Optimization of the extracavity 4 th harmonic generation process for low-intensity fundamental beam.

Transcription

1 Optimization of the extracavity 4 th harmonic generation process for low-intensity fundamental beam. Romanov Ivan MSc Thesis May 2018 Department of Physics and Mathematics University of Eastern Finland

2 Romanov Ivan Supervisors: Optimization of the extracavity 4th harmonic generation process for low-intensity fundamental beam, 49 pages University of Eastern Finland Advanced materials of photonics Prof. Yiry Svirko Dr. Vadim Kiyko Dr. Elena Kolobkova Abstract The frequency conversion processes and especially 4 th harmonic generation are discussed; however, the attention was paid to the deep UV-light generation. Suitable inorganic crystals for UV-light generation were presented. A conception of critical and non-critical synchronism was discussed. Intra- and extracavity generation optical schemes were compared in terms of low-intensity conversion. The low-intensity deep UV-laser based on Nd:YVO4 with diode pumping was constructed, laser operates with OA-modulator and the 4 th harmonic was done in KDP crystal. Most important parameters in terms of the conversion efficiency at low intensity fundamental beam are identified and ways to optimise the conversion efficiency are shown. We demonstrated experimentally that created low-intensity Nd:YVO4 laser combined with extracavity 4 th harmonic generation scheme is efficient disinfection tool. Keywords: nonlinear crystals, SHG, FHG, harmonics, UV-light, disinfection. ii

3 Preface To my Mom. Joensuu, the 23 rd of May 2018 Romanov Ivan iii

4 CONTENTS 1 Introduction Theory Materials for nonlinear optics Anharmonic oscillator Even and odd harmonics generation Inorganic crystals for UV-light generation Phase-matching and birefringence of crystals Birefringence Sellmeier equation. Critical and noncritical synchronism Temperature insensitivity Intra- and extracavity conversion schemes Equipment and experiments Results Coclusions References iv

5 CHAPTER 1 Introduction Since 1960, when the 1 st laser has been demonstrated, the laser technology progress is extremely fast and from my point of view the laser is one of the greatest inventions of XX century. The range of possible utilization of lasers is virtually unlimited, spanning from a simple pointers and laser printers to thermonuclear fusion and removing cancerous tumours devices [1]. Laser technology expands the borders in production and researches, improving our life quality and bringing the mankind closer to a new era. One can say that nowadays laser technology is in demand in the modern world. Roughly almost every manufacture uses a laser somewhere: metal processing [2], food [3, 4, 5], chemistry production [6], paper industry [7], lasers also widely used in medicine [8, 9], cosmetology [10] and many of research needs: spectrophotometers, which can be constructed in a tiny body or for instance provide a space observation and exploration wider with help of laser guide star [11]. Laser military devices give big advantages for an army and leading to new ways of prosecution of martial operations. A modern market of laser technology gets bigger and more diverse day after day. In industry lasers are usually used for operations of cutting or welding different materials: metals, non-metals or glasses [12, 13, 14]. These operations require high powers and intensities of a laser emission (1-10 kw/cm 2 ) in order to provide a proper result, as well as different operations of heat treatment, such as quenching, annealing and tempering. Other huge sphere for an application of lasers is medical sphere. Eye surgery [15, 16], neurosurgery [17], dental surgery [18] - laser technology provides a lot of benefits for medicine. After the laser was invented in ophthalmology operations on retina or glaucoma curing became just ordinary procedures. Using of a laser scalpel for organic tissues [19], when instead of a metal instrument the cutting tool is presented as CO2 laser (working powers is 2-5 kw/cm 2 ), made surgery safer and more effective. This instrument is able to make precise and sharp cutting of almost any part of human body, the depth of incision is about 1-3 mm with a diameter of a beam around 0.01 mm. In addition to that, laser scalpels fully sterilized, what is obviously a great benefit in surgery. Another great application of laser technology for medical purposes is biological welding [20], when the sintering of organic tissues edges is provided by a laser emission (working power is 25 W/cm 2 ). There are many other ways to find an implement of a laser technology for medical purposes: for example, a laser nephelometry made a process of investigation of blood cells a lot easier, when a laser tomography [21] allows getting a lot of useful information about the condition of a human body 1

6 from only one test. Diagnostics of some diseases (asthma, diabetes and some others) can be done by one exhalation with a usage of laser gas analysers [22]. This short rundown showing how big the roots of laser technology grown up in modern era and now it needed to be mentioned that lasers can be classified by the nature of the impact: invasive, when the interaction of the laser emission and a material causes some irreversible changes of this material, and non-invasive impact for analytical lasers and low-intensity emission in general. Non-invasive lasers found to be extremely useful in low-level laser therapy (LLLT) and photodynamic therapy (PT). Shortly the principle of LLLT can be described as stimulation of healing processes of different inflammatory processes by laser emission, when PT is a destruction therapy, the process of initiation of photosensitized free-radical reactions arising as a result of interaction of laser quanta with photosensitizer molecules in the presence of oxygen. It should be noted that although the effectiveness of LLLT is proven and widely uses in medical practice, the true mechanism of stimulation processes has not been fully studied and the theory of these effects is unclear [23]. In some works, a positive influence of LLLT for the same organic tissue was observed even for different spectral range of irradiation. The result of the influence depends mostly on intensity, peak power, wavelength and power density of a laser emission and time of interaction. UV-light laser sources widely used in medicine, but there is one big field for UV-lasers which is, in my opinion, unfairly stays behind the scene disinfection and decontamination. Conventional sterilization techniques (e.g. heating, chemical processing and so on) are not always applicable and expensive. Chemical-free way of sterilization is can be done by light irradiation with the use of UV-lamps, UV LEDs and blue LEDs. The question of efficiency of blue LEDs is controversial (see Fig. 1) when the usage of clear UV-light sources obviously has significant advantage: Figure 1: Bacteria kill spectrum. The process of disinfection by UV-light generally describes as an inhabitation of the replication ability of microorganisms by dimerization of DNA and RNA bonds with 2

7 the help of photochemical reaction caused by photon absorption in these molecules. In other words, UV-light makes microorganism unable to reproduce [24]. UV-LEDs might be more efficient than blue LEDs or UV-lamps but the lifetime of UV- LEDs is not long enough for sterilization applications. Results of research made by Kneissl and Rass are shown in Fig. 2: Figure 2: UV-LEDs lifetime diagram [25]. The most efficient in terms of sterilization UV-LEDs, capable to emit in UV-C and UV- B regions, have a lifetime about 500 hours. UV-lamps have a better lifetime (about 2000 hours) but emitting in relatively wide spectrum from 230 to 330 nm excluding other components of emission which are insignificant for sterilization. Other point is that UV-lamps are unsafe for human eyes and skin and the sterilization time is about 300 sec/cm 2. One can observe from Fig. 1 that the 4 th harmonic of Nd 3+ -based laser (266 nm) is extremely effective for killing microorganisms. This allows one to expect a tool based on a laser technology will be compact, efficient (sterilization time approximately less than 1 sec/cm 2 ), have a great lifetime (from at least, up to hours) and will be safe because low-intensity emission will be enough to provide a proper result in terms of disinfection and will not hurt anybody. The use of low-intensity UV-laser emission devices is very wide not only in terms of fields to be implemented but also just geographically due to the fact that it is in demand world-wide from cold Russia to hot Africa and wherever people are. This is important aspect because it means that the same laser device will show different characteristics of radiation depending on where it will be used. So, the device must be wellstabilized in order to keep the characteristics. The efficiency and characteristics of generation of the laser emission dealt with a lot of parameters and have their own influence on the ending result what will be discussed it this work. 3

8 CHAPTER 2 Theory This chapter describes basic concepts of light interaction with nonlinear inorganic crystals and shows how different parameters affect the harmonics generation efficiency with special attention to the frequency conversion at low-intensity fundamental beam. Attention was drawn to conversion of low-intensity emission. The even and odd harmonics generation have been described. Section 2.4 shows the relationship between the structure of a crystal and phase-matching condition. The phase-matching described and the dependence from different parameters is presented. Critical and noncritical types of synchronism as well as Sellmeier equation and coefficients are shown. The temperature dependencies for a noncritical synchronism are investigated in subsection Extracavity and intracavity conversion types are described in the last section of this chapter. 2.1 Materials for nonlinear optics. In the field of nonlinear optics inorganic dielectric crystals are the most common materials in usage to obtain nonlinear effects, such as generation of harmonics. There are also different substances to be used, for instance films, gases or liquids. Different nonlinear effects and suitable materials to obtain these effects are given in the Table 1 below: Table 1: Fundamental nonlinear optical phenomena. Nonlinear effect Harmonics generation Materials Mechanism Application Transformation of laser frequency Parametric scattering Crystals, semiconductors, inert gases, metal vapours Electronic nonlinearity Smooth frequency tuning. Generation of coherent IR, UV and X-ray light. Visualization of IR-image. Diagnostics of the surface of semiconductors. 4

9 Stimulated scattering Gases, liquids, crystals, metal vapours, optical fiber Scattering of light due to an interaction with electronic, molecular, acoustic or other oscillations. Transformation of laser frequency. Compression of laser pulses. Wave front reversal and correction. Improvement of spatial coherence. Nonlinear spectroscopy Self-action Gases, liquids, crystals, liquid crystals, optical fiber Electronic nonlinearity, molecular orientation, electrostriction, heating Self-modulation of light pulses. Generation of femtosecond pulses. Bistable optical elements. Components for optical computer. Modelling of neural networks. Inorganic crystals have several different useful advantages, such as chemical and thermal stability, high optical characteristics, developed database of different classes of crystals and well-known technologies of growing, what affect the price positively. These points make dielectric crystals relatively cheap but a high-quality material to be used, and in my work the attention faced exactly to inorganic crystals. The electric field of the light wave causes displacement of an electron shell of an atom relatively to the position of a nuclear, as the result of this interaction an atom obtains electric dipole moment. The response time of the electronic polarization is seconds, while the speed of ionic is about In nonlinear optic the role of the force that polarizes field is light wave that propagates through the material. In the interval UV light visible NIR the most important polarization is the electronic one. Within the IR-range, at wavelengths more than 10 μm, the ionic polarization should be taken into account also [26]. The dipole moment of the volume referred to polarization (P) of the medium can be presented in following form: (2) Where E is the electric field strength,, and are tensors the first-, the second- and third-order susceptibilities, respectively. The typical values for these susceptibilities are shown in Table 2 below: 5

10 Table 2: Typical values for tensors. Tensor Typical value χ (1) 1 χ (2) , m/v χ (3) , m 2 /V 2 In case of centrosymmetric crystals, the 3 d rank tensor χijk vanishes. In order to demonstrate this one can take into account that components i j k of the third-rank tensor are transformed in the same way as a product of the relevant Cartesian coordinates xi xj xk. For example, the component χ112 is transformed as xxy, component χ223 as yyz and so on. Operation of inversion relative to the centre of a crystal each tensor changes the sign to opposite χijk -χijk, but since in centrosymmetric crystals the properties are the same independently of the orientation of a crystal χijk = -χijk, which means that χijk -χijk 0. So, any crystal with a centre of symmetry has no quadric susceptibility. Nonlinear effects in this kind of crystals describes only with a linear tensor and cubic tensor. If the crystal has a quadric susceptibility, then the quadric polarization will be prevailed for the nonlinear response of a crystal. From 32 classes of crystal symmetry 20 of them possess quadric susceptibility, shown in Table 3: Table 3: quadric susceptibility symmetry classes Crystals Syngony Symmetry Isotropic Cubic T, Td Uniaxial Trigonal Hexagonal Tetragonal C3, C3v, D3 C6, C6v, D6, C3h, D3h C4, C4v, D4, D2d, S4 Biaxial Triclinic Monoclinic Orthorhombic C1 Cs, C2 C2v, D2 Tensors of linear and nonlinear susceptibilities depend on a frequency ω of the incoming light. The process of establishing of the polarization requires some time. Therefore, the susceptibility must fall behind in time after the influence of light and the polarization of a crystal should be considered not only at the moment of influence, but also in previous moments, which means that there is a time variance for the nonlinear susceptibility: 6

11 (2.1) After the Fourier transform of Eq. (2.1) was complete, the Eq. (2.2) shows: (2.2) Thus, tensors of linear and nonlinear susceptibilities become frequency dependent. The nonlinear susceptibility of a crystal is a property that makes possible to achieve the harmonic generation effect change of the frequency (energy) of the wave that interacts with a crystal. In case of Nd:YAG lasers with a help of KTP crystal generated 1064 nm IR-emission may be converted to 532 nm green light and as it was highlighted in the work [29] there are several important parameters for generation efficiency: fundamental power, nonlinear coefficient of conversion, length of a crystal and the degree of interaction. The nonlinear susceptibility of an inorganic nonlinear dielectric crystal depends on the polarization properties of this crystal which is mainly defined by the symmetry of crystal structure. Nonlinear properties of a crystal structure represent by a model of classical anharmonic oscillator vibrations, this model is described in subsection below. The polarization also depends on frequency of ordinary wave and the orientation of a crystal. Second and third order nonlinearities are described in the section Anharmonic oscillator. For non-metallic solid materials and atomic vapour, the Lorentz model of a harmonic oscillator describes linear optical properties very well. In order to provide a description of nonlinear properties, the Lorentz model was extended with a respect to noncentrosymmetric and centrosymmetric materials. As the result, the model explains that the second order nonlinear response corresponds to non-centrosymmetric materials, when a third order one correlated with centrosymmetric materials [27]. The motion equation of an electron in non-centrosymmetric materials is: (2.3) Where E(t) is applied electric field, -e is the charge of an electron and the damping force is in the form of. The restoring force: (2.4) 7

12 Parameter a is the strength of the nonlinearity. This form was obtained by assuming that 2.3 is a nonlinear function of the displacement of an electron from its equilibrium position and keep both linear and quadric terms in the Taylor series expansion of the restoring force in the displacement, so this can be described as a potential energy function: (2.5) The first term in 2.5 is a harmonic potential and the second is an anharmonic correction term shown in Fig. 3 below: Figure 3: Non-centrosymmetric potential energy function [27]. This model is suitable for only non-centrosymmetric materials due to the fact that the potential function 2.5 contains both even and odd powers. In order to describe centrosymmetric materials, odd component needed to be removed because the potential function needs to match the equilibrium U(x) = U( x). In order to describe electron oscillations of a centrosymmetric material we use the function of restoring force as: (2.6) Where b is a parameter of nonlinearity strength. The Eq. (2.7) corresponds to: (2.7) Thus, the Eq. (2.7) will be presented as: 8

13 Figure 4: Centrosymmetric potential energy function [27]. So, the extended Lorentz model of an anharmonic oscillator describes the nonlinear properties for centrosymmetric and non-centrosymmetric medias and explains what the order of a tensor in different crystal structures is presented. The connection between the oscillations and a nonlinear response described in detail in works [26, 27]. 2.2 Even and odd harmonics generation. The nature behind generation of the second harmonic (SHG), or in other words, frequency doubling can be roughly described with the Fig. 5: Figure 5: Energy-level diagram describing SHG. When two photons with the same energy interact with a crystal structure, the energy of these two photons will be effectively combined and as the result only one photon with the double frequency 2ω (the wavelength changes from λ to 1/2λ) will be generated. As it shown in Fig. 6 below, when a wave with frequency ω propagating through a nonlinear inorganic crystal, the output will contain an ordinary wave ω and a generated harmonic wave 2ω. In case of cascade generation, when there are two waves with frequency ω and 2ω propagating through a crystal again, the interaction in a second order nonlinear material will cause to generate waves with frequencies ω, 2ω, 3ω and 4ω. 9

14 Figure 6: Cascade generation in two 2 nd order nonlinear susceptibility materials. A laser beam propagating through a nonlinear material will create an electric field, which can be written as: (2.8) The polarization of a nonlinear crystal with χ (2) can be presented as: (2.9) In case of generation of the 3 d harmonic (THG) the process can be described as the interaction between 3 photons with the frequency ω. The result of this interaction is destruction of these 3 photons and generation of 1 photon with triple energy/frequency 3ω (see Fig. 7 below): Figure 7: Energy-level diagram describing THG. 10

15 In terms of harmonics generation, it would be more correct to describe the 3 d harmonic generation as a sum of fundamental and second harmonic beams, instead of tripling of fundamental beam. The polarization of 3 d -order nonlinear material can be written as: Or (2.10) And the total electric field presented as: (2.11) (2.12) After calculation of E (3) (t), it will contain 44 different frequency components (same positive and negative frequencies are counted): ω1, ω2, ω3, 3ω1, 3ω2, 3ω3, (ω1 + ω2 + ω3), (ω1 + ω2 - ω3), (ω1 + ω3 ω2), (ω2 + ω3 ω1), (2ω1 ± ω2), (2ω1 ± ω3), (2ω2 ± ω1), (2ω2 ± ω3), (2ω3 ± ω1), (2ω3 ± ω2); + negative of each ω. Thus, the tensor of the 3 rd rank has 44 components as maximum, when the 2 nd rank tensor has only 27 components. This factor makes odd harmonics be able to produce much more combinations in different frequencies: Figure 8: Interactions of two waves in the 3 rd order nonlinearity material. Fig. 8 illustrates the case of interaction of two waves with different frequencies ω1 and ω2 in third order nonlinear material. Compare to the interaction of two waves in second order nonlinear material (see Fig. 6), the output from a third order material is richer, providing 6 possibly generated waves, against 4 waves of second order nonlinear crystal output. As it seen from Fig. 6 and Fig. 8, the interaction in nonlinear material may lead to summation or to subtraction between the interacting waves, so the output may contain 11

16 waves like: 2ω1 + ω2 as well as 2ω1 ω2, these phenomena is known as sum- and difference- frequency generation and described in detail in many works, for example [26, 27]. 2.3 Inorganic crystals for UV-light generation. As it was shown in paragraph 2.2 polarization of a crystal strongly depends on the symmetry. 1 st of all, in order to be able to generate UV and deep-uv light the nonlinear material should be a noncenrosymmetric material, so there are 21 classes of NCS crystals shown in Venn diagram in Fig. 9. To provide stable and strong SHG there are 3 suitable configurations of nonlinear crystals: (C3b), (D3h), (Td) (bottom oval in Fig. 9). Figure 9: Venn diagram of the NCS crystal classes [28]. The crystal used to generate a 2 nd harmonic for UV light should have a large SHG coefficient. The efficiency of conversion to 2 nd harmonic is strongly linked with the magnitude of the SHG coefficient χij. Obviously, the larger χij coefficient value, the large the efficiency as itself. For the UV region this coefficient requires to be larger than the coefficient of SHG of KDP crystal (χ pm/v [41]) which is used as the standard. The next requirement for NCS crystal used for SHG in the UV region is moderate birefringence and this is one of the most important parameters. The value Δn (see subsection 2.4.1), which is the difference between two refractive indexes for a specific wavelength along z and x axis, should be in the interval from to 0.10 in order to achieve a phase-matching. The condition of the phase-matching is achieved when the equilibrium between the speed of propagation of the fundamental beam with the frequency ω and the speed of propagation of the generated beam with the frequency 2ω is established, i.e. n(ω)=n(2ω). This makes classes 23 and -43m unsuitable for SHG due to 12

17 a fact that moderate birefringence of this classes is absent, these classes related to isotropic crystals and Δn=0. Finally, the crystal used for SHG in UV region needed to be chemical stable and have a large laser damage threshold. Well, this condition required not only for the SHG in UV region but actually almost for every generation process. In case of generation of a nanosecond pulses from a Nd:YAG laser (λ=1064 nm) the NCS crystal should be able to withstand powers about 5GW/cm 2 for a single pulse. After all what was mentioned above the Fig. 10 and Table 4 act as the outline. In this picture on the spectrum from 2 nd to 5 th harmonic from 1064 nm emission (wavelengths of harmonics are the black lines) most suitable nonlinear crystals are shown, when in table 4 some useful parameters of these crystals are presented: Figure 10: Suitable nonlinear materials for generation UV and deep UV light from 1064 nm radiation. Table 4: Suitable nonlinear crystals for the UV-light generation scheme. Crystal Possible wavelength to be generated, nm Transmission range, nm Nonlinearity coefficient, pm/v Walk-off angle, o Hygroscopicity KDP (KH2PO4) 532, d36= high d31=d15==1.95 KTP (KTiOPO4) d32=d24=3.9 d33= non β-bbo (BaB2O4) CLBO (CsLiB6O10) LB4 (Li2B4O7) YAB (YAl3(BO3)4) d22= very low d36= high 532, d31= very low d11= non 13

18 2.4 Phase-matching and birefringence of crystals. The process of generation of harmonics is associated with many issues of reduction in conversion efficiency due to absorption by coloring centres, temperature shifts, changes in spectral width, changing of polarization and other effects in a crystal [26, 28, 30]. All these effects will cause the efficiency to reduce, while this factor has a significant meaning, for example in order to produce UV light from an IR-laser (for instance, Nd:YAG with λ = 1064 nm) it is necessary to obtain 4th harmonic which is equal to 1064/4=266 nm and requires the use of at least 2 nonlinear crystals in cascade. In case if the efficiency of one (or both) of the crystals will be relatively small, the process of generation of harmonics will be ineffctive. The greatest influence on efficiency is given by the phase matching condition, when the speeds of ordinary and emmited waves are equal. When a wave with frequency ω propagating through a crystal with a quadric nonlinear susaptability it causes local dipoles to re-emit the light with freqquency 2ω, these waves will now interfere and as the result this interaction can lead to a generation of a harmonic. So if there are two waves with different wavelengths in dispersing medium they will propagate with different speeds. We assuame that these waves has same polarization and propagating along z-axis. Each wavevector can be described as: k(ω) = ω*n(ω)/c k(2ω) = 2ω*n(2ω)/c, (3) where n(ω) and n(2ω) are refractive indices for ordinary and the 2 nd harmonic wave, respectively; c is the speed of light. The wave (or phase) detuning will be presented until the refractive indices n(ω) and n(2ω) are different and the efficiency of generation of harmonics will be extremely poor. In order to achieve the phase matching condition, the wave vectors k(ω) and k(2ω) need to be equal: Figure 11: Phase mismatch in terms of wave vectors. 14

19 Figure 12: Phase matching in terms of wave vectors. There are 4 different types of phase-matching for uniaxial crystals, see Table 5: Table 5: Types of phase-matching for uniaxial systems: Type of phase-matching Type I, positive Type I, negative Type II, positive Type II, negative Condition no(2ω) = ne(ω) e(ω) + e(ω) o(2ω) ne(2ω) = no(ω) o(ω) + o(ω) e(2ω) [ne(ω) + no(ω)]/2 = no(2ω) e(ω) + o(ω) o(2ω) [ne(ω) + no(ω)]/2 = no(2ω) e(ω) + o(ω) e(2ω) The dependence between the amplitude of 2 nd harmonic and the Δk, wave detuning, can be described with Eq. (2.3) and presented as a typical synchronism curve, which is shown in Fig. 13. where l is the interaction length. (3.1) 15

20 Figure 13: Typical synchronism curve. Thus, the greatest intensity of the 2 nd harmonic wave will occur when the phasemismatch Δk=0 or k(2ω) = 2*k(ω). As it was mentioned in paragraph 2.3 in order to meet the condition of phase matching the nonlinear crystal should have a property of Birefringence Birefringence The birefringence is a property of crystals, which manifests itself as a sependence of the propagation velocity (e.g. refraction index) on light polarization. In case of uniaxial crystals, light will be splitted to ordinary (o) with a refractive index no and extraordinary (e) with its own refractive index ne. The value of birefringence can be described as difference between no and ne, see Fig. 14 below: 16

21 Figure 14: Refractive indices as a function of wavelengths of a positive nonlinear crystal. Δn value is birefringence [31]. So, the birefringence is the difference between refractive indices for special wavelengths. This phenomenon takes place only in anisotropic, or in other words non-cubic symmetry class crystals. There are two main groups if non-cubic crystals presented in Table 6: Table 6: Singonies of uniaxial and biaxial crystals: Uniaxial Biaxial Hexagonal Orthorhombic Tetragonal Monoclinic Trigonal Triclinic For biaxial crystals there are three refractive indices nx, ny, nz, and the birefringence defines as a difference between nx and nz, what is shown in the next Fig. 15: 17

22 Figure 15: Refractive index as a function of wavelength for a positive biaxial crystal. Δn value is birefringence [31]. The birefringence properties of the material can be displayed, in case of uniaxial crystals, as a sphere for an ordinary wave and as ellipsoid for an extraordinary (emitted) wave (see Fig. 16). An ordinary wave is a wave whose refractive index does not depend on the direction of the wave vector. Fig. 16 shows two-dimensional projection of the refractive index quadric in the xzplane for a positive uniaxial crystal quartz (left), the difference between ne and n0 is positive. In case when n0 > ne, then the crystal is negative, as example of a negative uniaxial crystal a two-dimensional projection of the refractive indices of a KDP crystal shown in Fig. 16 on the right side: 18

23 Figure 16: Two-dimensional projection of the refractive index in xz-planes for quartz (left) and KDP (right). Red line corresponds to ordinary wave refractive index and the dashed line showing extraordinary one. The birefringence shown as Δn [31]. For biaxial materials there are three different refractive indices: nx, ny and nz, along x, y and z axis respectively. By convention nx < ny < nz. The crystal will be negative if the condition nz - ny<ny - nx is true, or positive in case if nz-ny>ny-nx condition held. The projections of refractive indices of KTP crystal in the xz-, xy-, and yz-planes are shown in Fig. 17. Figure 17: Two-dimensional projection of the refractive indices for KTP crystal in a) xz-, b) xy-, and c) yz-planes. Angle V is the angle between the z- and optic axis [31]. The refractive indices of a biaxial crystal may be indicated also in three-dimensional projection: 19

24 Figure 18: Three-dimensional projection of refractive indices of a biaxial crystal for 1064 nm (a), 532 nm (b) and possible phase-matching locus (c) [31]. The moderate birefringence is an important parameter of a crystal, because if the birefringence is too small and refractive indicatrixes are does not cross, the material will not be able to meet the phase-matching condition. But it must be noticed that too large birefringence (Δn > 0.10) causes a heavy walk-off effect, which is affects the intensity of generation of 2 nd harmonic beam. The walk-off effect is shown in Fig. 19, where k vector is the wave vector and S vector represents an intensity of 2 nd harmonic beam, the angle between these two vectors corresponds to walk-off angle: 20

25 Figure 19: a) Type I phase-matching condition for a negative uniaxial crystal; b) The walk-off angle ρ; in c) shown that as birefringence increases the walk-off angle also increases [31]. The birefringence of β-bbo is at 1064 nm (χ22 = 2.20 pm/v), the big birefringence leads to a walk-off of the beam and as the result the SHG conversion efficiency will be reduced Sellmeier equation. Critical and noncritical synchronism. As it was shown in paragraph 2.1 the ability of generating harmonics is determined by nonlinear susceptibility of a crystal. The efficiency of generation mostly depends on the phase-matching condition, see Fig. 13. In reality, the fundamental beam is not a plane wave and has its own divergence, it is not totally monochromic and usually pulsed. After all, the interaction between light and crystal involves changes in refractive properties of a material, what leads to several additional important generation process conditions parameters for nonlinear crystals: angular width: shows the generation efficiency for different orientation angles of the crystal, for biaxial crystals there are two angular widths for angles θ and 21

26 φ, for fast and slow axes, respectively. Fig. 20 and Fig. 21 present the efficiency of generation depending on an angle; spectral width: nonlinear response will vary for non-monochromatic fundamental beam; thermal width: in anisotropic crystals the coefficient of thermal expansion is different for each axis, so the temperature of a crystal causes to enlarge distances between atoms along one axis differently from distances along another axis, an example of thermal width is in Fig. 22. Figure 20: Noncritical synchronism curve for angle θ of KTP crystal, presented by [32]. Figure 21: Synchronism curve for angle ϕ [32]. 22

27 Figure 22: Conversion efficiency for angle θ oriented for: perfect phase-matching (up) and non-critical thermal synchronism (down) [32]. Noncritical synchronism characterizes with bigger value of width and with smoother dependence curve of efficiency from angular or thermal parameter with small changes of chosen parameter. Angular, spectral and thermal conditions of the generation process affect the phasematching between ordinary and extraordinary waves, due to changes in refractive indices of a material, what leads to a difference of phase velocities or in other words propagation speeds of waves [32]. In 1871 Wolfgang von Sellmeier presented a formula for calculating refractive index for different wavelengths in visible and NIR region for transparent and non-transparent medium [33]. Nowadays Sellmeier equation (2.4) changed from its original formula and widely used in optics [34, 35, 36, 39] when it is necessary to understand how will change the refractive indexes of a material: 23

28 (4) where sub index i corresponds to axis (x, y or z); λ is a wavelength; A, B, C, D are the Sellmeier coefficients of a material. The question of suitability of the Sellmeier coefficients is widely discussed and studied in many researches; from year to year scientific community presents new coefficients for same material to improve the accuracy of calculating widths. For example, in 1987 T. Y. Fan, C. E. Huang, B. Q. Hu et al. [35] presented their number of Sellmeier coefficients for KTP crystal (see Table 7) and showed that the error of calculating refractive indices with this number does not exceed 0.1% in the range μm. Table 7: Sellmeier coefficients for KTP crystal, μm [35]: n A B C D nx ny nz In 1988 Dyakov et al [36] presented a Sellmeier parameters for KTP crystal for range of μm (see Table 8) and showed that these coefficients allow to calculate fairly enough the tuning characteristics of a KTP crystal for all transmission range. Table 8: Sellmeier coefficients for KTP crystal, μm, [36]. n A B C D nx ny nz This example shows that the Sellmeier coefficients may vary, at least, for different regions of generation. It must be noticed that usually crystal manufactures provide a datasheet with fairly complete information about a crystal and the Sellmeier coefficients may be taken from there. Once the Sellmeier coefficients are known it is possible to obtain information about phase synchronism what is the cut of a crystal where the speeds of ordinary and extraordinary waves are equal. The direction of phase synchronisms describes with a 4 th order curve and presents conical spaces around one of optical axis (see Fig. 23). In order to choose the direction of synchronism one has to solve a problem of compromise between maximum generation efficiency and the maximum widths of synchronism for all the parameters. 24

29 Figure 23: Negative uniaxial phase-matching cones for type 1 (green) and type 2 (blue) [31]. Finding special position of ordinary and extraordinary waves propagation where the synchronism becomes independent from temperature or has an angular interval where the efficiency of generation of the second harmonic beam varies insignificantly with small changes of where Δp (p is one of the parameters: θ, φ, λ, T) is an important task from the point of the design of a device and its working conditions. Thermal independence of the refractive indexes is very important for practical devices. Necessity to introduce thermal stabilization element into a device increase its cost and complicates the design, thus manufacturing is less reliable. Some ways of the investigation of thermal insensitivity intervals are presented in the next subsection. Temperature independent harmonic generation process was realized in [32, 34, 37, 38, 40] Temperature insensitivity. The nature of a temperature insensitivity phenomenon can be described with following explanation. Any changes in temperature cause to deform the wave surfaces differently for fundamental and generated frequencies. This effect dealt with the difference in deformation of a crystal lattice at these frequencies. This effect leads to strong temperature dependence on the phase-matching angles, i. e. it makes phase-matching condition temperature-dependent. But there are some cases, when for some specific directions both of wave surfaces change and vary equally with the change of a temperature, so 25

30 the phase-matching condition is held, and the temperature insensitivity takes place for this case of interaction. In order to meet temperature independence, it is necessary to determinate the values of θ and φ angles where the phase matching is presented but the temperature derivatives of the difference between refractive indices of fundamental and generated waves disappears and equal to zero. According to Grechin et al [32], the mismatch Δk can be calculated with Eq. (5): (5) where p is one of the parameters θ, φ, λ, T. Obviously, in order the efficiency of generation was high the Δk parameter must be equal to zero. If the difference between angular derivatives is zero, then the angular insensitivity appears; if the difference between temperature derivatives is vanished, then the thermal insensitivity condition takes place. Authors of this work [32] reported that thermal independence for SHG in KTP crystal from nm fundamental beam was calculated to appear for φ=48.2 o and θ=43.7 o for SSF-type of interaction and φ=67 o and θ=71.1 o for SFF-type. The KTP crystal with cut φ=67 o and θ=71 o was checked for thermal insensitivity, and the result is shown in Fig. 24 below: Figure 24: Noncritical thermal synchronism curve for KTP crystal (φ=67 o, θ=71.1 o ). I2 is the intensity of the second harmonic beam [32]. 26

31 The experiment showed that with the temperature of a crystal up to 150 C, the efficiency of generation of the second harmonic beam changes by no more than 5%, the thermal width at FWHM is 210 o C, so the calculations provided fair result K. Kato [34] suggested another way to find special phase-matching direction, where the temperature insensitivity achieved for the SHG from fundamental beam wavelength λ= nm. According to author, when the temperature derivatives are known with a good accuracy one can solve Eq. (6) and find the width of thermal independence for type-ii SHG: { } (6) Where λ is the fundamental wavelength, nω/ T and n2ω/ T are the temperature derivatives of the refractive indices at the fundamental and second-harmonic frequencies, and the superscripts o and e refer to the polarization directions of the interacting wavelengths (n o > n e ). For solving this equation for type-i phase matching needed to be replaced with. Author also provided calculation formulas of the temperature derivatives for KTP crystal ( o C -1 ): (6.1) Table 9: Temperature derivatives of the refractive indexes of a KTP crystal [34]. λ Then the index mismatch factor was calculated with Eq. (6.2): (6.2) The results are shown in Fig. 25 below: 27

32 Figure 25: Index mismatch for type-i and type-ii interaction in KTP crystal [34]. Index mismatch becomes zero at the point (44.0, 45.7) for type-i phase matching. The experiment showed that 5-mm long, AR-coated sample of a KTP crystal with cut (θ=44.0, φ=45.7) is temperature insensitive up to 150 o C. A way to improve the interval of thermal insensitivity according to work [37] is to use several nonlinear crystals in cascade configuration. For instance, if one KDP crystal was used as frequency doubler the ΔT interval is 5.36% with the conversion efficiency 4.55%, but if the number of crystals used increased to 4, the ΔT parameter will increase up to 21.6% while the conversion efficiency will be reduced to 4.17% [37] due to a sufficient amount of reflected light from each surface of a crystals in cascade. Authors also presented and experimentally proved the possibility of achieving of self-compensating thermally induced phase mismatch by making the polarization of the interaction waves inversed in adjacent crystals what leads to changing of the signs of the first phase mismatch derivatives with the respect to temperature, so they become opposite and the mismatch compensated. 2.5 Intra- and extracavity conversion schemes. There are two basic cases to construct a device with a nonlinear conversion component: the first one is when a nonlinear crystal is inside a laser cavity, and the second is when a nonlinear crystal proceeds a generation of harmonics outside a laser cavity (see Fig. 26 and 27). Both optical schemes have their own advantages and disadvantages, but this section mostly related to comparative overview between extracavity conversion as main and intracavity one, because the research showed that extracavity generation configuration is more suitable for aims of this work. 28

33 It is generally accepted that intracavity conversion provides a higher efficiency due to a length of interaction of fundamental beam in nonlinear crystal. This conclusion perfectly works in case of generation harmonics from CW laser emission with high power [29, 41], but in case of low-intensity pulsed lasers the usage of a crystal inside the laser cavity may gain a lower efficiency compared to when the crystal located outside the cavity [42, 43 ]. According to Nikogosyan et. al. [41] extracavity schemes are more preferable for converting low-intensity radiation from a diode-pumped laser. Latest research from Yu- Jen Huang et al [42] showed that the external cavity conversion is more efficient than intracavity with the correct optimization of generation process. The generation schemes of a second harmonic that were used in [42] are presented in Fig. 26 and 27: Figure 26: Intracavity conversion scheme [42]. 29

34 Figure 27: Extracavity conversion scheme [42]. In the work [42] authors showed that even when the output power at 532 nm is higher for intracavity process (see Fig. 28), the efficiency of generation of the forth harmonic beam is higher within extracavity generation process (see Fig. 34). Figure 28: Output power of extra- (red curve) and intracavity (green curve) generation processes at 532 nm depending on the pulse repetition rate [42]. This effect may be understood from the following. The length of interaction of the fundamental beam in nonlinear crystal in intracavity conversion is bigger than in extracavity and that explains why the conversion efficiency from 808 to 532 nm is higher for I-SHG conversion. Another additional factor which is also contributes the efficiency in I-SHG is that the optical power within the resonator sufficiently higher. The compared efficiency at 40 khz for I-SHG and E-SHG is 22.2% against 17.3% [42]. 30

35 On the other hand, it was found that when the pulse rate increases from 30 to 100 khz the pulse duration for I-SHG and E-SHG changed from 10 to 42 ns and 8 to 14 ns, respectively, see Fig. 29: Figure 29: Pulse repetition rate curves for E-SHG (red) and I-SHG (green) [42]. With the change of a pulse width the energy changed also. For the intracavity configuration the pulse energy was varying from 177 to 42 μj and for extracavity from 133 to 29 μj. The widths of pulses are presented in Fig. 30 below: Figure 30: I-SHG pulse (left) and E-SHG pulse (right) [42]. The peak power change depending on pulse repetition rate is shown in Fig. 32: 31

36 Figure 31: Peak power of I-SHG pulse and E-SHG [42]. The Fig. 31 shows that the peak power of the extracavity pulse is almost three times larger than the peak power of the pulse obtained from intracavity configuration even when the output energy is higher for I-SHG (see Fig. 28). The peak power of E-SHG changes from 30.4 to 3.7 kw and for I-SHG from 11.6 to 1.4 kw. In the paragraph 2.1 it was shown that the higher the energy of the light field created the higher the nonlinear response of a crystal and the work of Yu-Jen Huang et al proved this in their experiments. To check their suggestions, they constructed configurations of the forth harmonic generation (FHG) of two types: I-SHG+E-FHG and E-SHG+E-FHG. Figure 32: E-FHG configuration [42]. The experiment showed that extracavity configuration provide better conversion efficiency compared to intracavity one, see Fig

37 Figure 33: Pulse energy and peak power for E-FHG from I-SHG (green) and E-SHG (red) [42]. Figure 34: Output power of E-FHG at 266 nm from I-SHG (green) and E-SHG (red) [42]. Authors made a point that they believe that the correct optimization of acusto-optic modulator without the parasitic lasing effect as well as the use of convex lens to focus the light made a great influence on high conversion efficiency result, they also mentioned that the high finesse of the laser cavity and overcoupling effect causes the efficiency of I-SHG to reduce due to the enlargement of the pulse width because off the tail (see Fig. 30). The conversion efficiency from 532 to 266 nm is 37.1% for extracavity configuration and 7.2% for intracavity one. The wall-plug efficiency of E-FHG for the AO Q-switched diode pumped Nd:YVO4 from 808 to 266 nm is 6.4% or 1.67 W was obtained for from 26 W of the pump power. If in the work [43] the generation of harmonics was held without any mirrors, authors of this work presented the next E-FHG scheme: 33

38 Figure 35: E-SHG + E-FHG scheme that was used in the work [43]. Both of nonlinear crystals were placed inside their own resonators and the mirrors were covered with special AR-coatings according to the required parameters of generation as well as the surfaces of crystals were coated with AR-films. Authors reported that they achieved the wall-plug efficiency of 10.7% or 1.82 W from the pump power of 18.8 W at 30kHz for BBO crystal adjusted for type-i phase match. The bigger efficiency of a laser from work [43] compared to a laser from work [42] can be explained by the increased length of interaction due to a multi-passing of the light in resonator, so the average power grows. The resonator also improves the conversion efficiency due to a fact that the after the interaction of fundamental beam with nonlinear crystal structure the harmonic wave will be emitted in both directions relatively to optic axis. The peak power of the laser pulses at 266 nm from Yu-Jen Huang et al. is about 7.2 kw at 30 khz and higher than the peak power of the pulse obtained from the laser from Zhai Suya et al which is 3.8 kw at 30 khz, while the energy of the pulse from Zhai Suya et al is higher μj, than 51 μj obtained by Yu-Jen Huang et al. In my opinion this difference could be caused by the difference in pulse profiles, see Fig. 36: Figure 36: Pulse shape from [42] (left) and [43] (right). The tail in the end of the laser pulse in the work [43] may be the reason why the peak power is smaller than the peak power from [42]. Nonetheless, the efficiency of conver- 34

39 sion in the scheme from Zhai Suya et al is bigger than in the scheme made by Yu-Jen Huang et al. The efficiency of generation of harmonics may be improved by correcting the pulse shape, making the peak power value bigger what will cause a stronger nonlinear response. In the work [42] authors mentioned that the forth harmonic beam parameter M 2 is very poor (Mx 2 2.5, My 2 1.8) was caused by large walk-off effect and precise green beam focusing in BBO crystal. I want to notice that both [42] and [43] works have a very important practical significance. As I see it, with a same conception of extracavity generation, the work [43] showing an interest symbiosis of extracavity configuration with own resonators for each crystal, what is actually an attribute of intracavity schemes, except the moment that in true intracavity process a nonlinear crystal is located in same resonator with active medium of a laser. With results from [42] it is possible to say that E-FHG is more efficient, than I-FHG. However, I think that an optical scheme of UV-device should be presented as extracavity, but I also think that some experiments about implementing of additional resonators needed to be provided, to investigate where this type of scheme is going to be more efficient. After all what was mentioned above, the optimization of harmonics generation process is an important operation to achieve high conversion efficiency. Adjusting the modulator, focusing the light beam, picking the right pair of crystals with a special cut suitable for some special insensitivity regimes and some more tricks are extremely useful to create a high-efficient laser emitter especially when it comes to low-intensity conversion. 35

40 CHAPTER 3 Equipment and experiments It has been shown in Chapter 2 that peak power of a fundamental pulse and phasematch condition play most important role in the efficiency of the frequency conversion. The peak power strongly connected with a nonlinear susceptibility of a material. The orientation of a crystal makes the phase-matching condition achievable. Thus, high nonlinear response of a crystal with a phase-matching condition held makes the generation process effective. In a practical part of this work my attention was paid to the width of a pulse of fundamental beam and constructing and adjusting of the most effective conversion scheme. 3.1 Equipment List of equipment: 1) Laser diode pump source ATC2529 ATC-semiconductor devices, 808 nm; 2) Active medium 1% doped Nd:YVO4, HR 1064 nm, HT 808 nm, 1 mm length; 3) Acusto-optic modulator 1-cm long fused silica, driven by RF 10-W generator at central frequency 80 MHz; 4) Two KTP crystals for generation of 532 nm emission adjusted for synchronism type II; 5) KDP crystal for generation of 266 nm emission. KDP crystals are not the best in terms of suitability for the generation of the 4 th harmonic in this case. The d36 coefficient is equal to 0.4 pm/v and comparing to BBO crystals that rutinely used for FHG with d22 = 2.2 pm/v the nonlinear susceptability of KDP is 5 times lower than BBO. In addition to low nonlinearity, this material has a relatively small width of angular synchronism: about 5000 angular seconds for nm emission for both ooe and oee types of interaction [41]. For the useful properties of KDP crystals one can determine the possibility of manufacturing big and long samples and a great laser damage threshold. Thus, KDP crystals are usually used for generation of harmonics in high-power lasers. Unfortunately, this crystal is the only one that I can use for this experiment. Constructed optical scheme is presented in the end of this section in Fig. 43. The process of construction of this scheme can be roughly divided into several steps (provided that holders, adaptors, suitable electronics and software, materials, instruments and etc. are available): 36

41 1) Find a position when a pump-light from fibre focuses directly in the centre of an active medium. The thermal lens that appears inside the material should be symmetric. This step was made with a help of camera and a laptop. 2) When the spot of the pump-beam is fixed, the distance between active medium and pump-beam focusing system needed to be installed in order to find the position when a waist of a pump-beam located in the middle of the active medium. Due to some construction issues it was hard to place the active element in calculated position of a beam waist at EFFL = 3.1 mm so the focusing system was placed right up to the surface of the copper active medium holder. Then by registering power meter readings the distance was adjusted by turning pitch/yaw screws of the focusing system holder for all 3 axes simultaneously (holder model - Thorlabs K6XS) until the power of generation of 1064 nm will be the highest. Fig. 37 illustrates the dependence of power of 1064 nm emission generation from the number of screw turns, 1 turn 0.3 mm P, mw P, mw Figure 37: 1064 nm emission power depending on the number of screw turns 3) After that the resonator was made as short as possible to make the pulse shorter. The length of resonator is 31 mm in a final scheme. 4) One of the most important operations adjusting of the acusto-optic modulator. When a high-frequency wave comes to a modulator, it makes the piezo-electric element to vibrate. This element is located closely to quartz and sends the ultra-sound wave in a material what will make the refraction index of this material change periodically so the Bragg s cell appears, and a light will be diffracted if it propagates through this area. Practically the modulator creates a canal with stable Bragg s cell, when an area around this canal cannot diffract the laser 37

42 beam fully and resonator will not be closed, so the peak power of a pulse will be reduced dramatically. The indicators of the correct adjustment of modulator is no power registered in continuous wave regime of modulator work and the glares of aligning laser are diffracted, when a spot of an origin laser beam is unaffected in a pulse regime of work. If one can see that the origin laser beam diffracting same way as aligning laser, it means that the beam is currently outside the canal but in the area near it, so some part is diffracting but some part is licking through, so the resonator will not be closed. Wrong position of a modulator greatly reduces pulse power, what is illustrated in Fig. 38 and Fig. 39: Figure 38: Pulse shape before the modulator was adjusted. Figure 39: Pulse shape after the modulator was adjusted. 5) After the resonator is short as possible and fully locked it is time to set up a shape and width of a pulse, by slightly turning of the modulator and resonator screws. Working pulse shape presented in Fig

43 Figure 40: Working pulse (FWHM = 5 ns). 6) When a pulse is short as possible, the laser beam must be focused inside the KTP crystal. A lens was installed strictly on the normal to radiation. 7) KTP crystals were placed in the position of the focusing lens waist. By turning the crystals and checking the power meter readings one can place both of the crystals and achieve a generation of a green light (Fig. 41). The generation from 270 mw of 1064 nm 16 mw of 532 nm emission was achieved. Figure 41: Generated green (532 nm) light 39

44 8) The next step is to focus the second harmonic light in a crystal for the forth harmonic generation. This operation is the same as in steps 6-7. In case of KDP crystal, the efficiency of generation strongly depends on the condition that the crystal is aligned to the optical axis of a laser. So, the first step is to meet this condition. After that by turning the crystal the UV-light spot can be registered on a sheet of a white paper (see Fig. 42). Figure 42: Generated deep UV-light. Figure 43: Optical scheme of the 4 th harmonic generation. 40

45 CHAPTER 4 Results The UV-laser that was constructed presented in Fig. 44, numbers are correspond the elements according to the list below: 1) Pumping diode; 2) Fiber; 3) Focusing system; 4) Resonator mirrors; 5) AO-Modulator; 6) Focusing lens for 532 nm generation; 7) KTP crystals; 8) Focusing lens for 266 nm generation; 9) KDP crystal; 10) Light filter. Figure 44: Constructed UV-laser. 41

46 From 130 mw of 1064 nm laser radiation in Q-switching regime of work 20 mw of 532 nm radiation was obtained. The efficiency of the extracavity conversion of IR-light to the second harmonic is about 15%. Unfortunately, even if the UV-light was obtained and registered visually, the measuring of its power is complicated due to 2 factors. The first is that some part of 1064 nm radiation is transpiring through the filter (about 5 mw). The second factor is that the conversion efficiency is very low, and the usage of prism involves a big amount of losses and it was impossible to register a power of the clean UV-light beam. While it is impossible to measure the intensity of UV-light its power was calculated. The intensity of harmonic generation can be calculated with Eq. (7) [41]: (7) Where A (m 2 ) is the spot size: (9) Power P (W) of the green light is: (10) Where F (Hz) is the frequency and τ (sec) is pulse duration. In case of perfect phase-matching the term simplifying the Eq. (7) one can obtain Eq. (11): from the Eq. (7) vanishes. After (11) Where Z0 = 1/(ε0*c) = 377 (Ω) is the impedance of free space, χ 2 is a nonlinear coefficient of a crystal, l is the length of interaction, nω and n2ω are refractive indexes for 532 and 266 nm emission respectively. Power of the green light beam was calculated to be 340 W/m 2. The intensity of the UVlight was calculated with Eq. (11) and found to be 11 mw/mm 2. The intensity was calculated for the perfect phase-matching condition, but in reality, this condition is not perfectly held and the real intensity is lower than calculated one. To check the disinfection properties of constructed UV-laser the experiment was conducted. 4 samples (Fig. 46) were taken from a petri dish that was inside the packing machine for a one hour (Fig. 45). The petri dish was taken from one of the food production company. 42

47 Figure 45: Petri dish that was inside a packing machine for 1 hour. Figure 46: The mold samples. The samples 1 and 2 were irradiated, samples 3 and 4 were used as a control group. The exposition time was 30 min. After the irradiation was done, the samples were placed in in a warm (T=28 30 o C) humid environment for 24 hours. 43

48 Figure 47: the mold samples after 24 hours. As it was expected, irradiated samples 1 and 2 showed the reduction of the growth rate comparing to reference samples 3 and 4. Results of the work: 1) UV-laser was constructed. The 4 th harmonic intensity is maximized for an existing configuration. 2) While a laser is constructing, in order to make a high-quality device the work should start from the beginning and be verified at every step. The conversion efficiency achieved for 266 nm is very low, but there are some problems to be solved. When a KDP crystals are used in high-power lasers and its nonlinear coefficient is very low, in order to increase the nonlinear response of the crystal and optimize the generation of the 4 th harmonic the green light was focused with a help of a short lens, instead of trying to use pluses from the length of interaction. As it was mentioned KDP crystal should be replaced with BBO one, because BBO is more preferable material in case like this and the conversion efficiency will be greatly increased. 3) The intensity of the 4 th harmonic generation was calculated and found to be about 11 mw/mm 2. 4) At low average power of the fundamental beam the only way to achieve an efficient generation of harmonics is to reduce the width of pulses or in other words to increase the peak power of a pulse. The pulse duration strongly depends on the next factors: the length of resonator, correct position of the Q- switching element and its working parameters. Pulse duration in the constructed conversion scheme is equal 5 ns. Modulator works with a frequency 11.5 khz. 5) The disinfection ability of the laser was checked experimentally. Even a very low-intensity emission of 266 nm showed a good result in terms of decontami- 44