Abstract

Monochromatic THz (far-IR) sources have important applications in chemical analysis, high-resolution spectroscopy, and bio-medical imaging. In order to realize these applications the THz sources must have the specifications of high coherence (narrow linewidth), sufficiently high output powers, and wide tuning ranges. Parametric processes based on second-order nonlinearities, especially difference-frequency generation (DFG) offer a simple method for efficient coherent and tunable THz emission by using nonlinear optical crystals [1]. LiNbO₃ was first used for THz generation based on phase-matched DFG [2]. Recently, DAST [3], GaSe [4], and ZnGeP₂ [5] have respectively been used to generate THz radiation based on this scheme. Among all the nonlinear crystals, GaSe has the highest figure of merit as a result of its lowest absorption coefficients among all the nonlinear crystals and large effective nonlinear coefficients in the THz region [4]. As a result, coherent THz waves tunable in the range of 56.8-1618 μm (176-6.18 cm⁻¹) with the peak power of 69.4 W were recently obtained [4]. In addition, GaSe was also used to efficiently generate mid-IR waves tunable in the range of 2.7-28.7 μm [6].

© 2004 Optical Society of America