Synchrotron terahertz absorption measurements and polariton gain modelling of 5 mol% MgO:LiNbO3 at cryogenic temperatures

Abstract

We determined the absorption coefficient of 5 mol% MgO:LiNbO₃ using synchrotron measurements of the crystal's transmission and reflection over the frequency range from 0.75 – 6 THz and temperatures 37 K to 300 K. The spectra cover almost the entire low-frequency branch of the 252 cm⁻¹ (7.4 THz) A₁ polariton mode, which is commonly utilized in terahertz lasers based on stimulated polariton scattering. A broad peak at 3.15 THz was observed, which we assigned to be a low-frequency defect mode that causes increased polariton damping. We used Gaussian gain theory to estimate the Stokes gain at 77 K and 300 K. The additional mode strongly affects the gain which explains the difficulty of MLN's THz generation beyond 3 THz. Our model also predicts possible tuning around the 4 THz region, which could further be improved by crystal cooling.