Directional terahertz emission from corrugated InAs structures.

The terahertz (THz) radiation from transient dipoles, formed by distinct diffusion coefficients between oppositely charged carriers as often observed in low band gap semiconductors, propagates with an anisotropic amplitude distribution perpendicular to the dipole axis along the diffusive motion. By directionally adjusting the electronic diffusion, we conceptualize groove-patterned THz emitters based on (100) InAs thin films and demonstrate the unidirectional radiation. Line-of-sight emission along the surface-normal direction is greatly enhanced in a distributed asymmetric trapezoid with its period similar to the electronic diffusion length of InAs. This directional enhancement is in clear contrast to the constant emission amplitude along the lateral direction, regardless of pattern scale, which manifests the role of groove patterns as microscale reflectors in laterally corrugating the carrier density. In contrast to the rather limited nonlinearity in (100) plane, the azimuthal angle dependence of the THz field amplitude in corrugated samples shows a combined effect of diffusive transport and second-order nonlinearity, whose compositional contributions varies in different structures.