Terahertz pulses from semiconductor-air interfaces

We report an unexpected observation of strong terahertz pulses, radiated perpendicular to semiconductor-air interfaces illuminated by femtosecond laser pulses at normal incidence. Symmetry arguments preclude radiation emitted strictly along the direction of the surface electric field and the dipoles, normal to the interface. We attribute our observations to the strong interface-induced angular redistribution of the radiation being collected by a small dielectric lens. For detection, a small symmetry breaking displacement within the terahertz diffraction spot is sufficient.