Investigation On Optical Properties And SHG In Nonlinear Crystals GaSe, GaSe: X(X=S, In) And GaSe_1-xS_x(x≤0.413)

For the first time experimental study of optical properties of pure GaSe,GaSe:X(X=S,In) and GaSe_1-xS_x(x = 0, 0.0042, 0.023, 0.09, 0.091, 0.182, 0.275, 0.375, 0.413) of sulfur content, relatively, are studded in a view of nonlinear optics application, so as comparative study of Er³⁺:YSGG and CO2 laser second harmonic generation with it use. The study consists of two parts as following.I. The IR optical properties of GaSe, GaSe:X(X=S,In) and GaSe_1-xS_x(x≤0.413) crystalsGrowing of the single crystals was carried out by the Bridgman-Stockbarger method from the synthesized polycrystalline compounds. (Chemical composition of GaSe_1-xSx crystals grown differ from the charge composition in lower Se and S content per 1-2 wt% each,here we used the data of real content measured by EPMA ) Pure GaSe,GaSe_1-xS_x (x = 0, 0.0042, 0.023, 0.09, 0.091, 0.182, 0.275, 0.375, 0.413) and Ga_1-xIn_xSe(x = 0, 0.0055, 0.0692, 0.133) crystals have been used in this study. Transparency 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 absorption spectra are displayed in Fig.1.Transparency range of these highly nonlinear (d₂₂ up to 75 pm/V) crystals is under the control on the technological stage of the crystals growth and is shifted to wavelength range shorter than 0.56μm with mixing ratio x increase to 0.413 as shown in Fig.2. It allows to design nonlinear devices under high peak power (intensity) picosecond Nd:YAG (1.06μm) and femtosecond Ti:Sapphire (0.7-1.1μm) and Cr:Forsterite (1.25-1.32μm) laser pump in difference to all known nonlinear crystals but low efficiency (d36=12.5 pm/V) AgGaS₂ crystal. Let us note, frequency conversion efficiency is proportional to d_ij² and short wavelength transparency end allows using visible range laser for alignment of frequency converters and optical systems with it use even during operation without remove of GaSe_1-xS_x crystals. Long-wavelength transparency end shits from 18μm to 14μm that does not limited significantly these crystals application because long-wavelength transparency window of the atmosphere is just at wavelength 14μm.GaSe_1-xS_x hardness is linear increasing to two fold to GaSe with x increase to 0.1 and the crystals become suitable for mechanical treatment and polishing, so as useful for outdoor applied system. At x=0.4 the crystals hardness is 4-fold to GaSe.II. The nonlinear properties of GaSe, GaSe:X(X=S,In) and GaSe_1-xS_x(x≤0.413) crystals(1) The SHG phase matching at room temperature It was found that GaSe_1-xS_x crystals do not change acentrosymmetric lattice structure till x≤0.413. It goes from linear shift of short-wavelength transparency end of the crystals with x and gradual change of phase matching angles for Er³⁺:YSGG and CO₂ laser second harmonic generation. It optical quality is also still high (absorption coefficientα≤0.1-0.2 cm^-1) thus the crystals are useful for frequency conversion.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·Hz^1/2 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⁽3+0:YSGG operating atλ=2.79μm with ?3 mm TEM00 beam also was used as a pump source. It high pulse output energy, up to 24.5 mJ, let us to measure SHG signal with pyroelectric energy meter with high signal/noise ratio without use of focusing lenses.Single crystal of pure GaSe and solid solution crystals GaSe_1-xSx(x≤0.413) were grown by the Bridgman-Stockbarger method. Direct measurements with CO2 laser show that all samples are characterized by absorption coefficientα≤0.1 cm^-1 but GaSe #16 crystal byα≈0.25 cm-1. Exfoliated samples used in this study have high optical quality faces and have been used without polishing and any another treatment. The solid solution crystals with different x form red to light yellow color with respect to the increase of the sulfur composition.Experimental phase matching angles for Er³⁺:YSGG laser SHG in higher quality GaSe crystals #1 and #2 are of 49.3°and 49.25°(Fig.4). Phase matching angles are in good coincidence with phase matching angle of 49.45°estimated with disperse formula of Ref. [5.3]. 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 #16 (α≈0.25 cm^-1) is of 50.0°that is 0.55°in upper position to Ref. [5.3]. Further, phase matching angle estimated with data available in earlier paper[5.12] 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. [5.6] published in 2005 is in lower position for 3.8°.SHG phase matching in GaSe_1-xSx is changed within only 10-11°with x from 0 to 0.413 and did not lower significantly the values deffeeo=d22cosθsin3?, deffeoe=d22cos2θcos3? (Fig.4). Bulk damages in GaSe disappear with even at sub-pro cent sulfur doping resulting in 2-fold increase of pump intensity and efficiency of frequency conversion to pure GaSe. Surface damage threshold is almost similar for all crystals studded. Possible reason is low gradient of pump power beam that is differ from optical axis direction not more than for 30°thus only thermal properties along crystal layers are of importance. But thermal conductivity along layers is high, second to ZnGeP2, and do not changes significantly with x. The sulfur doping and correspondent hardness increase leads to excluding of layer cleavage defects and opens real high nonlinear properties of GaSe"masked"by cleavage defects for long-length centimeter-sized crystals. That is why the GaSe_1-xSx crystals have potential for higher efficiency frequency conversion. (2) The temperature phase matching properties at 108K to 500KAs shown in Fig.5a. 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 GaSe0.909S0.091, 19′′/1°C for GaSe0.818S0.182 and 13′′/1°C for GaSe0.625S0.375 crystals, 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[5.10] 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 FWHM. Here, in difference to Er3+:YSGG laser SHG, at positive temperatures phase matching angle decreases with temperature (Fig.5b). And the slope for pure GaSe #1 crystal dθ/dT=-4.9′′/1°C is smaller to solid solution GaSe_0.909S_0.091 slope of -9.7′′/1°C and to GaSe_0.625S_0.375 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. [5.5] of 172°C·cm for pure GaSe crystal at 10.59μm.In summary, experimental study of optical properties of pure GaSe,GaSe:X(X=S,In) and GaSe_1-xS_x(x≤0.413) of sulfur content, relatively, are studded in a view of nonlinear optics application.Transparency range of GaSe_1-xS_x(x≤0.413) crystals is shifted to wavelength range shorter than 0.56μm with mixing ratio x increase to 0.413. Long-wavelength transparency end shits from 18μm to 14μm that does not limited significantly these crystals application because long-wavelength transparency window of the atmosphere is just at wavelength 14μm. GaSe_1-xS_x hardness is linear increasing to two fold to GaSe with x increase to 0.1 and the crystals become suitable for mechanical treatment and polishing, so as useful for outdoor applied system. At x=0.4 the crystals hardness is 4-fold to GaSe.We also developed that composition and optical quality variations in pure GaSe crystals is a important reason for the scatter in measured phase matching angles. As high as about 1°difference in phase matching angles for crystals withα=0.1 cm^-1 andα=0.25 cm^-1 is fixed in the study.the temperature dependent phase matching conditions for Er3+:YSGG and CO₂ laser SHG in GaSe1-xSx crystals throughout 108-500 K temperature range are also be researched. As low slope as from dθ/dT=-4.9′′/1°C to 22′′/1°C is determined at wide phase matching temperature width from 22.7°C.cm to 219°C.cm. Thus, in our knowledge, these crystals are of most attractive for design of high power frequency converters.