Abstract

Generation of THz pulses by optical excitation of semiconductor and metallic surfaces attracted renewed attention recently as novel THz emission mechanisms had been discovered [1-3]. Among these mechanisms, THz pulse emission due to transient photo-currents in the plane of the optically excited surface is still not fully understood and it is presently debated [1,2]. In this contribution we present a set of time-domain THz near-field microscopy experiments designed to clarify the origin of THz pulse generation. We correlate near-field distribution of the generated THz pulses with the spatial distribution of photo-excited charge carriers for the first time. This capability allows us to conclude that the in-plane gradients of the charge carrier density are responsible for the THz pulse generation process. We also demonstrate that contrary to the common understanding, the in-plane dynamics of the charge carriers can be the dominant source of THz emission in the excitation of semiconductor surfaces.

© 2015 IEEE