Investigation On Optical And SHG Properties Of Crystal GaSe: In

Optical transmission range and phase matching conditions for second harmonic generation of Er:Cr:YSGG and CO2 laser in doped GaSe:In(0.1, 1.23, 2.32 mass%) are studied in comparison with these in pure and sulfur doped GaSe:S(0.09, 0.5, 2.2, 3 mass%) crystals. The study consists of three parts as following.I. The characterization of GaSe and GaSe:X(X=In,S) crystalsAll pure GaSe and doped GaSe:In, GaSe:S crystals were grown by Bridgman technique in SPhTI of TSU, Tomsk and FGUP GOKB Ametist, Krasnodar, Russia. Nine samples are studied and parameters of selected crystals are listed in Table 1.Chemical composition of doped GaSe single crystals has been measured with electron probe micro-analysis (EPMA) with using LEO 1430 device. Phase purity has been tested with X-ray diffraction (XRD) analysis andε-GaSe polytype was found. As–grown highly doped GaSe:S crystals were ~1-2% deficient in chalcogen content in reference to initial chatge composition and weakly doped GaSe:S crystals show chalcogen deficiency as ~10-15%. In doped GaSe crystals were significant, up to 25%, deficient in indium content in reference to initial charge composition. In content of 0.5-1 mass% high optical quality crystals can be grown with 2 to 3-fold hardness to pure GaSe. Optical quality of GaSe:In crystals degrades rapidly with In doping at In content of over ~1mass% so that the crystals can not be used in nonlinear systems. Several highly doped GaSe:In crystals had spherical precipitates and/or local surface deformations. For present study, optical quality samples were cleaved up to thickness lc = 3 ?4mm.Microhardness measurements were performed with hardnessmeter PMT-3, Russia. The S-doped crystals have shown almost linear increase in hardness with doping level, fewer tendencies to cleavage and are suitable to handling during optical characterization. In particular, for the crystal No.4 (2mass% S content) the hardness is 15kg/mm2 that is 2-fold higher in comparison with pure GaSe hardness of 7-8 kg/mm2. The hardness of high quality In-doped crystals was only few higher that of pure GaSe, in particular for the crystal No.20 with 2.32mass% of indium content the hardness is 9.5kg/mm2, but for lower quality crystals the hardness is similar or few higher that of GaSe:S solid solytions. As high hardness, as 14kg/mm2 has been determined for the crystal No.17. It may be supposed that higher hardness is induced by significant In intercalation into interlayer space in GaSe:In crystals.No optimal levels of In or S doping are found to have minimum optical losses. GaSe:In crystals show rapid degradation of optical quality with In doping increase over 1-2 mass% that well correlate with the crystal hardness variation and possible intercalation. The cleaved crystals characterized byα≤0.1-0.2 cm-1 are almost uniform in optical quality and no domain structure is found. GaSe:S crystals, however, show no noticeable changes in optical quality that well correlates with supposed small S intercalation.II. The optical properties of GaSe and GaSe:X(X=In,S) crystalsTransparency spectrum were recorded with spectrophotometer TU-1901, Puing Corp, Beijing, China:Δλ= 0.2-0.9μm range, spectral resolutionΔα=0.05 nm and ATAVAR 360 FT-IR spectrophotometer, ThermoNicolet, USA:Δλ= 2.5-25μm,Δν=4 cm-1, averaging over 32 measurements. Estimated Transmission spectra of pure and doped GaSe crystals are displayed in Fig.1. From Fig. 1 no change is seen in transmission curves recorded for GaSe:In crystals as a function of doping level. Transmission range of GaSe:S crystals is significantly shifted toward shorter wavelengths with increasing of S content(see Fig.2). We can also explain the shift of transmission curves through the color change of the crystals. The color of solid solution crystals with different x changes from red to pink, then to light yellow with respect to the increase of the sulfur composition, while the color of Ga(1-x)SeInx crystals shows few change compare with pure GaSe. The reason for unmeasurable transmission spectrum variation for GaSe:In crystals becomes clear by calculation of x value in chemical formula of solid solution. Because of pronounced difference of atomic mass of In and S, the value 2 mass% of S or In is equivalent, respectively, to mole content of x = 0.09 for sulfur and x = 0.03 for indium. This means that the same doping level written in mass% gives the number of Ga atoms substituted by In atoms in GaSe:In (without intercalation) is more than three times lower the number of Se atoms substituted by S atoms in GaSe:S solution. Besides this, significant intercalation of In into interlayer space was found in GaSe:In crystals. Thus, the combination of these factors promises for much smaller shift of transparency curve for GaSe:In crystals in comparison with that in GaSe:S solid solutions.III. The PM conditions for SHG of GaSe and GaSe:X(X=In,S) crystalsPhase matching conditions for second harmonic generation of Er:Cr:YSGG and CO2 laser in crystals are studied. Schematic experimental setup of SHG is shown in Fig.3. A low-pressure line-tuneable CO2 laser with TEM00 mode selection, 600 Hz pulse-repetition frequency, up to 500 W peak power in leading 110 ns pulse at FWHM followed by 1μs tail is used in this experiment. ZnSe 50 mm focal length lens L1 is applied for focusing of ?3.5 mm TEM00 pump beam into the crystal mounted on a holder installed in vacuumed assemble that was arranged at about 1 m distance from the laser. The holder allows driving the crystal temperature from about 100 K to 500 K with±2.5°C accuracy. Step-motor-drive computer-controlled rotational stage RSA100, Zolix Instruments Co., Ltd, China, with positioning accuracy 4.5″is used for precision determination of the PM angles. RT pyroelectric detector D2 arranged with microchip preamplifier: 2-20μm sensitivity range, NEP=1.5?10-9 W/cm?Hz1/2 is applied to control the output pulse parameters. Digital two-channel storage oscilloscope TDS3052, Tektronix Inc.,Δf=500 MHz, is used to control pulse time shape-form. The residual pump radiation was blocked by two 3 mm LiF plate located P3, relatively, close to the nonlinear crystal and the detector. Homemade Q-switched 132 ns FWHM Er:Cr:YSGG operating atλ=2.79μm with ?3 mm TEM00 beam also was used as a pump source. Its high pulse output energy is up to 24.5 mJ, let's measure SHG signal with pyroelectric energy meter with high signal/noise ratio without using focusing lenses. In Fig.4 it can be seen that external PM angles for Er:Cr:YSGG and CO2 laser SHG in the In and S doped crystals have opposite trends with the doping from PM diagram for pure GaSe. In indium doped crystals PM angles for Er:Cr:YSGG laser SHG are increasing with doping but decreasing for CO2 laser SHG with about 3-fold lower gradient. Only repeated measurements and averaging of the results allow detection od as low as about 0.06°difference in PM angles for good quality doped and pure GaSe crystals. These features of the PM in GaSe:In crystals reveal a shift of the PM diagrams to longer wavelengths in difference to the shift to shorter wavelengths for GaSe:S crystals and explain why the changes were not found in PM angles for CO2 laser with In doping.The PM angles can be estimated with available dispersion data[25,35,69,76,95]. At T = 300 K our results obtained for pure GaSe crystals are in good relation with PM angleθ= 39.43°estimated for CO2 laser SHG with dispersion formula given in [95] but are 5.85°higher the PM angle estimated with dispersion data of [35]. Thus, the 0.05°difference between experimental data for CO2 laser SHG [82] and estimated with Sellmeier equations reported in [35] and also the 0.4°difference between experimental data and estimated with Sellmeier equations of [84] seem be an impact of low accuracy rotational stages used for crystal positioning.We also report the phase matching conditions for second harmonic generation within temperature range T = 108-500 K(shown in Fig.5 and Fig.6). In both GaSe:In and GaSe:S crystalsθext for SHG atλ= 2.79μm is increasing with heating. Contrary to that, small decreasing inθext for SHG atλ= 9.58μm is observed on T increasing in GaSe:In and GaSe:S crystals. From Fig.5 it can be seen that at T>250K the variation of PM angle with temperature can be approximated by linear function with the slope dθ/dT = 22′′/1°C for pure and In doped crystals. At T~108K the slope is half as that at positive temperatures. This seems be a consequence of birefringence decrease as it was observed for ZnGeP2. Measured PM temperature bandwidth is of 22°C?cm FWHM.From Fig.6 it can be seen that the PM angles for CO2 laser SHG are linearly decreasing with temperature with the slope dθ/dT = -4.9′′/1 C. PM temperature bandwidth at 9.58μm pump is of 219°C?cm FWHM, which is close to earlier reported value of 172°C·cm FWHM for pure GaSe crystal at 10.59μm pump.Thus, application of GaSe:In solid solutions in high average power outdoor nonlinear optical systems seems be prospective.