Terahertz generation in a thin GaAs slab in a tapered parallel plate waveguide by femtosecond laser excitation at 1560 nm

A comparative experimental study of terahertz generation by optical rectification of femtosecond laser pulses with a wavelength of 1560 nm in a 100 μm thick slab of GaAs sandwiched between the walls of a tapered metallic waveguide and in a bulk GaAs crystal has been performed. In both cases, the velocity mismatch between the optical and terahertz beams propagating in GaAs was overcome by using the Cherenkov phase-matching method. The dependence of the terahertz time-domain signal amplitude, spectral bandwidth, and dynamic range on the focusing conditions were measured and compared. For both samples, the most efficient generation was observed for the 50 mm focal length lens. A significant improvement in the directionality of the terahertz emission from the thin GaAs slab due to the tapered waveguide was predicted by FDTD simulations and this was also confirmed experimentally.