Observation of forbidden exciton transitions mediated by Coulomb interactions in photoexcited semiconductor quantum wells.

TLDR: Terahertz pulses are used to induce resonant transitions between the eigenstates of optically generated exciton populations in a high-quality semiconductor quantum well sample, yielding an effective terahertz transition between the 1s and 2s populations.

Abstract: We use terahertz pulses to induce resonant transitions between the eigenstates of optically generated exciton populations in a high-quality semiconductor quantum well sample. Monitoring the excitonic photoluminescence, we observe transient quenching of the $1s$ exciton emission, which we attribute to the terahertz-induced $1s$-to-$2p$ excitation. Simultaneously, a pronounced enhancement of the $2s$ exciton emission is observed, despite the $1s$-to-$2s$ transition being dipole forbidden. A microscopic many-body theory explains the experimental observations as a Coulomb-scattering mixing of the $2s$ and $2p$ states, yielding an effective terahertz transition between the $1s$ and $2s$ populations.