Mono-cycle terahertz pulses from intersubband shift currents in asymmetric semiconductor quantum wells

Both second-order frequency mixing in nonlinear optical media and photoconductive antennas have provided terahertz (THz) transients in a wide parameter range. Here, we demonstrate a novel type of ultrafast nonlinear optical response in asymmetric semiconductor quantum wells, originating from electron shift currents. Resonant intersubband excitation by a femtosecond mid-infrared pulse induces a transient spatial shift of electronic charge, which leads to the emission of a mono-cycle THz pulse. This mechanism is characterized and separated from conventional difference frequency mixing by nonlinear two-dimensional THz spectroscopy. The amplitude of the THz electric field emitted by the shift current reaches several percent of the mid-infrared driving field, significantly higher than in difference frequency mixing. Changes in time structure of the THz pulse upon nonlinear saturation of intersubband absorption allow for generating tailored THz transients. The present concept can be implemented in highly compact devices driven by mid-infrared pulses at megahertz repetition rates to provide versatile THz pulses for spectroscopy and optoelectronics. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement