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We report a study of the effect of postgrowth treatment of ZnGeP2 single crystals (low-temperature annealing, irradiation with fast electrons, polishing of working surfaces) and the conditions of exposure to repetitively pulsed laser radiation [wavelength (2091 or 1064 nm), pulse repetition rate, beam diameter, exposure time, sample temperature] on the laser-induced damage threshold (LIDT) of the surfaces of these crystals. It is found that thermal annealing of ZnGeP2 single crystals and their irradiation with a flux of fast electrons, which increase the LIDT at a wavelength of λ = 1064 nm, do not lead to a change in this threshold at λ = 2091 nm. It is shown that ZnGeP2 elements with lower optical losses in the spectral range 0.7 – 2.5 μm have a higher LIDT at λ = 2091 nm both immediately after fabrication and after postgrowth processing. An increase in the threshold energy density of laser radiation by a factor of 1.5 – 3 at λ = 2091 nm is revealed with a decrease in the crystal temperature from zero to –60 °C. The fact of reversible photodarkening of the propagation channel of laser radiation in ZnGeP2 in the predamage region of parameters is established by the method of digital holography.

A method for visualization of volumetric defects in a ZnGeP2 single-crystal by digital holography method using strontium vapor laser radiation is proposed. The possibility of obtaining a volume distribution of defects with dimensions of ≥15–20 μm and their identification in a crystal is shown. The identification in a ZnGeP2 single-crystal of such volumetric defects as growth bands and needle inclusions of zinc phosphides is carried out. Processing of holographic data has resulted in determination of the periodicity of the formation of growth bands, and evaluation of physical causes leading to the appearance of growth bands with a period of ~10–20 µm, ~25–90 µm, and ~200 µm.

Results of experimental investigations of the dispersion of the refractive index n(ν) and of the absorption coefficient α(ν) of a ZnGeP2 crystal in the frequency range 0.3–1 THz are presented. Their comparative analysis is performed. The spectra obtained can be used for real-time diagnostics of the material quality that requires integral characterization. The most informative spectral range in which considerable differences between the absorption coefficients are observed is determined to be 400–800 μm.

A method of visualization of the process of optical damage of a ZnGeP 2 single crystal using digital holography is suggested. The mechanism of the ZnGeP 2 single crystal damage is investigated at energy density ~0.3–0.6 J/cm ² . The temperature in the damage channel during its formation is estimated. The data obtained allow the conclusion on the thermal nature of ZnGeP 2 crystal damage to be made under the action of laser radiation with a wavelength of ~2.1 μm. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.

We have experimentally realised the average generation power of a parametric generator based on a ZnGeP2 single crystal, which constituted ~1.5 W at an efficiency of ~28.6 % in the wavelength range of 3.5 – 4.8 μm with an average pump radiation power of ~5.5 W and its energy density of ~ 0.47 J cm−2. It is shown that there are threshold conditions for the average pump power, providing the realisation of parametric generation. In this case, an increase in the generation efficiency with an increase in the energy density of pump radiation to ~ 0.4 J cm−2 is observed; with its further increase, the output power increases due to a decrease in the parametric generation threshold. Parametric generators (PGs) based on ZnGeP2 single crystals with such characteristics are promising for solving many applied problems, including generation of terahertz radiation when nonlinear crystals are pumped by radiation from these PGs at difference frequency.

Results of investigations are presented of the THz spectra of the refractive index n(v) and of the extinction coefficient k(v) for ZnGeP2 single crystals with different degrees of technological treatment – after growth by the vertical Bridgman technique, after heat treatment at temperatures above the Debye temperature for the highest-frequency phonons, and after modification of the single-crystal properties under irradiation by fast electrons (~4 MeV).

An optical layout of a two-cascade frequency converter of the mid-IR laser radiation into the terahertz (THz) radiation is proposed. In the first stage it is assumed to convert the Tm:YLF-laser frequency in a Cr+2:ZnSe polycrystal into the radiation with the wavelength ~2–3 μm. The second cascade can be presented as a parametric conversion of the frequencies of two laser sources operating in the ~2–3 μm range into the THz radiation via the difference-frequency mixing in a nonlinear optical ZnGeP2 crystal. The estimates of the terahertz output signal are reported.

In this work, the method of IR digital holography intended for detection of volumetric defects in ZnGeP2 single crystals has been tested. A method of visualization of the process of optical damage of a ZnGeP2 single crystal using digital holography is suggested.

Volume filamentary inclusions in ZnGeP2 have been visualized using the digital holography method. The chemical composition of the filamentous volume inclusions Zn3P2 and Ge have been determined using the method of X-ray diffraction analysis. It is shown that the presence of volume inclusions in the ZnGeP2 single-crystal leads to an increase in the reflection coefficient in the region of 12.5 μm. The dispersion dependences of the refractive index and the absorption coefficient of the studied ZnGeP2 samples at wavelengths of 300–1000 μm in the region of non-fundamental absorption have been obtained. It has been shown that the presence of volume inclusions in a single-crystal leads to an increase in the refractive index in the entire THz range under study by the value of Δnmax = 0,0008. The difference between absorption coefficients for the studied samples varies from Δα = 0.1 cm−1 to Δα = 0.15 cm−1 depending on the wavelength.

The reflection and transmission spectra of ZnGeP2 single crystals irradiated with 4-MeV electrons have been measured in the frequency range of 5-5000 cm−1 at temperatures in the range of 10-300 K. Based on the measured spectra, the spectra of complex permittivity ɛ*(ν) and absorption coefficient α(ν) have been simulated using the dispersion analysis method. It has been found that the electron irradiation decreasing the losses in the pumping range by a factor of 2–3 does not lead to additional losses in the region of generating terahertz radiation.

Optical scheme of three-stage converter of middle IR range laser radiation to the terahertz (THz) radiation of range is proposed. In the first cascade the 2-micron emission in Ho:YAG laser is generated. In the second cascade it is supposed using frequency conversion of radiation of the Ho:YAG laser in ZnGeP 2 single crystal to radiation with a wavelength of ~ 4-4.5 μm, and in the second cascade it is expecting of using parametric frequency conversion of the two parametric sources ~ 4-4.5 μm range to the THz range by generating radiation of difference frequency in the nonlinear optical crystal ZnGeP 2 . Estimation of the characteristics of the output terahertz radiation is given in this paper.

Structural defects were studied in the nonlinear optical material ZnGeP 2 (ZGP) grown by the vertical Bridgman technique from the melt. Progress in ZGP growth with sufficiently perfect structure allowed us observe the Borrmann effect and to apply X-ray transmission topography method based on this effect for the first time for ZGP. Types of defects were defined by comparison of the experimental and simulated topographs. In as-grown ZGP crystals the topography found growth striation, various dislocations, coherent and semi-coherent particles of second phases, inclusions, in the initial part of the crystal with compositions lying along ZGP–GeP 2 pseudobinary cut. Inclusions forming solute trails in the end part of ZGP ingots with compositions lying along the ZGP–ZnP 2 pseudobinary cut. The dislocation density changed from N d ~3.5×10 3 cm −2 to ~2.5×10 2 cm −2 from the onset to the end of the ingot, respectively. FWHM of rocking curves for as-grown ZGP ranger from 13″ to 35″. A high optical absorption coefficient in the 0.65–2.5 µm spectral region, always existing in as-grown ZGP, can be reduced by thermal annealing and electron irradiation down to ~0.02 cm −1 at 2.06 µm.

Physical mechanisms of formation of radiation losses in the terahertz range in ZnGeP2 crystals have been experimentally studied in the wave number range of 5–350 cm−1 at temperatures of 10–300 K. The dominant contribution of two-phonon difference processes to the loss formation in the given frequency range has been shown.

Spatial inhomogeneity of the ZnGeP2 single crystal grown in the new installation that allows thermal conditions in the working volume and pressure of the vapor phase to be changed and forced convection in the melt to be created is investigated. It is demonstrated that continuous correction of the crystallization temperature regime allows the crystal growth rate to be stabilized and its deviation from the nominal velocity of growth container motion to be decreased thereby decreasing the spatial inhomogeneity of the growing crystal.

Possible parametric oscillation of 3-THz pulse at synchronous pumping of the ZnGeP2 crystal by a train of short second-harmonic pulses from the CO2 laser has been analyzed. Calculation shows that at changing laser pulse duration τ between 4 and 500 ps and correspondingly pumping energy density (0.5–3.5 J cm−2) THz pulse peak power varies from 3 to 70MW with maximum at τ =9 ps.

An optical system for conversion of mid-infrared laser radiation into terahertz radiation is proposed. The proposed system uses lasing with a wavelength of 2 μm in a holmium:yttrium–aluminum–garnet (Ho:YAG) laser in the first stage of the system, conversion of the Ho:YAG laser radiation in two parametric light sources based on ZnGeP2 single-crystals with fixed and tunable wavelengths in the range of 3–4.5 μm in the second stage, and conversion of the radiation of these two parametric sources in the 3–4.5-μm region into the terahertz range by generating difference-frequency radiation in the nonlinear ZnGeP2 optical crystal in the third stage. We estimate the characteristics of the output terahertz radiation.

A defect structure of nonlinear optical material ZnGeP 2 , grown by the vertical Bridgman technique from the melt, was studied. The state-of-the-art results in ZnGeP 2 growth with sufficiently perfect structure allow one to register the presence of Borrmann effect and to apply the X-ray topography method based on this effect for the first time. Microscopy and X-ray transmission topography based on the Borrmann effect revealed growth striation, precipitates, forming lineage structures along the growth axis, dislocations and unknown linear defects, which should be more elaborately studied in future. The observed defects are formed because of deviation from ZnGeP 2 stoichiometry during synthesis and growth and unfavorable thermal conditions during growth. The precipitates are observed only with significant deviations from stoichiometry. The width of rocking curves for the as-grown crystals is 13–35 seconds of arc, which shows a good structural perfection, in spite of the revealed defects. The thermal annealing and electron beam irradiation decrease the optical absorption coefficient at 2.06 μm to 0.02 cm −1 .

Analysis is given for a possibility of singly resonant parametric oscillation in the submillimeter range at synchronous pumping of the ZnGeP2 crystal by a train of 100-ps second-harmonic pulses from the CO2 laser with the radiation energy 1.0 J. The calculation shows that using the ZnGeP2 crystal and the second harmonic of the CO2 laser with the energy density 1.8 J cm−2, one can get the peak submillimeter radiation power from 3.6 to 12 MW in the range from 95 to 300 µm (1.0–3.3 THz). The expected peak power values are larger than the experimental ones obtained by other nonlinear optics methods.

We describe a modified two-temperature process for reproducible high-volume (up to 500 g) synthesis of the nonlinear-optical semiconductor ZnGeP2, which enables the preparation of nominally stoichiometric material. The major reaction intermediates in the two-temperature ZnGeP2 synthesis are ZnP2, Zn3P2, GeP, and Ge. Using x-ray diffraction, we refined the interplanar spacings in the tetragonal structure of ZnGeP2 and fcc structure of GeP and indexed peaks missing in the PDF cards 33-1471 (ZnGeP2) and 21-353 (GeP).

For the first time for a ZnGeP2 crystal, the three-wave interaction of picosecond pulses was considered as the difference frequency generation. The group velocities and absorption of radiation were taken into account in performing a spectral analysis. The calculations of a radiation pulse were made for the following parameters of a CO2 laser: 100-1.25 ps and 15-1100 GW cm-2. At the 804.5 µm wavelength of difference frequency, the spectral characteristics have been obtained and the pulse power has been found as a function of crystal length. The half-cycle pulse generation is possible at a particular crystal length.

A real defect structure is studied for ZnGeP 2 crystals grown by the vertical Bridgman technique from the melt. Optical microscopy reveals growth striations and precipitation of second phases emerging because of deviation from stoichiometry. The revealed growth striae have a complex structure caused by temperature oscillations in the growth setup. The other type of defect, precipitates, is observed as a colony of trails orienting in the longitudinal direction. The colony of trails occurs with a concave interface and as a rule, is located in the central part of the lateral section of crystal. The most probable composition of the precipitates is a mixture of ZnGeP 2 , Zn 3 P 2 , and Ge. The colony of the precipitate trails results in significant optical losses over the whole transparency range of ZnGeP 2 .

High-yield two-temperature synthesis and subsequent growth of ZnGeP 2 crystals are investigated. By X-ray phase analysis it has been demonstrated that two-temperature synthesis of ZnGeP 2 has been achieved using binary zinc and germanium phosphides formed at a Zn–Ge mixture temperature of about 900°C and a P pressure of 7–10 atm. Using heat balance equation a ratio of thermal conductivities in solid and liquid ZnGeP 2 near the melting point has been determined. The value of the determined ratio is K L / K S ≅2.3. Analysis of the most favorable crystallographic directions for ZnGeP 2 growth has been performed. These directions are [1 1 6], [1 3 2], [1 1 0] and [1 0 2]. Data for optical absorption of as-grown and annealed ZnGeP 2 crystals have also been presented and discussed.

The two-temperature technique has been examined for ZnGeP2 synthesis. The effect of the cold zone temperature on the Zn diffusion and the ZnP2 formation outside the ternary compound reaction zone has been studied. A qualitative analysis of the Zn diffusion equation taking into account the P mass transport has allowed us to identify conditions where no Zn diffusion is observed. The effect of the ampoule surroundings on the ZnGeP2 crystal growth has been investigated for the vertical Bridgman and vertical gradient freezing methods. The steady-state thermal conductivity equation has been solved for different ampoule surroundings. The proposed numerical model has established the reasons for the polycrystalline structure and physical properties heterogeneity of ZnGeP2 crystals, viz. the isotherm concavity and variable growth rate with respect to the ampoule translation speed.

a pulsed radiation power of the order of 1W in the submillimeter range (102 -110 μm) was reached for the first time, with the aid of a ZnGeP 2 crystal, at the difference frequency of CO 2 laser radiations.

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.

Summary form only given. For mid IR region ZnGeP/sub 2/ crystals have excellent potential for application in nonlinear optics due to unique combination of physical properties. In 1987 second harmonics generation (SHG) efficiency of 49% was realized with this crystal pumped by 2-ns pulses of CO/sub 2/-laser with intensity /spl sim/ 1GW/cm/sup 2/. In the present work SHG was investigated with the ZnGeP/sub 2/ crystals pumped by CO/sub 2/-laser pulses with much higher radiation intensities. To prevent the crystal surface optical damage we applied the significantly shorten laser pulse duration as compared with known data. In our experiments CO/sub 2/-laser system "Picasso-2" was used generating the train of pulses with duration 20 - 100 ps and total energy up to 15 J. The system provided laser intensity up to 100 GW/cm/sub 2/ at the crystal samples surface /spl sim/15 /spl times/15 mm/sup 2/. The work was supported by the Russian Fund for Basic Research (grant #03-02-16094).

Thermodynamic analysis of the vapor phase over ZnGeP2 in Zn-Ge-P-Cl system has been carried out. The analysis showed that this system can be used for the vapor growth of ZnGeP2. Homoepitaxial layers of ZnGeP2 were grown in a closed system using chemical vapor transport. Electrical and photoluminescence properties of the layers were studied, and crystal lattice parameters were measured. Comparison of properties for bulk and vapor grown ZnGeP2 crystals were carried out. It was found that the vapor grown crystals have more perfect structure than the bulk ones, particularly, they have significantly lower vacancy concentration.

In 7.7 divided by 39.6 cm 1 range investigated for the first time refraction and absorption spectra of AgGaSe 2 and AgGa 1- x In x Se 2 crystals allowed to compute a phase matching characteristic and efficiency values of difference frequency generation with total laser intensity from 60 MW/cm 2 up to 15 GW/cm 2 . The maximum efficiency values were close to 1*10 -3 . Powerful half-cycle pulse generation is proposed with the investigated crystals.

Single crystals (beta) -BaB2O4 (BBO) are new perspective nonlinear-optical material used for development of laser sources tunable in wide spectral range from near IR to UV. It is well known that BBO single crystals cannot be grown from their own melt because of formation of (alpha) - phase of BaB2O4 at melting point of 1095 degree(s)C. Therefore the solution-melt (flux) technique is used for growth of low-temperature (beta) -BaB2O4 phase (temperature of (alpha) yields (beta) phase transition is 925 degree(s)C). This method is applied still quite seldom because of high labor-consuming. BBO single crystals were grown by top-seeded solution growth technique with high temperature gradient with simultaneous decrease of temperature. The main difficulties of the growth of BBO single crystals are related to high viscosity, tendency to stratifying and vitreous formation, incongruent evaporation of solution-melt, and possibility of simultaneous co- crystallization both (alpha) - and (beta) -phases from the supercooled solution-melt. Any of them result in formation of various growth defects with concentrations depended on peculiarities of technological conditions.

This paper describes detailed analysis of optical and dielectric properties of ZnGeP 2 crystals in middle IR and FIR spectral ranges, and also first experimental results on 1 W level 102 - 110 micrometers emission generation by means of difference frequency generation of CO 2 laser line pairs. It is shown that MW level is achievable now, and AgIn 0.35 Ga 0.65 Se 2 crystal can be used for efficient generation of FIR emission.

Infrared tunability of parametric superluminescence is studied in ZnGeP2. Q-switched and actively mode-locked Er3+:YSGG laser (2.79 micrometers ) is used to investigate the temperature tunability. Angular tunable wavelength gap near degeneracy in type II phase matching is partially covered by crystal temperature rise thereby eliminating beam walk-off/displacement unavoidable in angular tuning. The result is explained by changes in crystal phonon absorption, birefringence and refractive indices with rise in temperature.

a pulsed radiation power of the order of 1W in the submillimeter range (102–110 µm) was reached for the first time, with the aid of a ZnGeP2 crystal, at the difference frequency of CO2 laser radiations.

The creation of a high-intensity source in the 4.6–5.4-µm range is a problem that has attracted the attention of researchers for several decades. This paper describes the optimization of the process of generating the second harmonic of the radiation of a pulse-periodic TEA CO2 laser by choosing a nonlinear crystal and the temporal shape of the laser-pump pulse. Energy-conversion efficiency of up to 20% has been experimentally obtained in a ZnGeP2 crystal with mean power of the radiation in the λ=4.8µm region of up to 0.24 W and a pulse repetition rate of 10 Hz.

In an attempt to bring the high power and versatility of carbon dioxide lasers to bear on atmospheric measurements for pollutants such as carbon monoxide, nitrogen oxides, and sulfur dioxide, the authors propose parametric frequency converters for carbon dioxide laser radiation in the spectral range required. Field measurements are discussed of the densities of a number of atmospheric gas components, including CO, and of the transparency of the atmosphere to beam paths using a beam-path gas analyzer containing a frequency doubler for a carbon dioxide laser based on the zinc phosphide-germanium phosphide monocrystal. The KAMAK MERA-60 computer system 23 does programmed tuning of the carbon dioxide laser radiation wavelength by rotation of a diffraction grating.

Results from a study of electrophysical and luminescent properties of zinc-germanium diphosphide single crystals, diffusion doped with copper, are presented. The nature of the dominant defects formed upon copper diffusion in ZnGeP2 is determined using a thermodynamic analysis of defect formation processes in ZnGeP2 and a ZnGeP2: Cu solid solution performed within the framework of the quasichemical analysis method. It is demonstrated that by choosing the copper diffusion method the hole concentration in the ZnGeP2 can be varied over the range 1012−1016 cm−3. A retrograde character was observed in copper solubility in ZnGeP2 compounds. Defect formation processes in ZnGeP2 upon copper diffusion depend on the degree of atomic ordering in the cation sublattice at the diffusion annealing temperature.

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%.