We report results on comparative study of SHG in powder of promising nonlinear γ-Ga<sub>2</sub>S<sub>3</sub> 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-Ga<sub>2</sub>S<sub>3</sub> 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<sup>-1</sup>, long-wave (mid-IR and THz) source based on difference frequency generation (DFG) in collinear configuration in GaSe<sub>0.91</sub>S<sub>0.09</sub>: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 GaSe<sub>0.91</sub>S<sub>0.09</sub>: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.
Double peak phase matching was recorded during experimental study of parametric frequency converters based on GaSe, its solid solutions crystals like GaSe<sub>1-x</sub>Sx and ZnGeP<sub>2</sub>. Its physical origins are identified as presence of twin, large difference between cut and PM angles, self-heating effect at appropriate conditions, mismatched second (Φ) phase matching angle and microdefects.
Model study of not phase matched and phase matched optical rectification or down-conversion of Ti:Sapphire laser pulses at 950 nm into THz and far-IRrange in pure and S-doped GaSe single crystals is carried out. First, the ordinary and extraordinary wave dispersions of the GaSe refractive indices were measured by terahertz time-domain spectroscopy (THz-TDS). Measured data were approximated in the form of Sellmeier dispersion equations for 0.62 – 2000 μm range with using available shorter wave data.
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/cm<sup>2</sup> 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.
Detailed model study of THz generation by CO<sub>2</sub> laser down-conversion in pure and solid solution crystals GaSe<sub>1-x</sub>S<sub>x</sub> is carried out for the first time. Both forward and backward collinear interactions of common (eo-e, oe-e, oe-o, oo-e, ee-o) and original (ee-e, oo-o) types are considered. Possibility of realization, phase matching angles and figure of merits are estimated for line mixing within 9 μm and 10 μm emission bands, as well between them. Dispersion properties of o- and e-wave refractive indices and absorption coefficients for GaSe, GaS and GaSe<sub>1-x</sub>S<sub>x</sub> crystals were preliminary measured by THz-TDS, approximated in the equation form and then used in the study. Estimated results are presented in the form of 3-D figures that are suitable for rapid analyses of DFG parameters. The most efficient type of interaction is eo-o type. Optimally doped (x = 0.09-0.13) GaSe<sub>1-x</sub>S<sub>x</sub> crystals are from 4 to 5 times more efficient at limit pump intensity than not doped GaSe crystals.
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 GaSe<sub>1-x</sub>S<sub>x</sub>, 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<sup>-3</sup> and it is within the range of 10<sup>-2</sup>-10<sup>-3</sup> 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
Er<sup>3+</sup>:YSGG and 10.6μm CO<sub>2</sub> 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.
Experimental and modeling results on CO laser frequency conversion are presented. A Q-switched multiline CO laser
with pulse repetition rate 20-150 Hz of sub-microsecond pulses and electron beam sustained discharge frequency-tunable
mode-locked CO laser were used in the experiments on second harmonic generation (SHG) in high-quality ZnGeP<sub>2</sub> and
GaSe crystals. Internal SHG efficiency exceeded 12.4 % in 12 mm ZnGeP<sub>2</sub> crystal. The SHG in 4 mm GaSe crystal was
observed with internal efficiency of 0.3%. A possibility of difference frequency generation of fundamental and firstovertone
CO laser lines to cover spectral range of ~4.0-5.0 μm is discussed. It is estimated that the difference frequency
generation of neighboring lines of both fundamental and first-overtone bands allows one to obtain oscillation in THz
spectral range within ~200-3000 μm.
Mastering of the middle IR range is attractive for many applications, such as lidar gas analyzers, optoelectronic
countermeasures for suppression of IR detecting, optical communication systems, and scientific and medical instrument.
However, until now this has been held back by the lack of commercial coherent radiation sources with necessary energy
parameters and efficiency. AgGa<sub>1-x</sub>In<sub>x</sub>Se<sub>2</sub> and Hg<sub>1-x</sub>Cd<sub>x</sub>Ga<sub>2</sub>S<sub>4</sub> are well-known middle IR crystal families introduced in
recent years. The main advantage of them is that their physical properties including refractive indices and birefringence
can be engineered by varying the contents of Ga, In, Cd and Hg. Consequently, the phase matching range can be
extended and the 90° non-critical phase matching in three-wave interaction can be realized within a certain wavelength
band. In consideration of influence of composition ratio, acceptable composition ratio and group velocity mismatch of
ultra-short pulses on nonlinear properties of AgGa<sub>1-x</sub>In<sub>x</sub>Se<sub>2</sub> and Hg<sub>1-x</sub>Cd<sub>x</sub>Ga<sub>2</sub>S<sub>4</sub> are investigated for the first time. The
corresponding phase-matching diagrams and spectral dependence of the acceptable composition ratio on wavelengths for
second harmonic generation and optical parametric oscillation pumped by the popular Nd<sup>3+</sup>:YAG (1.06 &mgr;m) and
Ho<sup>3+</sup>:ILF (2.08 &mgr;m) lasers are estimated and demonstrated with accounting of available Sellmeier coefficients. Group
velocity matching for second harmonic, optical parametric generation under the pump of the two lasers in AgGa<sub>1-x</sub>In<sub>x</sub>Se<sub>2</sub>
and Hg<sub>1-x</sub>Cd<sub>x</sub>Ga<sub>2</sub>S<sub>4</sub> are carried out as well. All relative results are compared and analyzed within a number of sampling
values or continuum of composition ratios. In addition, the utilities of AgGa<sub>1-x</sub>In<sub>x</sub>Se<sub>2</sub> and Hg<sub>1-x</sub>Cd<sub>x</sub>Ga<sub>2</sub>S<sub>4</sub> for second
harmonic generation are also discussed finally.
The physical properties of pure GaSe and the crystals doped with 0.01÷3% Al, In, Te, and S have been observed comparatively to reveal the potentials for frequency conversion of laser emission. It has been shown that GaSe:S(greater than or equal to 3%) is the most promising material for practical applications.
LiInSe<sub>2</sub> and mixed LiIn(S <sub>0.5 </sub>Se <sub>0.5 </sub>)2 crystals are grown by vertical Bridgman-Stockbarger method. Optical properties of deep red LiInSe<sub>2</sub>, yellowish LiInS<sub>2</sub>, and orange LiIn(S <sub>0.5 </sub>Se <sub>0.5</sub>)<sub>2</sub> crystals are investigated. Second harmonic generation has been evaluated in LiInSe<sub>2</sub> element 6.3 mm long under pumping by CO<sub>2</sub> laser, 30 ns pulses, and as high external peak power efficiency as 11.7% has been obtained under optimal conditions.