Terahertz generation control by metal substrate in sandwich structure with thin LiNbO3

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 LiNbO₃ or LiTaO₃, 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 and diffraction of laser pump. To avoid these limitations it was proposed to use a planar sandwich structure with thin LiNbO₃ core (to generate THz radiation and guide laser beam) and Si cladding (to output THz radiation with low absorption). This scheme showed experimentally the highest efficiency available today. 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 LiNbO₃ layer. In this paper the authors explore this scheme experimentally.