Terahertz magnetoconductivity of excitons and electrons in quantum cascade structures

Abstract

We examined the quasiparticles formed by the photoexcitation of $\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{Ga}\mathrm{As}$ terahertz quantum cascade structures using terahertz time-domain spectroscopy. At low temperature and excitation density the measured conductivity was excitonic, with a $1s\text{\ensuremath{-}}2p$ transition energy indicative of three-dimensional excitons correlated across the quantum well barriers. Free electrons increasingly dominated the conductive response at higher lattice temperatures and excitation densities. Under an external magnetic field transitions from the $1s$ level into $2p$ states with different magnetic quantum number were observed, while at high excitation densities the electron cyclotron resonance became more prominent.