Observation of Coulomb-Assisted Dipole-Forbidden Intraexciton Transitions in Semiconductors

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 2\(s\) and 2\(p\) states, yielding an effective terahertz transition between the 1\(s\) and 2\(s\) populations.