Extreme Values of Photoelastic Effects in Btas and BTGS Crystals

Indicative surfaces (ISs) of the spatial distribution of piezo-optic and elasto-optic effects (POE and ELOE) in Ba3TaAl3Si2O14 (BTAS) and Ba3TaGa3Si2O14 (BTGS) langasite-type crystals were constructed based on piezo-optic im and elasto-optic pin coefficient matrices obtained from first-principles calculations.We presented relations for the ISs of longitudinal and transverse POE and ELOE, as well as relations for the ELOE induced by shear strains.It is shown that the POE and ELOE properties of BTAS and BTGS crystals are essentially anisotropic, while the largest values of the transverse ELOE of BTAS and BTGS crystals (0.213 and 0.209, respectively) do not coincide with the directions of the crystallophysical axes of these crystals.According to these ELOE values, these crystals exceed other langasite-group crystals, La3Ga5SiO14 (LGS) and Сa3TaGa3Si2O14 (СTGS), by a factor of 2-3.For all the above crystals, ELOE surfaces induced by shear strains have been constructed.The maximum values of these surfaces (0.108-0.131) are nearly two times lower than the maxima of the transverse ELOE in BTAS and BTGS crystals, but the acousto-optic efficiency can be relatively high due to the low acoustic-wave velocities caused by shear strains.