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We analyse the functional capabilities of new crystals, BaGa2GeS6 (BGGS) and BaGa2GeSe6 (BGGSe), which are used for nonlinear optical frequency conversion in their transparency range. The wavelengths at which maximum conversion efficiencies can be obtained and the tuning range for difference-frequency generation are found. It is shown that there are wavelength combinations at which the effective nonlinearity coefficient varies only slightly in a wide frequency band.

The paper reports the efficient terahertz generation using nonlinear optical rectification process and comparative studies of optical, dielectric, and surface scattering properties of 1.0 mm thin GaSe: S crystal in transmission/reflection modes. We employed 800 nm wavelength of 140 fs and 50 fs pulse durations obtained from Ti: Sapphire oscillator and amplifier systems at 80 MHz and 1 kHz repetition rates, respectively, and evaluated the THz generation potential and optical properties such as refractive index, absorption coefficients, relative permittivity in transmission mode. In the next step, the crystal was subjected to a commercial THz system of 0.3 picosecond pulses at 100 MHz repetition rate in reflection geometry and once again recorded the similar parameters and compared with the transmission mode data. In addition, these data were used to measure the scattering losses from the surface of the crystal in terms of surface roughness, optical impedance, and Fresnel’s reflection coefficients. The maximum generated THz power was of the order of 4.5 μW with conversion efficiency (η) ∼ 2.2 × 10−3%. The measured THz efficiency was found equivalent to 20 mm thick GaSe crystal used in the difference frequency mixing technique. Finally, we have measured the Rayleigh roughness factor (g) of the crystal surface at different THz frequencies. When g < 1 (for small roughness) the Fresnel reflection loss factor becomes 0.75 at 1.4 THz frequency range which helps to enhance the generated signal.

We present the thorough studies of optical properties of BiB3O6 (BIBO) crystal in the millimeter-wave (subterahertz) range. We observe a large birefringence Δn = nZ -nX = 1.5 and the values of absorption coefficients of all three axes to be less than 0.5 cm−1 at the frequency of 0.3 THz. The difference from visible range in angle ϕ between the dielectric axis z and crystallophysical axis X is found to be more than 6°. The simulated phase-matching curves in the xz plane of the crystal show the optimal value of the angle θ to be around 25.5°±1° for an efficient millimeter-wave generation under the pump of 1064 nm laser radiation.

We report the method and results of a complex analysis of phase-matching and nonlinear properties for biaxial crystals of all point groups of symmetry, which determine their functional possibilities in solving various problems of nonlinear frequency conversion of laser radiation.

A two-cascade frequency conversion of CO-laser radiation is demonstrated in a single sample of a nonlinear ZnGeP2 crystal. The crystal is pumped by a repetitively pulsed cryogenic lowpressure CO laser operating on ~150 vibration – rotational transitions in the wavelength range 5.0 – 7.5 μm, which corresponds to the frequency range of a half octave. In the first conversion cascade, generation of second harmonic and sum frequencies of various pairs of CO-laser radiation give ~350 emission lines in the wavelength range 2.5 – 3.7 μm. In the second cascade, by mixing the radiation converted in the first cascade with the residual radiation of the CO laser we have obtained ~90 lines in the range 4.3 – 5.0 μm and more than 80 lines in the range 7.5 – 8.3 μm. Thus, using a single sample of the nonlinear ZnGeP2 crystal pumped by the radiation of a single CO laser we have produced a source of broadband (more than one and a half octaves) laser radiation, simultaneously operating at ~670 lines in the wavelength range 2.5 – 8.3 μm.

Frequency doubling of mode-locked frequency-tunable CO laser emission in high quality 17-mm long ZnGeP2 nonlinear optical crystal was studied. The internal efficiency of frequency doubling exceeded 25%, which was obtained by stabilization of mode-locking regime and increasing peak power of pump radiation in the scheme ``master oscillator – power amplifier''. A feasibility of difference frequency conversion of both fundamental and first overtone CO laser lines to cover ~ 4.0 – 5.0 μm spectral range and a possibility of getting spectral range of ~ 1.25 – 10 μm by mixing of all CO laser lines were estimated.

Dispersion of refractive index and absorption coefficients in flux-grown high-resistivity KTiOPO4 crystals between 0.2–2.5 THz are verified at room temperature by a THz-TDS. Measured dispersion components n x , n y and n z are approximated for the first time in the form of Sellmeier equations. Phase matching for down-conversion into the THz range under a visible and near IR pump is found possible only in the principle plane by and types of three-wave interactions. Low frequency THz generation is favorable due to the low absorption coefficient down to 0.2 cm−1, below 0.5 THz.

A differential absorption lidar based on a tunable TEA CO2 laser emitting at 42 lines of the 'hot' 0111 — 1110 band in the range from 10.9 to 11.4 μm is developed for detecting natural gas leakages from oil pipelines by measuring the ethane content in the atmosphere. The ethane detection sensitivity is 0.9 ppm km. The presence of methane does not distort the measurement results. The developed lidar can detect the natural gas leakage from kilometre heights at the flying velocities up to 200 km h-1 and a probe pulse repetition rate of 5 Hz.

Lithium triborate LiB3O5 (LBO) crystals are widely used for frequency conversion of the near-IR lasers within main transparency windows. Their optical properties at these wavelengths are well studied. However, very little work has been published on the properties in the terahertz (THz) range. There was a lack of data on the refractive indices, the absorption coefficients spectra and their temperature dispersions. There are no reports of THz applications. Present work reveals all these topics including the prospects for use LBO crystals as down-converters of the near-IR lasers radiation. Optically finished samples of flux-grown LBO crystals were studied by THz-TDS. The refractive index dispersions were recorded and then approximated in the form of Sellmeier equations for the temperatures of 300 and 81 K. The phase-matching curves for the IR-THz and THz-THz frequency conversions were calculated. It was found that the absorption coefficients of LBO decrease significantly with cooling to cryogenic temperatures, but the overall character of optical properties changes is intricated. Experimental results are discussed in detail considering potential characteristics of THz down-converters.

The anisotropy of optical properties of high quality beta barium borate crystal (β-BaB2O4, β-BBO) was studied in the main transparency window by using classic spectroscopic methods and in the range of 0.2 – 2 THz by using THz time-domain spectroscopy. β-BBO crystals were grown by the top-seeded solution technique in a highly resistive furnace with a heat field of 3-fold axis symmetry. At room temperature (RT), absorption coefficient in the maximal transparency window in grown crystals did not exceed 0.05 cm-1. Strong absorption anisotropy was observed in 3 – 5 μm and the THz range. At 1 THz absorption coefficients for e and o wave were, respectively, 7 cm-1 and 21 cm-1 at RT; 2 cm-1 and 10 cm-1 at 81 K. At the most attractive for out-of-door applications range < 0.4 THz the absorption coefficient is found to be very low: below 0.2 cm-1 at RT and 1 cm-1 at 81 K. Refractive indices dispersions measured by THz-TDS were approximated in the form of Sellmeier equations. Birefringence is found quite large for phase matched difference frequency generation (DFG) or down-conversion into the THz range (THz-DFG) under near IR pump at RT and 81 K. Type II (oe-o and eo-o), and type I (ee-e) three wave interactions can be realized at RT. THz-DFG of Nd:YAG laser and KTP OPO can be realized by type II (oe-o) three-wave interaction. For selected spectral ranges of femtosecond Ti:Sapphire laser efficient phase matched and group velocity matched optical rectification can be realized by another two types of three wave interactions. Accounting other well-known attractive physical properties of β-BBO crystal, wide application in THz technique can be forecasted.

The results of radiation resistance measurements for twelve nonlinear crystals are presented. The crystals include the well-known nonlinear CdGeAs2, ZnGeP2, AgGaSe2, GaSe, AgGaS2, and Ag3AsS3 crystals operating in the middle IR range, new mixed AgGaGeS4 and Cd0.35Hg0.65Ga2S4 crystals, two-phase (orange and yellow) HgGa2S4 crystal, and the doped GaSe:In crystal. The mixed crystals and the two-phase HgGa2S4 crystal are transparent in the range from 0.4 — 0.5 to 11.5 — 14.5 μm. The measurements were performed using a pulsed single-mode highly stability TEA CO2 laser with an output pulse duration of ~30 ns. The damage thresholds of new nonlinear AgGaGeS4 and Cd 0.35Hg0.65Ga2S4 crystals and of the HgGa2S4 crystal (the orange and yellow phases) were found to be 1.5 — 2.2 times higher than for the crystals operating in the middle IR range.

The optical properties of a new promising nonlinear mixed AgGaxIn1-xSe2 crystal were investigated. Its absorption coefficients and refractive indices were determined at wavelengths 0.8 — 19 μm. For the first time the coefficients of the Sellmeier dispersion equations were determined for different indium contents. The corresponding phase-matching diagrams for second-harmonic generation and an optical parametric oscillator pumped by a Er3+:YAG laser were calculated and plotted. An experiment on the generation of the second harmonic of a TEA CO2 laser and a Q-switched laser was carried out and a frequency doubling efficiency of 10.9% was obtained. AgGaSe2 and ZnGeP2 crystals were also investigated for comparison.

Optical properties of nonlinear PbIn6Te10 (PIT) crystal were studied for the first time within 30–1200 µm (0.2–15 THz) by FTIR and THz-TDS. It was found that PIT possesses the most attractive transparency features for efficient generation of long-wave mid-IR emission including 20–60 µm (5–15 THz) region by optical rectification. This crystal is also found useful for phase matched DFG generation into the long-wave THz region. In particular, by model study phase matched down-conversion of CO and CO2 laser lines into 130–1200 µm (0.3–2.3 THz) range is shown possible by ooe and eoe type of three wave interaction. Noncritical 90° phase matching for eoe type of interactions can be realized by selecting samples with specified composition.

A pulsed TEA CO2 laser was used in an investigation of the influence of the pump radiation parameters (mode composition, wavelength, pulse duration), of the focusing conditions, of the properties of the material (absorption coefficient), and of the operating conditions (temperature) on the efficiency of conversion to the second harmonic and on the angular dependences of phase matching in ZnGeP2 crystals. The calculated results were found to be in good agreement with the experimental data.

High quality lithium borate (LBO) samples cut along (1 0 0), (0 1 0) and (0 0 1) axes were studied by terahertz time-domain spectroscopy (THz TDS) between 0.2–3 THz. It was found that in the direction of crystallographic axis X the optical absorption coefficient is the lowest amongst all known anisotropic nonlinear crystals, and that birefringence is as large as 0.42. Dispersion equations for the entire transparency range of LBO were developed for the first time. Phase matching for down-conversion into the THz range was found to be possible. Phase matching availability, low optical loss in the transparency band, and high optical damage threshold make LBO one of the most promising nonlinear materials for THz generation.

A second harmonic generation model of space-time thermo-optic coupling was proposed in thermal anisotropic nonlinear crystals, where an iterative approach was used to approximate the temperature anisotropy. The coupling process was simulated with the aid of derived analytical formulas in a gallium selenide crystal, pumped by a pulsed carbon dioxide laser. The intensities of the fundamental and harmonic beams, conversion efficiencies and temperature distributions were demonstrated during the evolution processes. The results showed that a compensation for the phase mismatch induced by the thermal effect can mitigate the reduction of conversion efficiency better than temperature control.

High optical quality undoped and sulfur-doped gallium selenide crystals were grown from melts by the modified vertical Bridgman method. Detailed study of the damage produced under femtosecond pulse exposure has shown that evaluation of the damage threshold by visual control is unfounded. Black matter spots produced on crystal surfaces do not noticeably decrease either its transparency or its frequency conversion efficiency as opposed to real damage identified as caked well-cohesive gallium structures. For the first time it was demonstrated that optimally sulfur-doped gallium selenide crystal possesses the highest resistivity to optical emission (about four times higher in comparison with undoped gallium selenide).

The linear and nonlinear optical properties and the radiation resistance of a LiInS2 single crystal are studied. Phase matching conditions are calculated for the first time for a LiInS2 crystal.

We have succeeded in generating the second harmonic of the radiation from a DF laser for the first time, using single crystals of ZnGeP2. For crystals with lengths of 10.1 and 13.6 mm, the overall external efficiencies of the entire oscillator system were 4 and 6.2%. The internal efficiencies of second-harmonic generation in the crystals were 7.6 and 11.8%, respectively.

The second harmonic of radiation from a molecular NH3 laser was generated for the first time in CdGeAs2 single crystals. The internal efficiency of the frequency doubling reached 5% for radiation at the wavelength of 11.7 μ, when the efficiency of conversion of CO2 laser radiation power (used for direct optical pumping of the NH3 laser) into 6-μ radiation was highest.

The frequencies of CO2 and CO lasers were summed in a ZnGeP2 crystal as a result of a type I three-frequency interaction: the phase-matching angle was θpm =56° and the angular width of the phase-matching curve was 2°50'. The output power of the sum-frequency radiation was ~0.6 mW.

The second harmonic of CO2 laser radiation was generated for the first time in ZnGeP2 single crystals under phase-matching conditions when the pump (fundamental-frequency) radiation was tuned within the ranges 9.19–9.7 and 10.15–10.8 μ. A study was made of the spectral, angular, and temperature widths of the phase-matching curve, of the dependence of the efficiency of conversion to the second harmonic on the pump wavelength and temperature of a crystal, and of the rate of reduction in the phase-matching angle as a result of heating.

Frequency conversion of nontraditional CO2 laser radiation bands was performed for the first time. The second harmonic of the 4.3μ (1001–1000) band was generated and this was summed with the second sequence band at 10.4μ (0002-1001). The phase matching angles were 55°50' and 54°20', respectively. The spectral and angular phase-matching bandwidths were investigated. The efficiency of doubling of the P(26) line in the 4.3μ band by conversion in a ZnGeP single crystal exceeded 10%.

A study was made of second-harmonic generation in a high-quality ZnGeP2 crystal (absorption coefficient 0.1 cm−1 ) . The second-harmonic power ranged from 2.45 to 4 mW when the CO laser power was 81.25–194 mW. The maximum external efficiency of second-harmonic generation reached 3%.

Efficient (49 ± 5%) second harmonic generation was demonstrated for the first time in the infrared. The frequency of CO2 laser radiation with a pulse duration of ~2 nsec and an intensity of up to 1GW/cm2 was doubled in an unbleached ZnGeP2 crystal. The internal energy efficiency was ~80%.

An experimental investigation was made of cascade generation of the fourth harmonic of CO2 laser radiation and of the second harmonic of CO laser radiation in single crystals of zinc germanium diphosphide. The radiation was converted to the spectral range 2.3–3.1 μ. The overall efficiency of conversion of CO2 laser radiation to the fourth harmonic was ~ 0.1 %, but it should be possible to increase it to 20%. The efficiency of conversion of CO laser radiation to the second harmonic was (2.5–10)×10−3%.