Extended Cauchy equations of congruent LiNbO3 in the terahertz band and their applications

The refractive indices of ordinary and extraordinary waves (n(o) and n(e)) of congruent LiNbO3 (CLN) in the terahertz (THz) band are measured at different temperatures. Extended Cauchy equations are proposed to describe the temperature- and frequency-dependent refractive indices. With less than 0.5% deviation, both no and ne can be obtained accurately. The values of n(o) and n(e) are similar to 7 and similar to 5.4, respectively, which are much larger than those in the visible band. The extremely high birefringence (similar to 1.6) may lead to some interesting applications of linear, electro-optic, and nonlinear optical effects of CLN. As an example, true zero-order THz waveplates (WPs) could be obtained with considerable low loss based on thin CLN wafers. The working frequency can be tuned by changing the environmental temperature. Moreover, wide bandwidth achromatic THz WPs were also achievable by stacking four CLN wafers together with precise orientation control. (C) 2016 Optical Society of America