Low-temperature terahertz absorption measurements and polariton laser gain modelling for 5 mol% MgO:LiNbO₃

We determined the THz absorption coefficient of a 5 mol% MgO:LiNbO₃ crystal using synchrotron transmission measurements over the frequency range from 0.75 to 6 THz and temperatures from 37 to 300 K. A broad peak at 3.15 THz was observed at all temperatures, which we assigned to be a low-frequency mode associated with defect centers in the crystal that causes unexpected increased absorption. We estimate stimulated polariton scattering Stokes gain at varying temperatures. We found that the additional mode strongly affects the gain, which explains the difficulty of using MgO:LiNbO₃ for THz generation beyond 3 THz. The implications of our findings for operating THz lasers based on MgO:LiNbO₃ beyond 3 THz, including temperatures down to 77 K, are discussed.