We report results on comparative study of SHG in powder of promising nonlinear γ-Ga2S3 crystal. Digallium trisulfide powders with particle size from 20 μm to 500 μm were tested in comparison with powders of well-known LBO, BBO, KABO, KDP, and LN crystals under the pumping by 7 ns 1064 Nd:YAG laser. Laser-induced damage threshold of different powder fractions were determined. The γ-Ga2S3 shown high damage threshold and large SHG intensity: 56 times to that in LBO powder, 15 in BBO, 50 in KABO, 67 in KDP, and 3 in LN (for particle size: 20–50 μm), that renders it amongst the most promising crystal for frequency conversion of high-intense nanosecond radiation of near-IR lasers by optical rectification technique.
We launched into a development of a new stand-off sensing system that can detect atmospheric and hazardous gases in real atmosphere utilizing THz technology. Narrow line width, <0.1 cm-1, long-wave (mid-IR and THz) source based on difference frequency generation (DFG) in collinear configuration in GaSe0.91S0.09:Al(0.03 at. %) using seeded YAG laser and KTP OPO as pump sources was designed. The low optical loss coefficient and large hardness, together with the simplicity of the processing, make GaSe0.91S0.09:Al(0.03 at. %) as a high-reliable and effective THz-wave generator suitable for out-of-door application. We demonstrate incoherent terahertz wave detection by stand-off room temperature Schottky diodes located at over 110 m using open waveguide formed by multitude HPE lenses, mirrors and/or unpolished reflectors.
Design of top S-doped GaSe growth technology is completed. New methods for characterization of high optical quality crystals are proposed that allowed selection optimally doped crystals. Frequency conversion of fs pulses into 6.5–35 μm and into 0.2–4.5 THz is realized. S-doped crystals demonstrated advantages from 50–70% in the first experiments up to 8.5–15 times in the following experiments depending on experimental conditions.
Ab-initio study on modification of commerce terahertz spectrometer with time resolution Z-3 (Zomega, USA) by substitution of ZnTe and GaP detectors and LT-GaAs generator for homemade of pure and S-doped GaSe is carried out. It was established that in spite of not optimized parameters pure and doped GaSe:S(0.3 mass%) crystal are comparable, relatively, in generation efficiency and detection sensitivity to commerce units due to lower nonlinear optical loss and much higher damage threshold. The advantages are in force from pump fluences of below 5 mJ/cm2 for pure GaSe. The closer S-doping to optimal concentration, the lover fluences resulting in the advantages. Pure and S-doped GaSe demonstrate higher reliability and larger dynamic range of operation. Recorded absorption spectra well match known spectra.
The modified Bridgman method with heat field rotation was used to grow ε-polytype single crystals of pure and 1, 2 and 10 mass % S-doped GaSe or solid solution crystals GaSe1-xSx, x = 0.002, 0.091, 0.412. The interaction of ultrashort laser pulses of ∼ 100 fs duration at 800 nm and 2 μm with the grown crystals was studied at room temperature. Up to 3.4-fold advantage of S-doped crystals in limit pump intensity (no decrease in the transmission) was found under 800 nm pump at S-content increase up to 10 mass %. The advantage became a half less at 2 μm pump due to a decrease of two-photon absorption in pure GaSe crystals. The spectral dependence of transient absorption is recorded with 37 fs resolution and interpreted. It was ascertained that first observable damage of high quality crystals is caused by dissociation of submicrometer thick surface layer to initial elements and do not influence the frequency conversion efficiency until alloying of dissociated Ga. Local microdefects, multiphoton absorption and transient transmission processes are identified as key factors responsible for damage threshold.
The сentimeter-sized GaSe crystals doped with 0.01, 0.05, 0.1, 0.2, 0.5, 1, 2 at.% of Al and 0.025, 0.1, 0.5, 1,2 at. % of Er have been grown by the modified Bridgman method with heat field rotation. The crystals have been studied in comparison with GaSe crystals doped with 0.1, 0.5, 1, 2, 3, 5, 7, 10.2 wt.% of S, 0.01, 0.1, 0.5, 1, 2, 3, 5 wt.% of In and 0.01, 0.1, 0.5, 1, 2 wt. % of Te grown by the conventional Bridgman method. The distribution coefficient of Al in the grown GaSe:Al (≥0.1 at.%) crystals has been estimated to be ∼8⋅10-3 and it is within the range of 10-2-10-3 in Er-doped crystals. For the first time, the optimal doping levels have been estimated for Al and Er in GaSe as 0.01- 0.05 at.% for Al and ~ 0.5 wt.% for Er, respectively.
Damage threshold of non-linear GaSe crystals under IR fs (Ti:Sapphiere 800 nm laser and 1.1-2.9μm OPG) and ns (2. 79
Er3+:YSGG and 10.6μm CO2 laser) pulse pumping is studded in details. Local micro defects and field induced effects (GaSe dissociation, multiphoton absorptions and transient transparency origin effects) are identified as responsible for damage threshold in this case. Local (including nano scaled) defects and thermal effects are identified as reason of damage threshold under ns pulse pumping.