The Investigation On Temperature Disperse Of Phase Matching For Second Harmonic Generation In ZnGeP₂,HgGa₂S₄,GaSe_1-xS_x

The crystal may be heated by residual absorption of the pump laser radiation in high power nonlinear devices. It will casuse the change of the refractive index,and result in unphase-matching. So it leads to a reduction in conversion tefficiency and stability. temperature turning allows realizing of phase matching unreachable by angular turning and fine adjustment to non-critical phase matching. The straight requirement of the developing concentration for solid solution crystals is often not realized in the solid solution crystal growth process limiting their use or efficiency, but the negative influence of composition ratio variations on phase matching can be compensated by a crystal temperature control. Temperature stability and tunability are thus important parameters in view of desired operation of a crystal. Second harmonic generations (SHG) of the tunable CO₂ laser and Er³⁺:YSGG are demonstrated with crystals of ZnGeP₂,HgGa₂S₄,GaSe and GaSe_1-xS_x (x≤0.369). Phase-matched properties are investigated. The study consists of four parts as following.(1) ZnGeP₂Schematic experimental setup of SHG is shown in Fig.5.1.Employing tunable Q-switched CO2 laser, temperature property of SHG phase-matched angles in ZnGeP₂ crystal is investigated. The dimension of ZnGeP₂ crystal is 6. 9×10.3×4.3mm³, which is cut at 70 degree. As shown in Fig 5.2, our experiment data are well consisted with the theoretical predictions at the 9μm waveband of CO₂ laser for the different temperatures. But there exists a big difference between the experiment and simulation at the 10μm waveband except at the temperature of 500 K.The difference of 10-15°in phase matching angles for CO₂ laser second harmonic generation in ZnGeP₂ reported by Russian and USA researches arises from positive temperature coefficient of birefringence through it residual absorption of incident CO₂ laser radiation, i.e. self heating effect. It goes from measured negative change of phase matching angle as dθ/dT = -4.3′/°C at 10.3μm and dθ/dT = -6.4′/°C at 10.6μm at temperature phase matching width ofΔT=50°C?cm FWHM and controlled temperature change of ZnGeP₂ crystal up to 150-200°C(Fig 5.3).The ZnGeP₂ crystal heating up to 150-200°C is simultaneously resulted in shift of long-wavelength end of phase matching diagram from 10.2 to 10.8μm that allows second harmonic generation for entire CO2 laser operation range. So, the differences in pump powers is a main reason for differences in phase matching conditions reported by different researches but as it goes from model estimations thermo-optical equations developed by Prof. Bhar is still valid in every case.(2) HgGa2S4SHG phase-matched angles in XZ plane of AgGaGeS₄ crystal with dimension of 10×15×2.1mm³ are 29.7 and 27.6°for pump wavelengths 9.48 and 9.6μm, respectively. Measured experimental data for SHG two crystals are well within the different diagrams. The differences among different diagrams can be caused mainly by crystal cut and inaccuracies approximation, and by different crystal composition and/or physical properties due to different growth technologies.In our experiment a mechanically polished single crystal HgGa₂S₄ plate with dimensions 10×12×3.1 mm³ and color variations from light green to orange over the big plane area was used. Chemical composition has been determined by electron probe microanalysis (EPMA) of LEO-1430 device with scanning area 0.1×0.1 mm², 5 nm deep, for several positions at the 10×12 mm² sample surface. The actual composition is Hg1.02±0.08Ga1.90±0.04S4 without any reasonable relation to local area color. Optical transparency at short-wavelength end in visible range has been measured with Shimazdu UV 3101PC (0.3～3.2μm ) as shown in Fig.5.4. Different color crystals are located at different positions as shown in Fig.13 inset: 1 is a local part of most light-green colour, 2 is light-green, 3 and 4 are yellow; 5, 6 and 7 are orange, 8 is deepest orange part. At 10% level the short-wavelength end of transparency range found at 528 nm for deepest orange part of the crystal is shifted to longer wavelength in reference to 520 nm for yellow and 512 nm for light-green part of the crystal. Long-wavelength transparency end is studied with Specord 80 IR (2.5-25μm) and the same result 14.3μm for different colors is record. To confirm correlation between the colour variation and phase matching (PM) conditions we have carefully measured phase-matched angle (PMA) for SHG under CO₂ laser pump at the points 1, 4 and 7 on the crystal surface shown in Fig.4a inset. The relation between different color crystals and angular mismatch under the pump wavelength 9.512μm is shown in Fig5.5a. To improve the accuracy the changes of SHG output powers versus angular mismatch were also developed and approximated. In such a way an influence of measurement instabilities resulted in scattering of experimental points near the tops of diagrams on the determination of PMA is significantly reduced. From Fig5.5a one can see that maximal difference in PMA for different colour local points is ¹.5°. Our data for light green local point 1 are close to Takaoka's data [CLEO'98, CWP39, P.253-254] for his green colored or something yellowish HgGa2S4 crystal. Our experimental data for yellow and orange points 4 and 7 are shifted in the direction to Petro's diagrams (Opt. Commun., 2004, 235, 219-226) for yellow and orange colour crystals. Temperature tuning of SHG PM pumped by different wavelength 9.48,9.51,9.53μm is shown in Fig5.5b. With the increase of the crystal temperature, phase-matched angles increase slightly. Temperature dispersion of dθ/dT= 0.36′/°C at pump wavelengthsλ=9.48-9.53μm is recorded.Based on the analysis of the available sellmeier equations and experimental data, dispersion equations for different colour crystals are proposed aswhere y is a weighting coefficient. At y=1 and 0, the dispersion of refractive indices is governed by Sellmeier equation from Takaoka's and Petro's. As shown in Fig5.6a, under the relation, the value y=0 well describes our experimental data for local position 7. The values y=0.55 and 0.32 are related to positions 4 and 1, respectively. When we change the weighting coefficient y, phase-matched diagrams simulated are well agreed with available experimental data. With the help of short-wavelength end of HgGa₂S₄ crystal transparency at 10% level shown in Fig.4a and the new dispersion relations, short-wavelength end of different color HgGa₂S₄ crystal transparency at 10% level can be concluded and shown in Fig.6b. The dispersion relations open the possibility for accounting real refractive index variation synchronous with crystal colour variation and are applicable for PM conditions determination in wide spectral range, which are helpful for the development of different colour HgGa2S4 crystals in device applications.(3) pure GaSe and GaSe1-xSx (x≤0.369) Experimental phase matching angles for Er³⁺:YSGG laser SHG in higher quality GaSe crystals #1 and #2 are of 49.3°and 49.25°(Fig5.7). Phase matching angles are in good coincidence with phase matching angle of 49.45°estimated with disperse formula of Ref. [G51]. This paper is published on the end of 90th when best quality GaSe crystals haveα=0.05-0.1 cm^-1, i.e. same to crystals # 1 and #2. Phase matching angle for lower quality GaSe #3 (α≈0.25 cm^-1) is of 50.0°that is 0.55°in upper position to Ref. [G55]. Further, phase matching angle estimated with data available in earlier paper[G59] of 1982 is in much higher position atθ=57.2°. On the side, phase matching angle of 45.5°estimated with disperse formula of Ref. [4.61] published in 2005 is in lower position for 3.8°At positive temperatures Er³⁺:YSGG laser SHG phase matching angle in GaSe #1 crystal linearly increases with temperature with slope dθ/dT=22′′/1°C, 19.4′′/1°C for GaSe_0.91S_0.09, 19′′/1°C for GaSe0.825S0.175 and 13′′/1°C for GaSe0.631S0.369 crystals (fig.8a), thus open lower temperature dependence of phase matching for solid solution crystals versus mixing ratio x. It can be explained by predominant influence of the birefringence decreasing with temperature to shift of phase matching diagrams for solid solution crystals to longer wavelength range with temperature like in ZnGeP₂ and to bigger phase matching angles with x. At the temperature range close to LN temperature the slopes become about 2 fold lower to positive temperature coefficients. Measured phase matching temperature width in GaSe_0.91S_0.09 is of 22.7°·cm FWHMIn other words, pure GaSe and GaSe1-xSx (x≤0.369) crystals are also insensitive to high power Er³⁺:YSGG laser pump but control of phase matching conditions through temperature control is possibly reasonable for shorter wavelength pump.Here(fig.8b), in difference to Er³⁺:YSGG laser SHG, at positive temperatures phase matching angle decreases with temperature. And the slope for pure GaSe #1 crystal dθ/dT=-4.9′′/1°C is smaller to solid solution GaSe_0.91S_0.09 slope of -9.7′′/1°C and to GaSe0.631S0.369 of -10.6′′/1°C. Follow to data for Er³⁺:YSGG laser SHG it opens shift of phase matching diagrams for solid solution crystals with x both to shorter wavelength range and bigger phase matching angles. Phase matching temperature width for GaSe_0.91S_0.09 crystal is of 222.4°C·cm at 9.58μm that is close to data Ref. [G54] of 172°C·cm for pure GaSe crystal at 10.59μm. For GaSe_0.91S_0.09 phase matching angle for CO₂ laser second harmonic generation is dθ/dT=-9.7′′/°C that is same sign but 30 times lower to ZnGeP₂. Again it is of opposite sign but about 2 times lower to HgGa₂S₄ , For pure GaSe dθ/dT=-4.9′′/°C that is about a half to solid solution GaSe_1-xS_x (x≤0.369) crystals.