Abstract

Continuously-tunable and coherent THz and millimeter sources are in great demand for applications including high-resolution spectroscopy, communications, and bio-medical imaging. However, these applications cannot be put into practice until compact THz lasers are available. On the other hand, parametric processes based on second-order optical nonlinearities such as difference-frequency generation (DFG)^1,2 offer a simple method for efficient THz and millimeter emitters. So far, LiNbO₃, ZnGeP₂ and DAST¹ have been used based on this scheme. However, all of them have very large absorption coefficients in the THz domain. Among all second-order nonlinear crystals GaSe has the lowest absorption coefficients in the THz domain.⁴ As one of the important NLO crystals, GaSe can satisfy phase matching (PM) conditions in a large wavelength range.⁵ Moreover, this crystal possesses large second-order susceptibility. All these properties make GaSe a winner for efficient generation of THz and millimeter waves based on DFG. In this report, we have implemented a new coherent THz and millimeter source with the output wavelength continuously tunable in the range of 56.8-1618 μm (5.27–0.18 THz) in a GaSe crystal.

© 2002 Optical Society of America