Antireflection microstructures fabricated on the surface of a LiGaSe2 nonlinear crystal.

LiGaSe2 is a propitious material for nonlinear parametric conversion in the mid-infrared (mid-IR) range. Its refractive index of n = 2.25 in the 2-12 µm wavelength range results in significant losses due to Fresnel reflection. However, the conventional method of increasing the transmittance with antireflection coatings (ARCs) significantly reduces the damage threshold of the material. Fabrication of the antireflection microstructures (ARMs) is an alternative approach for increasing the surface transmittance. In this work, ARMs were fabricated on the surface of a LiGaSe2 crystal using a single-pulse femtosecond laser ablation method. An average transmittance of 97.2% in the 2-8 µm spectral range and the maximum transmittance of 98.6% at 4.1 µm were achieved.

Anisotropic Thermal Expansion and Electronic Structure of LiInSe2.

Optical quality cm-sized LiInSe2 crystals were grown using the Bridgman-Stockbarger method, starting from pure element reagents, under the conditions of a low temperature gradient of 5-6 degrees/cm and a slight melt overheating. The phase purity of the grown crystal was verified by the powder XRD analysis. The thermophysical characteristics of LiInSe2 were determined by the XRD measurements in the temperature range of 303-703 K and strong anisotropy of the thermal expansion coefficients was established. The following values of thermal expansion coefficients were determined in LiInSe2: αa = 8.1 (1), αb = 16.1 (2) and αc = 5.64 (6) MK-1. The electronic structure of LiInSe2 was measured by X-ray photoelectron spectroscopy. The band structure of LiInSe2 was calculated by ab initio methods.

Fabrication of antireflection microstructures on the surface of GaSe crystal by single-pulse femtosecond laser ablation.

GaSe crystals are promising as nonlinear optical converters in the mid- and far-IR ranges. However, it is challenging to increase the GaSe surface transmittance of 77% with conventional antireflection coatings because of poor surface quality, leading to coating adhesion problems. Antireflection microstructures (ARMs) offer an alternative way of increasing surface transmittance. In this work, ARMs were fabricated on the surface of a GaSe plate by single-pulse femtosecond laser ablation. An average GaSe surface transmittance of 94% in the 7-11 µm range and a maximum transmittance of 97.8% at 8.5 µm were obtained. The proposed method can be used to increase the efficiency of GaSe-based nonlinear converters.

Structures and optical properties of two phases of SrMgF4.

SrMgF4 has an extremely large bandgap Eg of 12.50 eV as obtained from reflection dispersion. The symmetry of this crystal is monoclinic P21 at room temperature and transforms to the orthorhombic Cmc21 phase near 478 K as the temperature increases. The acentric character of the low-temperature (LT) phase is confirmed by pyroelectric luminescence at T < 440 K. The fundamental absorption edge of the LT phase is located at 122 nm (10.15 eV). A considerable difference between the absorption edge and bandgap Eg is due to the strong exciton absorption. The first-principles electronic structure, refractive indices, nonlinear susceptibility and polarizability were calculated for both LT and high-temperature (HT) phases. Band-to-band transitions are direct for the LT phase but indirect for HT. In spite of relatively low birefringence (∼0.017) and nonlinear susceptibility (∼0.044 pm V(-1), an order lower than that in KDP), SrMgF4 crystals are considered promising for nonlinear optics thanks to their transparency far in the vacuum ultraviolet spectral region.