Impact of femtosecond interband pumping on terahertz gain of quantum-cascade structures

Abstract

Ultrashort terahertz pulses are a key prerequisite of terahertz time-domain spectroscopy. The generation of high-power terahertz pulses in quantum-cascade structures is limited by gain clamping, which can, however, be overcome for a short period of time using ultrashort interband pumping. In order to clarify this possibility, the terahertz gain of a quantum-cascade structure under irradiation by femtosecond pulses of various energies should be quantified. Using the density matrix and many-body theory, the influence of femtosecond interband pumping on the terahertz gain of quantum-cascade structures is characterized. The resulting dynamical gain spectra of a three-well GaAs/AlGaAs benchmark quantum-cascade structure show increasing terahertz gain up to an order of magnitude under pumping by 100 fs optical pulses with intensity in the active region 100 MW cm − 2. The gain switching process is characterized by a 1 ps rise time and 8 ps recovery time. The short rise and recovery times and large increment of the terahertz gain can be used to develop sources for terahertz time-domain spectroscopy.