_,283 (1966). 13. S. D. Allen, M. Braunstein, C. R. Giuliano, and V. Wang, Natl. Bur. Stand. Pub. 414, 66 (1974). 14. V. Wang, C. R. Giuliano, S. D. Allen, and R. C. Paster, Natl. Bur. Stand. Pub. 435, 118 (1975). 15. A. A. Manenkov, Natl. Bur. Stand. Pub. 509, 455 (1977). 16. M.J. Soileau, Ph.D. Dissertation, University of Southern California, unpublished (1979). 17. B. G. Gorshkov, Yu. K. Danilenko, A. S. Epifanov, V. A. Lobachev, A. A. Manenkov, and A. V. Sidorin, Sov. Phys. JETP 45, 612 (1977). 18. E. W. Van Stryland, M. J. Soileau, A. L. Smirl, and W. E. Williams, Phys. Rev. B 2T3,2144 (1981). 19. M.J. Soileau, W. E. Williams, E. W. Van Stryland, T. F. Boggess, and A. L. Smirl, Opt. Eng. 22_,424 (1983). 20. E.J. Friebele and D. L. Griscom, in Glass II, edited by M. Tomozawa and R. Doremus (New York, Academic Press, 1979), p R. P. Ingel and D. Lewis III, J. Am. Ceram. Soc. 69, 325 (1986). 22. P. D. Marker, R. W. Terhune, and C. M. Savage, in Proc. 3rd Int. Conf. Quantum. Electron., Paris, Dunod, 1559 (1964). 23. C. R. Giuliano, Appl. Phys. Lett. 5, 137 (1964). 24. G. H. Cullom and R. W. Waynant, Appl. Opt. _3,989 (1964).

34 M. Hercher, J.Opt. Soc, of Am. 54, 563 (1964). 26. M. S. Bruma, J. Opt. Soc. of Am. _54,563 (1964). 27. V. N. Parfenov, L. N. Pakhomov, V. Yu Petrun'kin, and V. A. Podlevskii, Sov.Tech. Phys. Lett. 2_,286 (1976). 28. J. p.anthes and M. Bass,Appl.Phys. Lett. 31, 412 (1977). 29. F. E.Haworth and R.M. Bozorth, Physics 5, 15 (1934). 30. A.Von Hippel, Phys.Rev. 5, 1096 (1938). 31. A.Von Hippel,J.Appl.Phys. J5,815 (1937). 32. A.Von Hippel and R. S.Alger, Phys.Rev. 76, 127 (1949). 33. C.M. Zener, Proc.Royl. Soc. 145, 523 (1934). 34. W.V. Houston, Phys.Rev. 57, 184 (1940). 35. H. Frchlich, Proc.Roy. Soc. 188, 521 (1947). 36. F. Seitz, Phys.Rev. 76, 1376 (1949). 37. H. B. Callen, Phys.Rev. 76, 1394 (1949). 38. G. I. Haddad, Avalanche Transit-Time Devices (Artech House,Inc., Dedham,Massachusetts, 1973). 39. G. M. Zverev, T. N. Mikhailov, Y. A. Pashkov, and N. M. Solov'eva, Sov. Phys.JETP 26, 1053 (1968). 40. Y. Yasojima,M. Takeda, and Y. Inuishi,Japan J.Appl.Phys. 7, 552 (1968). 41. R. Cooper, Progress in Dielectrics, 5, 130 (1963). 42. D.W. Fradin, Laser Focus, 41 (February 1974). 43. Y. Yasojima, Y. Ohmori, N. Okumura, Y. Inuishi, Japan J. Appl.Phys. 14, 815 (1975). 44. B. Gorshkov, A. Epifanov, A.Manenkov, and A. Panov, Natl. Bur. Stand. Pub. 638, 76 (1981).

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36 CHAPTER II EXPERIMENTALAPPARATUS The experimental details of laser-induced breakdown and nonlinear absorption measurements will be described in this chapter. A general experimental configuration for these measurements is given in Fig This simplified schematic consists of a Nd:YAG laser system, a variable attenuator, pulsewidth monitor, and finally, detection electronics connected to an IBM compatible AT minicomputer. Two different Nd:YAG laser systems 1 were used to make the measurements reported in this work. The first laser system, Quantel's Model YG481C, is an actively Q-switched Nd:YAG oscillator/amplifier laser system with an output energy of about 800 mj at the fundamental wavelength (1064 nm) (Fig.2.2). The system can be operated at several repetition rates (1,2, 5, and 10 hz)with a laser pulsewidth of 18 nsec (FWHM). Frequency doubling of the output of this system is achieved by using angle tuned Type II KD*P crystals. This laser system is described more completely in the second section of this chapter. The second laser system, Quantel's Model YG40, is a passively modelocked neodymium-yag laser operated at 1064 nm. This system consists of an oscillator, an electro-optic switchout, and an amplifier. The energy output of this laser is 21

37 22 Beam Splittei Sample Nd:YAG Laser System Variable Attenuator Transmitted Energy Monitor Pulsewidth Monitor Incident Energy Monitor Data Acquisition Box and I.B.M. Computer AT's Fig. 2.1 Oval experimental schematic of laser-induced breakdown and nonlinear absorption measurements.

38 23 10 mj with a repetition rate of 0.5 hz. This system is capable of producing single optical pulses with temporal widths varying from 30 to 200 psec (FWHM). The variation of the temporal pulsewidth is obtained by using various interchangeable etalons as the output coupler of the oscillator cavity. A brief description of this system will be given in the third section of this chapter. A complete description of all optical components used for propagating and attenuating the laser beams is given in the fourth section of this chapter. Attempts were made to use the optical components without introducing additional diffraction, phase aberration, and walk-off in the path of the laser beams. The energy monitors, detection electronics, and data acquisition system will be discussed in the fifth section of this chapter. Energy calibration and characterization of laser pulses at 532 nm and 1064 nm will be explained in the sixth section of this chapter. The Nanosecond Nd:YAG Laser System The laser system consists of four major parts, set on a solid steel optical bench: an oscillator head, an amplifier head, a Q-switch unit, and a second harmonic generator. They were purchased from Quantel International as one unit (Model S G418C). The schematic of the laser system is shown in Fig The oscillator head (OSC) includes the Quantel SF laser housing, a laser rod (Nd3 + doped YAG) of 7 mm diameter by

39 24 PH OC Fig. 2.2 The nanosecond Nd:YAG laser system.

40 mm long with ends cut at 2 degrees, and two linear flashlamps. The gas in the flashlamps iskrypton which isunder two atmospheres of pressure. The amplifier head (AMP) is very similar to the oscillator head except that it includes four linear flashlamps and the diameter of the rods changes to 9.5 mm. The laser rods have antireflection (AR)coatings. The flashlamps and the rods in laser heads are cooled by closed circulating of deionized water. The heated deionized water is cooled by having cold tap water running through a coil within the reservoir (water-to-water heat exchanger). The active Q switch unit consists of three parts: a KD*P Pockels cell, a Glen-Taylor polarizer prism, and a quarter-wave plate. The polarizer ishighly transparent to p-polarization of laser light and highly reflective to the s-polarization. When there is no voltage applied to the Pockels cell, a laser beam propagating within the oscillator cavity makes a double pass through the quarter wave plate for a net rotation of 90 degrees. Thus, the beam obtains s-polarization and is rejected by the polarizer prism, and no lasing occurs. This is called closed cavity. The schematic is shown in Fig When a charge of approximately 3600 volts is applied to the Pockels cell, it acts as a quarter-wave plate. A further rotation of 90 degrees takes place by double passing through the Pockels cell. Therefore, the total rotation becomes 180 degrees and the beam obtains the original polarization (p-polarization). Now the beam can propagate into the oscillator cavity and

41 26 Back Pockells X Mirror Cell 4 Polarizer Laser Head Output Coupler ( o n i i l y H u i i t m i H I t H c f l Fig. 2.3 Round trip schematic in the case of closed cavity.

42 27 lasing occurs. This is called open cavity, shown in Fig The time delay between the opening voltage to the Pockels cell and the maximum population inversion (peak of fluorescence) is about 100 microseconds. The fine adjustment for time delay and applied voltage to Pockels cell can be achieved by two control knobs located on top of the electronics box of the Q-switch unit. To prevent instability of pumping in the laser system, the power supply was kept running continuously at a repetition rate of 10 hz. The Q-switch electronics were adjusted to control the on or off and changing the repetition rate of the lasing. The second harmonic generator (SHG) consists of a Type II KD*P crystal and a harmonic oven. The oven keeps the crystal at a constant temperature of 35 C. Keeping the crystal at a fixed temperature minimizes the effects of energy absorption in the harmonic crystal and provides long term instability. The rear oscillator mirror (RM) is 20 mm in diameter and has a radius of curvature of fivemeters. It has a dielectric coating on the front surface, achieving a reflectivity of nearly 100% at 1064 nm. The smooth pulse option (SPO) is a 3mm thick etalon with a single face antireflection coating. This plate is mounted between the rod and rear mirror, with the uncoated surface toward the laser rod. The SPO removes the low frequency modulations of the temporal shape of the pulse. The experimental evidence will be shown in Section 6 of this chapter. Two apertures (PH)of diameters 4.5 mm and 2.5mm are

43 28 Back Mirror Pockells Ceil 3600 Volts A y '4 Polarizer Laser Head i tit Output Coupler 3600 Volts c l Fig. 2.4 Round trip schematic in the case of opened cavity.

44 29 located on each side of the laser oscillator head. Selection of these two particular size apertures limits the transverse spatial modes of the laser beam to TEMqq. The output coupler (OC)of the laser oscillator cavity is a 3mm thick plate with a partial reflecting coated surface. Two dichroic mirrors (DM) are located at 45 degrees (Fig.2.2) after the second harmonic generator. These mirrors are coated for high transmission (better than 90% )for fundamental wavelength (1064 nm) and high reflection (better than 99%) for 532 nm. Conducting the beam through these two dichroic mirrors separates the fundamental wavelength from the second harmonic (532 nm). The Picosecond Nd:YAG Laser System This laser system, model YG40, is produced by Quantel International. The complete details of this laser system are reported inwilliams 2 and Woodall. 3 Here, the main aspects of the system will be discussed briefly. The laser consists of an oscillator, an electro-optic switch, and an amplifier (Fig.2.5). The active media is a Nd:YAG rod cut at Brewster angle with a diameter of 6mm and a length of 65 mm. Homogeneous pumping is provided by a helical flash tube. The oscillator cavity ispassively modelocked by a cell of circulating dye and produces a train of picosecond pulses at 1064 nm. The modelocker dye used is Kodak 9740 which is diluted with chlorobenzene. This system has a variable thickness output mirror that obtains an instant change of the

45 30 Aperture Dye Cell Aperture Electro-Optic Swltchout 1 M Output Coupler M Fig. 2.5 The picosecond Nd:YAG laser system

46 31 modelocked pulse duration. The pulsewidth of 30, 60, 100, or 200 picoseconds (FWHM) can be achieved by using the output mirror thickness of 0.32, 1.5, 3.2 and 5mm. There are two apertures which are located at each side of the oscillator head;they allow only the transverse TEMQO mode to propagate in the oscillator cavity. The presence of the TEMQO will be shown in the next section by spatial pinhole scans and perfect fitting of results to the Gaussian distribution. The electro-optic switch is used to select a single pulse from the modelocked train. The energy of the pulse is about 0.5mj and by passing through the amplifier the energy can be increased up to 10 mj. This depends on the power supply voltage which is applied to amplifier flashlamps. It is reported 3 that measurements of a slight change (about 10% )in spot size of the laser beam can be obtained by varying the voltage of the amplifier power supply. In this work, the amplifier voltage setting was kept at a fixed value in order to avoid this problem, and the energy of the beam was externally attenuated. The Optical Components The optical components used in this work were mainly mirrors, attenuators, beamsplitters, lenses, and filters. Since high power lasers were used, an attempt was made to use optics with good surface quality and high damage thresholds. For conducting the laser beam, dielectric mirrors from the CVI Corporation were usually used. 4 These mirrors are made of

47 32 polished fused quartz or BK-7 glass substrates with dielectric coatings of high reflectivity (better than 99%) at either 1064 or 532 nm. The surface figure of the mirrors is A/10 at 632 nm with damage thresholds greater than 5 GW/cm for nanosecond laser pulses at 1064 nm. Depending on the size of the laser beam, different sizes of mirrors were used (25 or 50 mm in diameter and 9.5mm thick) such that only the central 50% of the diameter was being irradiated. In case of turning and conducting the HeNe laser beam, commercial mirrors with aluminum coating were used. Combinations of a polarizer, a half-wave plate, and another polarizer (analyzer)were used as avariable attenuator in this work. The two polarizers were set in fixed mounts with transmission axes parallel to the optical table. The half-wave plate was mounted in a rotating frame and set between the polarizer and analyzer. By moving the half-wave plate 45 degrees with respect to its principal axis, it provided attenuation on the order of This attenuator did not introduce any measurable walk-off in the beam path. It was checked by looking at the laser light at a far distance (10meters) behind the attenuator system. The polarizer was purchased from CVI and is composed of two right angle BK-7 prisms with the hypotenuse coated with multilayer dielectric and cemented with index matching optical cement. The four surfaces have anti-reflection coatings and the clear aperture size is 10 mm in diameter. The analyzer is a Glen-Air spaced

48 33 calcite polarizer which was purchased from Special Optics.^ The half-wave plate is zero order with an antireflection coating (at either 1064ranor 532 nm) made of schlieren grade natural quartz. Plane windows with antireflection coatings on the back surface for 45 degrees incident irradiance were used as the beam splitter. The window materials were fused silica with a surface flatness of A/10 at 632 nm. The beam splitters were 50 mm indiameter and 10 mm thick. The laser beam was focused into the bulk of the tested samples using a lens. Six different Special Optics lenses were used in this work. They were positive lenses with best form geometrical design to minimize spherical aberration. The focal lengths were 40, 51, 75, 150, 501, and 999 mm. Detectors and Data Acquisition Systems The optical detectors used for these measurements were PIN photodiodes. The ones with active areas of 0.25 cm were purchased from United Detector Technology, model PIN-6D. The larger area detectors (1cm) were manufactured by EG&G Electro-Optics. 7 They were integrated into a specially designed detector circuit 8 that amplified the signal, detected the peak, and held it (peak-and-hold). This optical detection device had a response time of about 20 nanoseconds and a broadband spectral range of 350 to 1100 nm. Optical narrow bandpass filters (with bandwidths of 10 nm) were used in the

49 34 front of the active area of each detector to allow only the frequency of interest to be detected by the device. Each detector was carefully shielded to eliminate the available optical noises and radiated electromagnetic interferences in the laboratory environment such as laser flashlamp, room light,etc. In thiswork, the detection system was automated using IBM PC/ATs. The schematic of the data acquisition is shown in Fig The computer fires the laser, reads the signals of up to 12 detectors, and is capable of controlling three stepping motors. A detailed analysis of the electronics and menu driven software that were used in this data acquisition system is beyond the scope of this work. Therefore, only a brief description of the overall function follows. All the detectors connected to the data acquisition box are interfaced to the IBM computer via an analog-to-digital converter. When the program isexecuted, the computer sends an enable signal to the Q-switch unit to fire the laser. The energy level of the laser pulse is tested by detector 1,which is connected to a potentiometer. Once a laser shot is within the window setting of detector 1, the computer is allowed to read the values of the rest of the detectors. This process is repeated until a certain number of shots is obtained for each group. The computer program calculates the average and standard deviations of data (detector values or certain detector ratios). After the completion of the required number

50 35 Nd: YAG Laser System Enable Signal I.B.M. Personal Computer At's Det. *1 Data Acquisition Box Input From Detectors To Transition Drivers & Stepping Motors Fig. 2.6 The block diagram of the data acquisition system,

51 36 of shots in each group, the sample position and incident energy attenuator are automatically incremented by two Aerotech 9 translational drivers connected to the data acquisition box. Calibration and Characterization of Laser Beams The energy of laser pulses was directly calibrated with respect to a pyroelectric joulemeter. The energy meter 10 used was a Gentec ED100 with a responsivity of 138 volts/joule. To check the calibration, the energy of the laser pulse was also calibrated against five different pyroelectric detectors. The agreement inthe results was good within ±4%. For the picosecond laser pulses, the width of each pulse was determined by autocorrelation measurements. 11 The description of the pulsewidth monitor isgiven ingreater detail in Williams 2 and Perryman. 12 The overall procedure was that the width of each pulse was monitored by determining the ratio of the square of the energy in fundamental (1064 nm) to the energy in the second harmonic. (The harmonic crystal used in the ratiometer was LiI0 3.) This ratio has been shown to be inversely proportional to the temporal pulsewidth 13 and can be written as (<?f)/( sh> = ct P A f 2-1 ) where c is a constant related to the second order susceptibility of the second harmonic crystal, the length of the crystal, etc. In Eq. (2.1),t isthe pulsewidth and A is the area of fundamental laser pulse (1064 nm). The ratio was calibrated

52 37 by measuring the pulsewidth using Type I optical autocorrelation techniques. 14 The results of measuring the pulsewidth are shown in Fig In the case of nanosecond laser pulses,the temporal shape of the beams was directly monitored by the fast PIN photodiode detector and a Tektronix storage oscilloscope, 15 model The PIN photodiode was manufactured by Hewlett Packard, 16 model The device had a response time of less than 1 ns and broad spectral range (400 to 1100 nm). The temporal shape of the laser pulses at 1064 and 532 nm are given in Figs.2.8 and 2.9. In these experimental measurements, the laser beams were usually focused into the bulk of samples using a lens. In the case of a long focal length lens (f > 25 cm), the spatial spot size was directly measured at the sample position. The spot size measurements were performed using pinhole scans across the laser beams. A 5micrometer diameter pinhole was slowly scanned both horizontally and vertically over the beam cross section. The results were fitted to the best Gaussian distribution for each scan, as shown in Figs and There was a slight ellipticity in the spatial shape of the laser beams, with the two widths differing by about 10%. The spot size was taken to be the square root of the product of the two fitted widths, since this effective area is the same as the elliptical cross section.

53 d Delay (psl Fig.2.7 The autocorrelation scan for the picosecond,laser pulses at 1064 nm. The solid line represents a theoretical Gaussian fit to experimental data points. The deconvolved temporal width of the 1064 nm pulse is45psec (FWHM).

54 39 Fig. 2.8 Temporal profile of nanosecond laser pulses at 532 nm. The FWHM of the beam is about 20 nsec.