Abstract

Narrow-bandgap III-V semiconductors are proving to be excellent terahertz (THz) emitters when excited by ultrashort optical pulses. The specific nonlinear process contributing to the emission is inherently complicated by the fact that many different mechanisms contributing to the nonlinearity exist [1]. Surface field surge currents, photo-Dember and third order surface field-seeded optical rectification (SOR) are the most commonly acknowledged source phenomena. SOR relies on the static electric field resulting from charges populating the material surface states, which is responsible for the band bending and formation of a depletion region at the interface [2]. SOR in Indium Arsenide has been singled out as the dominant generation mechanism at high excitation energies. THz generation is typically limited by the saturation of the relevant field-matter interaction mechanism [3], the damaging threshold of nonlinear crystals or the lack of suitable phase matching between THz and optical fields in real materials. We have recently demonstrated in [4] that for SOR, the quadratic dependence between the emitted THz energy and the pump energy is re-established at extreme excitations. Increasing the impinging optical pump further, however, becomes impractical as the optical damage threshold of the semiconductor is approached. It would, therefore, be greatly beneficial to instead enhance the surface field to enable a continued increase in the emitted THz field.

© 2019 IEEE