Investigation On Optical Properties,Damage Threshold And Parametric Frequency Conversion In GaSe: Te (Al,S,AgGaS₂)

Comparative experiment on SHG in AgGaS2-, Al-,Te-, S-doped GaSe and pure GaSe is carried out at 2.12?2.9μm fs OPG and 9.2?10.8μm ns CO2 laser pump. It was found that GaSe:S crystals possesses the best set of physical properties for mid-IR application ,GaSe:Al and GaSe: Te are attractive for THz application. The study consists of three parts as following.I. The IR optical properties of GaSe and GaSe:X(X=Al,Te, AgGaS2, S) crystalsThe transparency spectra of the samples were measured over 32 times averaging by the spectrophotometer TU-1901(Puing Corp, China) with spectral resolutionΔλ= 0.05 nm in 0.2?0.9μm range and ATAVAR 360 FT-IR spectrophotometer (ThermoNicolet) withΔν= 4 cm?1 in 2.5?25μm range. The typical spectra of doped GaSe crystals are shown in Fig. 1.and Fig. 2.It can be seen in Fig. 1 the optical transmission characteristics for GaSe are almost not affected by Te-doping in difference to S and Er-doping. Only short-wavelength transparency end for GaSe:Te films has evident but small tendency to longer-wavelength shift versus doping (Fig. 1a) that can be just recognized in transparency spectra for mm-thick GaSe:Te (Fig. 1b). From transmission measurements with fs OPG operating at 2.6μm it was determined that low (≤0.05 mass%) Te-doped GaSe crystals are characterized by low absorption coefficient ofα= 0.1~0.2 cm-1 like GaSe. Higher (up to 5 mass%) Te-doping did not change significantly the absorption coefficients if measured at chosen local regions free from Te inclusions.The transmission curve for GaSe is weakly depended of Al-doping. In Fig. 2 it can be seen that GaSe:Al is characterized by exceeded absorption at wavelength≤0.62μm to that in GaSe. Besides, a small tendency for longer-wavelength shift is possibly caused by the defect band(s) and sub-microdispersed inclusions. No noticeable changes in the phonon absorption spectra were found. The optical quality of GaSe:Al crystals is rapidly degrading with increasing Al-doping which is different to that of GaSe:S crystals with S-doping. The GaSe:AgGaS2 and GaSe:S (2 mass %) have same short-wavelength transparency end, but the GaSe:AgGaS2 crystal is characterized by something higher optical losses. From the transmission measurements with fs OPG operating at 2.6μm, it was found that the GaSe:AgGaS2 and GaSe:Al (≤0.05 mass %) crystals are characterized by low absorption coefficient ofα≤0.1?0.2 cm?1 and they are well suitable for the mid-IR applications. The thin GaSe:Al (≤0.5 mass %) films withα≤1 cm?1 are still suitable for the nonlinear applications at fs pulse pumping. Schematic experimental setup of SHG is shown in Fig.3. The traditional SHG optical set-up and a homemade low-pressure line-tuneable CO2 laser with TEM00 mode, 400?1000 Hz pulse-repetition rate, and up to 500 W peak power in leading 150 ns pulse followed by 1μs tail was applied in this experiment. One BaF2 lens with focal length of 65 mm was used for focusing the ?3.5 mm pump beam on the crystals which was mounted on a step-motor-drive computer-controlled rotational stage (RSA100, Zolix Instruments Co., Ltd, China) with positioning accuracy 18″and 1 m far from the CO2 laser. The other BaF2 lens with focal length of 35 mm was applied for focusing the SHG beam on the sensitive area of detectors. The residual CO2 laser radiation was blocked by two LiF plates with 4 mm thickness which is close to the nonlinear crystal and the detector. The UV-FIR monochromator (Zolix SPB300, Beijing, China) with the grating of 67 gr/mm and RT pyroelectric detector (MG-30, Russia) with D≥7×108 cm?Hz1/2/W at 2?20μm range are applied to measure the wavelength and the SHG pulses over 1000 pulses averaging. The BBO OPG (Topas-C, Lithuana) was used as the other pump source. Under the pumping of 3.7 W and 103 fs Ti:Sapphire laser system, Topas-C OPG generates 60?90 fs signal and idler wave pulses which is tunable within 1.1?1.6μm and 1.6?2.9μm, respectively, with total average output power of up to 0.5 W in ?1.4 mm output beam and≤1% instability. The Ti:sapphire fs laser system included pump Nd:YLF 1 kHz PRF laser with BBO SHG pumps the master Ti:sapphire laser (Mira 900-B, Coherent) whose output is amplified by the Ti:sapphire OPA (Legend Elite, Coherent). The UV-FIR monochromator (Zolix SPB300, Beijing, China) with the grating of 300 gr/mm is employed to measure the wavelength and the PbS photoresistor (DPbS2900, Zolix, China) with D≥5×108 cm?Hz1/2/W,τ≤200μs, and voltage response≥3×104 V/W at 0.8?2.9μm range is applied to record the OPG SHG pulses over 1000 pulses averaging.A dependence of output signal versus angle for 9.6μm CO2 laser type I SHG in GaSe:Te(2 mass%) is depicted in Fig. 4. In this figure it can be seen that ?-angle dependence of the output related to efficient second order nonlinear susceptibility deff = d?22 cos sin3 for type I (e-oo) SHG in hexagonal -GaSe. This six-petal flower-type figure was not changing crystal to crystal and with pump beam scanning over the crystal surface. Thinner samples exfoliated from this and other Te and S-doped crystals also shown identical ?-angle dependence thus confirm hexagonal structure in the crystal bulks. It was found that output SHG power is related to the crystal length lc as l2c; thus demonstrates that no centrosymmetrical -GaSe presences in the crystal bulks. In the whole, these data confirm that GaSe:Te is hexagonal structure like GaSe at least with≤5 mass% Te doping. Same conclusion was made for GaSe:S(≤2 mass%).Fig. 5 shows type I PM for SHG as a function of pump wavelength. SHG in GaSe was not performed so far at pumping wavelengths shorter than 2.36μm. In this study we observed the SHG under the pumping by idler waves of fs OPG down to 2.12μm. In Fig. 5 it is seen that our experimental data for SHG in GaSe and GaSe:Te are most consistent with data estimated by use dispersion relations reported in [11,30]. All data in details are pictured in Fig. 6. From Fig. 6 it goes that our experimental data on SHG PM in GaSe and GaSe:Te at pump wavelength≥2.79μm are also in agreement with majority of available experimental data [46,47,49,50,51] but excluding data on CO2 laser SHG reported in [48]. SHG PM angles at2.5~2.65μm pump shifted to the curve estimated with dispersion relations from [44] that are reported as valid for 2.4~28μm range. SHG PM angles at 2.25μm pump, so as experimental data for SHG at 2.36μm pump reported in [48], are in agreement with data in [44, 47] but they are 2°upper from the curve estimated with the data in [45]. SHG PM conditions in GaSe and GaSe:Te and wide spectral bandwidth (12 nm) of the pump emission did not allow us to carry out correct measurement of PM at pump wavelength≤2.25μm. High reflectivity for pump and SHG waves at incident angles exceeding 75°drastically decreased output signal. Due too high gradient dθ/dλSHG spectrum was severely deformed in relation to wide spectral width pump spectrum. These two negatives prevent correct identification of PM angles. In Fig. 6 it is also seen that our experimental data for CO2 laser SHG are 1°lower to PM curve estimated with dispersion relations in [44]. It can be proposed that small changes in transparency spectrum is following by small changes in the lattice properties and PM conditions similar to the case of Er-doping resulting in all three small transformations: in rocking curve widening, transparency spectrum changes and DFG PM [44]. Finally, it can be concluded that our experimental data on SHG PM are in best agreement with data estimated by use the dispersion relations reported in [47]. Wide-used dispersion relations from [45] have to be corrected for wavelengths≤2.65μm. Fig. 7 presents the type I external SHG PM angle as a function of pumping wavelength. The SHG was not performed so far in the GaSe crystals by short-wavelength pumping which is shorter than 2.36μm. In Fig. 8, it can be seen that the PM angles in GaSe:Al for 2.25?2.9μm SHG have small tendency to upper PM angles. However, this tendency is very negligible or absent for CO2 laser SHG. Large SHG PM angles for GaSe and GaSe:Al at 2.12μm pump did not allow us to measure PM angles correctly. It is because of the high reflectivity of crystal surfaces at PM angles exceeded 75°to decrease the output signals drastically. It can be concluded that our experimental data for SHG in GaSe and GaSe:Al are in best agreement on the dispersion relations reported in [45, 47]. The PM for GaSe:AgGaS2 and GaSe:S (2 mass %) crystals grown by the traditional technique was measured over entire range of 2.12?2.9μm and at CO2 laser wavelengths. Their PM is characterized by general trend with small difference in PM angles. The surface damage threshold was measured at 2.12?2.9μm by fs OPG and ns CO2 laser radiation, which compares the incident power and the output power. The damage threshold of higher Te-, Al- and S-doped GaSe measured by fs pulses is about 10 to 15% lower than that of pure GaSe crystals and is independent on the OPG wavelengths. The damage threshold of GaSe:S (10.2 mass %) measured by 150 ns CO2 laser is 1.4- to 1.5-fold to that of pure GaSe. There is no difference between the damage thresholds of the GaSe:AgGaS2 and the GaSe:S (2 mass%). Similarly, no difference found between the damage thresholds between GaSe and GaSe:Al (≤0.05 mass %).The nonlinear coefficients were measured by comparing the SHG efficiency of doped GaSe to that of the pure GaSe by longer wavelength 2.79μm pumping (fs OPG) to exclude the interference by nonlinear absorption. No significant difference was found experimentally between the nonlinearities of GaSe:AgGaS2 (real composition GaSe:S (2 mass %) and the GaSe:S (2 mass %) crystals grown by the traditional technology. The results obtained from the SHG efficiency in GaSe:S crystals, d22 = 0.89 of GaSe:S (2 mass %) and d22 = 0.8 of GaSe:S (10.2 mass %) to that in pure GaSe, are in good agreement on d22 of the same doping level GaSe:S reported in [15]. In the calculation of nonlinear coefficient d22, theθ-angle dependence of deff was accounted. Thus, one can propose that the 11% decrease in d22 for GaSe:S (2 mass %) to that in pure GaSe is due to the lighter S-ion doping. It will lead to about 21% decreasing in the figure of merit and the SHG efficiency. On the other hand, this decreasing in efficiency will be overcompensated by 80% increasing in the damage threshold of GaSe:S (2 mass %) to that of pure GaSe. That is why, as high as 1.4-fold efficiency can be predicted for GaSe:S (2 mass %) crystal to that in pure GaSe. More over, the SHG PM angle under 2.79μm pump is about 7°lower in GaSe:S (2 mass %) to that in pure GaSe that leads to further 14% increase in deff and 20% in efficiency.In this study, no attempt was made to improve the crystal faces quality, optimize the pump beam parameters or the crystal length and finally to maximize SHG efficiency. Nevertheless, for majority of 0.5 to 2 mm crystals SHG average output power of 15 mW to 25 mW is measured under the pumping with 105 mW fs OPG output at 2.4 m that was well below the damage threshold. In particular, SHG average power of 15±3 mW is measured on the output of 1 mm crystals and 0.55 mm GaSe:AgGaS2, 19 mW on the output of 0.92 mm GaSe:Te(0.5 mass%) ,21 mW on the output of 0.89 mm GaSe and 25 mW on the output of 0.9 mm GaSe:S(2 mass%). The experiment on CO2 laser SHG in≤5 mass% Te doped GaSe resulted in slowly decreasing efficiency with doping level down to roughly 101?5% to that in GaSe. GaSe:S(2 mass%) 3 mm length crystal demonstrated over 2-folded advantages in CO2 laser SHG to that in 3 mm GaSe.Al-doped GaSe possesses up to 2.5?3-folded hardness to that of GaSe and 25% higher to the AgGaS2-doped GaSe. The GaSe:Al crystals are characterized by extremely low conductivity of≤10?7 Om?1?cm?1 among all of the doped GaSe crystals. It can be cut and polished at arbitrary direction. Its low conductivity and high hardness are attractive for fs pulse THz generation. GaSe:Te(≤5 mass%) are identified as hexagonal structure crystals likeε-GaSe. The resistivity of GaSe:Te(≤0.5 mass%) crystals is two or three orders higher to that for GaSe that makes them attractive for THz application. For the first time PM is studied in GaSe:S crystals at≤2.36 m pumping. It is shown that GaSeS crystals possesses best set of physical properties for mid-IR SHG to that for pure and doped GaSe.