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

TL;DR

This study demonstrates that terahertz pulses can induce Coulomb-assisted dipole-forbidden intraexciton transitions in semiconductors, revealing new pathways for controlling excitonic states.

Contribution

It provides the first experimental observation and microscopic explanation of Coulomb-assisted 1s-to-2s exciton transitions in semiconductors.

Findings

Transient quenching of 1s exciton emission

Enhanced 2s exciton emission despite dipole-forbidden transition

Coulomb scattering mixes 2s and 2p states, enabling effective 1s-to-2s transition

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