A series of single crystals of solid solutions KTi1-xZrxOAsO4 ( x = 0.025, 0.05, 0.075, 0.1) have been grown by the Czochralski method. The structural analysis of this series of samples showed that at titanium partial substitution (0.025 <= x <= 0.1) Zr4+ occupies the T1 position of the Ti1O6 octahedron, as well as the T2 position of the Ti2O6 octahedron, at the same time the volume of octahedra increases. But at 0.05 <= x <= 0.1 the volume of the Ti1O6 octahedron stops growing, and only the volume of Ti2O6 increases, consequently, at x = 0.05 Zr saturation is observed at position Ti1. The Raman spectrum of a pure KTA crystal is very different from the spectra of the entire range of solid solutions. In the transmission spectrum of samples with partial substitution of titanium atoms by zirconium atoms, there is practically no wide absorption band at 3.5-4 mu m typical for pure KTA, the maximum crystal transparency in this region is achieved for x = 0.075. It was found that the introduction of large Zr ions into the KTiOAsO4 structure leads to a distortion of the lattice and an increase of the band gap.

The performance of infrared (IR) nonlinear optical (NLO) materials is significantly affected by the thermal conductivity kL, but studies on the structure and property relationship of kL in these materials are very rare. In this work we evaluated the kL in IR NLO multicomponent chalcogenides LixAg1-xGayIn1-ySe2 with a smooth change in the compositions x and y by using a machine learning approach and laser flash measurements, combined with available experimental results. The found patterns of kL dependence on the atomic mass, bond length and electronegativity provide an effective understanding for navigation in the process of searching for new chalcogenide crystals with an optimal set of parameters that allow them to be effectively used as a frequency converter of laser radiation in the IR range. Moreover, the compositions Li0.5Ag0.5GaSe2, Li0.81Ag0.19InSe2 and AgGa0.5In0.5Se2 are demonstrated to exhibit a balanced combination of the parameters kL, NLO effects, energy band gaps, and birefringence for IR NLO applications.

Efficient near-infrared (NIR) optical devices based on non-linear crystals are demanded for diverse applications ranging from bio-imaging and diagnostic tools to optical parametric conversion. However, significant reflection energy losses, caused by high refractive index of the crystal material, limit their performance. Here, based on example of promising BaGa4Se7 nonlinear crystals we demonstrated simple, high-performing and single-step approach to fabricate anti-reflective structures onto their surfaces through direct femtosecond-laser multi-pulse processing. We found that under multi-pulse exposure laser-induced periodic surface structures (LIPSS) with nanotrenches period around 380 ± 10 nm and orientation perpendicular to the irradiating laser beam polarization can be created. The minimal structural damage of the laser-patterned crystals was justified by combining transmission electron microscopy with Raman/X-ray diffraction spectroscopies. Fourier-transform infrared spectroscopy was revealed 1.12-fold transmission increase within the 1.3–2 µm spectral range for LIPSS-textured BaGa4Se7 crystal as compared to the untreated one. The proposed method to enhance the efficiency of BaGa4Se7 non-linear optical crystal in the practically relevant NIR spectral range holds significant potential for non-linear optical applications.

Rb2KWO3F3 elpasolite was synthesized via the solid-state reaction route. The phase purity of the obtained sample was verified by the XRD analysis with Rietveld refinement in space group Fm-3m, yielding the unit cell parameter a = 8.92413 (17) Å. The electronic structure and chemical states of the constituent elements were investigated using X-ray photoelectron spectroscopy. The binding energy of the W 4f7/2 core level (34.95 eV) was found to be characteristic of the W6+ oxidation state, while the values for Rb 3d, K 2p, O 1s and F 1s levels were consistent with those reported for related oxide and oxyfluoride compounds. First-principles density functional theory calculations were performed to model the electronic structure. The fac-configuration of the WO3F3 octahedra was identified as the most energetically favorable. The calculations revealed a direct band gap of 4.38 eV, with the valence band maximum composed primarily of O 2p orbitals and the conduction band minimum formed by W 5d orbitals. This combined experimental/theoretical study shows that the electronic structure and wide bandgap of Rb2KWO3F3 are governed by the WO3F3 units and are largely insensitive to the Rb/K substitution. The wide bandgap identifies this class of oxyfluorides as a promising platform for developing new UV-transparent materials.

Large GaSe crystals were grown and various antireflection microstructures (ARMs) were fabricated on their cleaved surfaces using optimized femtosecond laser ablation, which provided the antireflection effect in a wide wavelength range of 4-16 mu m. The influence of ARMs created on the GaSe surface on the change of the laser-induced damage threshold (LIDT) of the crystal at a wavelength of 5 mu m was evaluated. The 5-mu m Fe:ZnMgSe laser with the pulse duration of 135 ns was used for the LIDT test in conditions close to single pulse exposure. The measured values of LIDT of 56 +/- 6 MW/cm2 and 51 +/- 9 MW/cm2 for two GaSe substrates, respectively, were comparable with the known data of single pulse LIDT of GaSe. The average LIDT intensities of 54 +/- 6 MW/cm2 and 52 +/- 7 MW/cm2 for the ARMs at two GaSe plates, respectively, were close to LIDT intensities for the corresponding GaSe substrates. The ARMs with lower structural quality had lower LIDT (50-52 MW/cm2) in comparison with the high-quality ARMs (58-60 MW/cm2). High LIDT for high-quality ARMs can be caused by increased selenium content in the ARMs. In any case, all the tested ARMs on the GaSe plates with different surface quality are workable for development of widely tunable mid-infrared nonlinear optical converters.

The paper demonstrates the experimental implementation of a broadband picosecond radiation source with a wavelength of 2.0−2.2μm based on a cascade parametric amplifier. To do this, in the first cascade, as a result of parametric luminescence in the MgO:PPLN fan-out crystal, pumped by Nd:YAG laser pulses (90 ps, 10 Hz), radiation with a wavelength of 2.1μm and a pulse energy of ∼185 μJ at a pump energy of 6.5 mJ was generated. To increase the output radiation energy to a level of ∼840 μJ, a second parametric amplification cascade based on two KTiOPO4 crystals with a walk-off angle compensation was used. The results of numerical simulation of the spatial and energy characteristics of the radiation being generated are in good agreement with the experimental results. The developed picosecond 2.1μm radiation source is designed to be used for mid-IR range supercontinuum generation in non-oxide crystals.

Phase transitions can change the crystal structure and modify the physical properties of crystals. In this work, we investigate the phase transition behavior in BaGa4Se7, an important middle infrared (mid-IR) nonlinear optical (NLO) crystal, in the temperature range from room temperature to 1173 K. Interestingly, the BaGa4Se7 crystal undergoes a reversible ferroelastic phase transition at T = 528 K, resulting in the presence of a newly discovered phase (gamma-phase) at the higher temperature. The experimental temperature dependence of optical birefringence, as well as the first-principles birefringence and NLO coefficients, reveals that the gamma-phase exhibits larger birefringence and better NLO properties compared with those of the low-temperature phase (alpha-phase). This work demonstrates that phase-transition-induced structural modification can improve the mid-IR NLO properties, which would provide an effective avenue to obtain materials with good optoelectronic performance.

A new approach to the precision determination of single-crystal unit cell parameters is proposed and demonstrated using tetragonal Ag0.8Li0.2InSe2.

Li0.55Ag0.45InSe2 crystals with orthorhombic structure were grown by the Bridgman-Stockbarger method with Li0.37Ag0.63InSe2 microcrystals of tetragonal modification found on their surface.The effect of substitution of silver and lithium ions on the evolution of structural properties in Li0.55Ag0.45InSe2 and Li0.37Ag0.63InSe2 phases was investigated. A fundamental difference in the change of unit cell parameters in the temperature range of 100-300 K has been revealed. A negative temperature expansion coefficient along the c axis was detected in the tetragonal modification Li0.37Ag0.63InSe2.

The achievement of balanced properties among nonlinear optical (NLO) effect, laser induced damage threshold (LIDT) and optical birefringence is the key to obtain the excellent NLO crystals for harmonic mid-IR coherent radiation generation. In this work, we successfully synthesize and grow a new NLO chalcogenide crystal Li0.81Ag0.19InSe2 by utilizing a molecular design strategy with rational replacement between Li and Ag. This crystal not only enlarges the very low birefringence in AgInSe2 from similar to 0.017 to similar to 0.047, but also improves the rather small NLO effect in LiInSe2 from 11.8 pm/V to similar to 26 pm/V while maintaining the LIDT almost unchanged. Based on the large size Li0.81Ag0.19InSe2 single crystals of optical quality, the spectroscopic properties are measured and the main parameters important for NLO output are determined. All these results demonstrate that Li0.81Ag0.19InSe2 is a promising mid-IR NLO crystal with balanced properties.

We present results of a complex experimental and theoretical study of peculiarities of the electronic structure and optical constants of Li2In2GeSe6 compound crystallizing in monoclinic non-centrosymmetric space group Cc. In particular, we have measured the XPS spectra of core-level and valence electrons of atoms composing a high optical quality Li2In2GeSe6 crystal for both pristine and treated with middle-energy Ar+ ions surfaces. Further-more, the XPS spectrum as measured for the valence-band range of the Li2In2GeSe6 crystal is compared with the total density of states (DOS) theoretically calculated within a density functional theory (DFT) framework as implemented in the augmented plane wave plus local orbitals (APW+lo) method. The present theoretical APW+lo results reveal a very good agreement regarding the shapes and energy positions of main features of the theoretical total DOS curve and the experimental XPS spectrum when the first-principles DFT calculations are made using the modified Becke-Johnson (mBJ) functional in the form of Tran-Blaha and involving the Hubbard correction parameter U treated for highly correlated In 4d electrons (TB-mBJ+U approach). This technique gives also band gap value of 2.26 eV, which is in good agreement with that experimentally measured for this com-pound. The present TB-mBJ calculations indicate that, at the top and in the upper region of the valence band, Se 4p states make the biggest contributions; the central region is formed mainly by Ge 4p states, while In 5s states dominate at the bottom and in the lower portion of the Li2In2GeSe6 valence band. With respect to peculiarities of filling the valence band by Se(Ge) 4p states, the present calculations are verified experimentally. Furthermore, the optical properties of Li2In2GeSe6 are studied theoretically in detail employing the TB-mBJ+U technique.

Nonlinear infrared (IR) crystals for radiation conversion are of paramount importance for realization of advanced laser spectrometers for medical diagnostics, environmental monitoring, and advanced sensing. However, performance of such crystals suffers from substantial surface reflectivity coming from rather high (over 2.5) refractive index of the key nonlinear materials used. Here, based on the example of promising BaGa4Se7 nonlinear crystal, we attested direct surface patterning with ultraviolet (257 nm) femtosecond laser pulses used to engrave anti-reflective microstructures (ARMs) directly on both output sides of the crystal. Imprinted surface nanotrenches arranged into a fish-net morphology with a periodicity down to 500 nm was found to increase transmittance of the crystals from 65% to 84% within a practically relevant shortwave IR spectral range. Formation of the ARMs with an optimized geometry is expected to weakly reduce the laser damage threshold of a pristine crystal material as it was also evidenced from supporting full-wave simulations and tests.

In this work, a number of new infrared nonlinear optical (NLO) crystals of LixAg1-xInSe2, in which the ratio x of Li/Ag varies in a wide range from 0 to 1, are investigated. Structural analysis reveals that the space group of LixAg1-xInSe2 evolved from I4̅2d in AgInSe2 to Pna21 in LiInSe2 as x increases from low values (0, 0.2, 0.37) to large values (0.55, 0.78, 0.81, 1). Compared to other Li/Ag coexisting chalcogenides such as LixAg1-xGaS2 and LixAg1-xGaSe2, the structural distortions in LixAg1-xInSe2 are much more prominent. This may explain the limited crystallization region in the phase graph of the tetragonal structure LixAg1-xInSe2. The fundamental optical absorption edges in these LixAg1-xInSe2 compounds are determined from the direct electronic transitions and the band gaps Eg gradually increase as the lithium content increases, consistent with the first-principles calculations. The composition x = 0.78 is calculated to have a good set of optical properties with a large NLO coefficient (dpowder = 28.8 pm/V) and moderate birefringence (Δn ∼ 0.04). Accordingly, the Li0.78Ag0.22InSe2 crystal is grown by the modified Bridgman-Stockbarger method, and it exhibits a wide transparency range from 0.546 to 14.3 μm at the 2% transmittance level.

The factors that determine the prospects of a crystal for frequency conversion of laser radiation in a wide range to the mid and far infrared (IR) ranges are nonlinear parameters and laser damage threshold (LDT). The latter is determined by the band gap and thermophysical characteristics. This work was devoted to studying the features of the structure change of the Li0.81Ag0.19InSe2 crystal in the temperature range 90-490 K. The temperature dependences of the thermal expansion co-efficients, dispersion and optical characteristics in this range were studied. A relationship between the temperature change in the structure and other parameters has been established. In Li0.81Ag0.19InSe2 crystals, pyroluminescence was observed during heating or cooling in the range of 100-300 K, which indicates the crystal is attributed to pyroelectrics. The dispersion characteristics (nx, ny and nz) for wavelengths of 2 and 7 mu m were studied, the values of birefringence Delta n (nz -nx) were calculated in the temperature range 300-500 K. The detected fluctuations of Delta n in the temperature range from 300 to 478 K may be associated with structural changes.

Direct laser processing technologies utilizing femtosecond (fs) pulses allow to create diverse surface morphologies promising for tuning surface reflectivity. In this work, antireflection microstructures (ARMs) representing so-called laser-induced periodic surface structures (LIPSSs) with different orientation, period and roughness were fabricated on the surface of a GaSe crystal for the first time by direct fs-laser patterning. The morphological and structural properties of the fabricated LIPSSs were systematically characterized by combining scanning electron microscopy and Raman micro-spectroscopy, while their optical properties were assessed by Fourier-transform infrared spectroscopy and Finite-Difference Time-Domain simulations. The LIPSSs-based ARMs formed on both sides of the GaSe monocrystals were shown to provide 20% increase in total transmittance of the 2 mm thick GaSe crystal (being compared to pristine one) within 5–14 µm spectral range. Moreover, numerical simulations show no additional light localization effects in the near-surface layer of the LIPSS-patterned surface, highlighting applicability of the laser processing methods for improvement of the optical characteristics of the nonlinear crystals without deterioration of their optical damage threshold.

We study structural and morphological transformations caused by multipulse femtosecond-laser exposure of Bridgman-grown ϵ-phase GaSe crystals, a van der Waals semiconductor promising for nonlinear optics and optoelectronics. We unveil, for the first time, the laser-driven self-organization regimes in GaSe allowing the formation of regular laser-induced periodic surface structures (LIPSSs) that originate from interference of the incident radiation and interface surface plasmon waves. LIPSSs formation causes transformation of the near-surface layer to amorphous Ga2Se3 at negligible oxidation levels, evidenced from comprehensive structural characterization. LIPSSs imprinted on both output crystal facets provide a 1.2-fold increase of the near-IR transmittance, while the ability to control local periodicity by processing parameters enables multilevel structural color marking of the crystal surface. Our studies highlight direct fs-laser patterning as a multipurpose application-ready technology for precise nanostructuring of promising van der Waals semiconductors, whose layered structure restricts application of common nanofabrication approaches.

Министерство науки и высшего образования Российской Федерации Российское химическое общество им.Д.И.Менделеева Секция по химической термодинамике и термохимии Научного совета РАН по физической химии Сибирское Отделение Российской Академии Наук Институт неорганической химии им.А.В.Николаева СО РАН