Direct electro-optic effect in langasites and α-quartz

Strain-constant (clamped) electro-optic coefficients r11S of langasite La3Ga5SiO14 (LGS), langatate La3Ga5.5Ta0.5O14 (LGT), catangasite Ca3TaGa3Si2O14 (CTGS) and α-quartz are measured at 1540 nm in the frequency range of 3-25 MHz. Experimental ratio of clamped and unclamped electro-optic coefficients r11S/r11T is 0.97 for LGS, 0.91 for LGT, 0.31 for CTGS, and 0.49 for quartz. Most of direct electro-optic effect in LGS and LGT is associated with lanthanum ions: clamped electro-optic coefficient r11S in lanthanum-free CTGS is 14 times less than in LGS. Low piezoelectric contribution to unclamped electro-optic coefficient r11T makes LGS and LGT promising materials for electro-optic devices, whose performance can be deteriorated by piezoelectric effect, especially, for high-voltage optical voltage sensors.