Low-power optically controllable THz attenuator

Abstract

We have demonstrated a low power, optically-controllable, THz attenuator capable of high contrast ratios using a mixed type-I/type-II quantum well sample. When high free-carrier densities are optically excited in the quantum wells by a cw-laser, the transmitted THz intensity can be controllably reduced. Normally in quantum well samples high carrier densities cannot be achieved using low power excitation densities, because the carrier lifetime is so short. This is not the case for out sample which consists of 20 periods of a narrow and a wide GaAs well. After electron-hole pairs are created via optical excitation in the narrow well, they are separated in space, because the electronics are rapidly transferred into the wide well via an x-valley in the barrier material. The carrier lifetime at low sample temperatures i therefore extremely long, 0.48 ms, leading to high carrier densities. Using an optical excitation power of 2.1 mW from a cw-HeNe laser, the transmitted THz intensity can be reduced by 60 percent.