부산대학교 도서관

Femtosecond laser pulses propagating in electro-optic crystal can generate wideband terahertz (THz) radiation via optical rectification. In the crystals with high optical nonlinearities and wide band gaps (small multiphoton absorption), such as LiNbO 3 or LiTaO 3 , the optical group velocity is more than two times larger than the highest phase velocity of terahertz waves. To achieve phase-matching in such (superluminal) crystals the mechanism of Cherenkov radiation may be used. The main drawbacks of this method are typically strong terahertz absorption (∼ 20 cm −1 for LiNbO 3 ) and diffraction of laser pump. To avoid these limitations it was proposed to use a planar sandwich structure with thin LiNbO 3 core (to generate THz radiation and guide laser beam) and Si cladding (to output THz radiation with low absorption) [1]. This scheme showed experimentally the highest efficiency available today (∼ 0.1%) [2]. In further development of the scheme it was proposed to use a metal substrate to collect the terahertz emission into one direction and to control its spectrum by varying an air gap between the metal substrate and the LiNbO 3 layer [3]. In this presentation we explore this scheme experimentally.