Edge-driving of LiNbO₃ slabs for high-power terahertz generation

Terahertz radiation from optical rectification in lithium niobate is a valuable tool for investigation and control of ultrafast dynamics in complex materials. However, the generated fields need to travel to a surface for free-space emission, and terahertz absorption therefore becomes a limiting factor at high average power and high peak energies. Here, we report a scheme for simultaneous in-situ measurement and optimization of nonlinear conversion and terahertz absorption in a large LiNbO₃ crystal in Cerenkov geometry. We scan the position of the terahertz generation inside the material and record the emitted power and spectrum as a function of distance to the emission edge. We find that terahertz absorption follows a Lambert-Beer law with a coefficient that depends on doping and temperature effects from four-photon absorption of the pump light. Minimizing the distance between terahertz generation and free-space emission provides about 30% more efficiency.