Phase Transitions and Nonlinear Optical Property Modifications in BaGa4Se7.

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 (γ-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 γ-phase exhibits larger birefringence and better NLO properties compared with those of the low-temperature phase (α-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.

Evolution of Structures and Optical Properties in a Series of Infrared Nonlinear Optical Crystals LixAg1-xInSe2 (0 ≤ x ≤ 1).

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.

Thermophysical properties of lithium thiogallate that are important for optical applications.

Lithium thiogallate LiGaS2 is one of the most common nonlinear crystals for mid-IR due to its extreme beam strength and wide transparency range; however, its thermophysical properties have not yet been practically studied. Large crystals of high optical quality are grown. DTA revealed features at 1224 K below melting point (1304 K) that are associated with the oxygen containing compounds of the LiGaO2-x S x type. The thermal conductivity of LiGaS2 (about 10.05 W (m-1 K-1)) and band gap value (3.93 eV at 300 K) are found to be the highest in the LiBC2 family. Isotropic points in the dispersion characteristics for the refractive index are found and LiGaS2-based narrow-band optical filters, smoothly tunable with temperature changes, are demonstrated. Intense blue photoluminescence of anionic vacancies V S is observed at room temperature after annealing LiGaS2 in vacuum, whereas orange low-temperature emission is related to self-trapped excitons. When LiGaS2 crystals are heated, spontaneous luminescence (pyroluminescence) takes place, or thermoluminescence after preliminary UV excitation; the parameters of traps of charge carriers are estimated. The obtained data confirm the high optical stability of this material and open up prospects for the creation of new optical devices based on LiGaS2.