Li<sub>2</sub>BaSiSe<sub>4</sub>: A new metal-mixed selenide with outstanding second-harmonic generation

Abstract

Nonlinear optical (NLO) materials as important frequency-conversion devices in solid state lasers, have attracted unprecedented attentions in laser science and technology. In infrared (IR) region, NLO materials display more important roles in view of critical application prospect including laser guidance, resource exploration and long-distance laser communication etc. AgGaS2, AgGaSe2 and ZnGeP2 are selected as the commercial IR NLO materials, but they exihibit low laser damage thresholds (LDT) induced by their small bandgaps, which limit their further applications in high energy laser systems. Therefore, it is still a big challenge to obtain new IR NLO materials, which could balance the demand of large second-harmonic generation (SHG) and high laser damage threshold. As a NLO material, the non-centrosymmetrical structure is the prerequisite condition, and mix-metal chalcogenides have attracted much attention since their structures exhibit a variety of acentric units arising from the combination of different center metals, which show different sizes, coordination preferences, and packing characteristics. Previous investigations indicate that the introduction of alkali and alkali earth metals is good for obtaining large band gap because they may avoid the d-d and f-f electron transition like popular LiGaS2, BaGa4S7 etc. Moreover, compared with sulfides, selenides produce larger SHG responses. Therefore, in this work, in aim at the balanced optical properties, we selected electricity-positive alkali metal Li, alkali earth metal Ba, silicon in IVA group and selenium as the raw materials. Through high temperature flux method, we have obtained a metal-mixed selenide Li2BaSiSe4. The raw materials are weighted with the molar ratio of 2:1:1:4, and then loaded into the graphite crucible, finally sealed in a silicon tube under ethane-oxygen flame. The tube is put in a program-computed furnace with the temperature program as blow: (1) heat from room temperature to 190°C in 3 h and keep at this temperature for 300 min; (2) heat from 190 to 450°C in 10 h and keep at this temperature for 20 h; (3) heat from 450 to 850°C in 10 h and keep at this temperature for 30 h; (4) cool from 850 to 400°C in 100 h and turn down the furnace. Optical characterizations contain single crystal X-Ray diffraction (XRD), powder XRD, Raman spectrum, UV-vis-NIR diffuse reflection spectrum, and SHG responses measurement at 2.09 μm. Moreover, theoretical calculations have also made to further investigate the relationship between properties and structure, including band structure, density of states, birefringence and SHG coefficients. Single crystals with millimeter sizes used for single crystal XRD and Raman spectrum are picked from the synthesized polycrystal under a microscope. The powder samples used for powder XRD characterization and UV-vis-NIR diffuse reflection are obtained from the ground polycrystals. The powder SHG responses are measured with sieved crystals in six different size ranges: 38−55, 55−88, 88−105, 105−150, 150−200, 200−250 μm, with AgGaS2 in the same particle size ranges as the references. The band gap of Li2BaSiSe4 is obtained from UV-vis-NIR diffuse reflection data converted by Kubelka-Munk function, which is measured to be 2.47 eV, which is larger than AgGaSe2 (1.83 eV). The SHG response is measured with 2.09 μm laser, and the result shows that Li2BaSiSe4 is type-I phase matchable and is equal to that of commercially applied AgGaS2. In conclusion, the introduction of Li and Ba could enlarge the band gap compared with classical AgGaSe2, but the SHG response need more rational designation to maintain at a high level.