Terahertz spectroscopy of two-dimensional electron-hole pairs: probing Mott physics of magneto-excitons

Density-dependent Coulomb interactions can drive electron-hole ($e-h)$ pairs in semiconductors through an excitonic Mott transition from an excitonic gas into an $e-h$ plasma. Theoretical studies suggest that these interactions can be strongly modified by an external magnetic field, including the absence of inter-exciton interactions in the high magnetic field limit in two dimensions, due to an $e-h$ charge symmetry, which results in ultrastable magneto-excitons. Here, we present a systematic experimental study of $e-h$ pairs in photo-excited undoped GaAs quantum wells in magnetic fields with ultrafast terahertz spectroscopy. We simultaneously monitored the dynamics of the intraexcitonic 1$s$-2$p$ transition (which splits into 1$s$-2$p_{\mathrm{+}}$ and 1$s$-2$p_{\mathrm{-}}$ transitions in a magnetic field) and the cyclotron resonance of unbound electrons and holes up to 10 Tesla. We found that the 1$s$-2$p_{\mathrm{-}}$ absorption feature is robust at high magnetic fields even under high excitation fluences, indicating magnetically enhanced stability of excitons. We will discuss the Mott physics of magneto-excitons as a function of temperature, $e-h$ pair density, optical pump delay time, as well as magnetic field, and also compare two-dimensional excitons in GaAs quantum wells with three-dimensional excitons in bulk GaAs.