Second-order Nonlinear Optical Properties Of Low Symmetry Crystal BaTeMo₂O₉

Nonlinear optical (NLO) materials are widely used in optical devices such as frequency conversion, optical parametric oscillation (OPO), optical modulation and telecommunications. A lot of researches have been done since the first observation of second-harmonic generation (SHG) in 1961. Up to now, materials for visible and UV light can meet the need of practical application primarily, such as LiB3O5 (LBO), CsLiB6O10 (CLBO) and KTiOPO4 (KTP). In the infrared (IR) region, a number of materials such as ZnGeP2 and AgGaS2 have been reported, but some drawbacks still exist that limit their applications. For instance, these crystals are hard to grow with good optical quality. They also have low thermal conductivity, large anisotropy of thermal expansion, and low transparency in the visible region. Therefore, great efforts have been devoted to the development of new NLO crystals for the IR region.BaTeMo2O9 (BTM) is a new infrared NLO compound, its Kurtz powder second-harmonic generation (SHG) efficiency is about 600×α-SiO2. High-quality bulk single crystal of BTM, with dimensions up to 30×23×18 mm3, has been grown by flux method in our group. Its basic properties have been studied and an experimental setup for Maker fringe platform to determine the second-Order optical nonlinear coefficients was constructed. This makes the ground to study second-order nonlinear optical properties of monoclinic BaTeMo2O9 bulk single crystal.We studied the second-order nonlinear optical properties of BTM systematically, and the main works are as follows:1,Overview of the origin and the development of the nonlinear optics. A brief introduction of the nonlinear optical crystals has been carried out. Several methods to test the second-order nonlinear optical coefficients are summed up. The phase matching method is easy to operate and calculate and is commonly used in the measurement of the nonlinear coefficients. The Maker fringe technique can be used to measure almost all of the nonlinear coefficients, and is currently the most commonly used method.2,A brief introduction of the basic physical properties of crystal BaTeMo2O9 is presented. High-quality bulk single crystal of BTM, with dimensions up to 30x23x18 mm3, has been grown by flux method in our group. It has a good transparency from 0.4 to 5μm at room temperature. It is a negative biaxial crystal with a large birefringence. The crystal physic axes (X, Y, Z, with the setting nx