Accurate measurement of Lithium Tantalate birefringence in whispering gallery resonators

Lithium Tantalate (LTA) is an important ferroelectric crystal widely used in piezoelectric, electro-optic, and nonlinear optical devices. Although this crystal is very common, there is an ongoing debate regarding its birefringence magnitude, and even its sign. While some sources claim it to be negative, others maintain that it is positive. The issue is further complicated by the sample-to-sample variation in the crystal composition, ranging from congruent to stoichiometric. Here we report the experimental exploration of this question based on high-resolution spectroscopy of optical whispering gallery resonators fabricated from this crystal. These sensitive measurements unambiguously indicate that in the near-infrared range at room temperature the stoichiometric LTA is negative, while the congruent LTA is positive.