Rydberg excitons in the presence of an ultralow-density electron-hole plasma

TL;DR

This study investigates how ultralow-density electron-hole plasmas affect Rydberg excitons in Cu₂O, revealing bleaching of exciton lines and band gap reduction, with potential for controlled exciton manipulation.

Contribution

It provides a quantitative analysis of plasma effects on Rydberg excitons at ultralow densities, introducing an effective Bohr radius to describe plasma blockade.

Findings

Rydberg exciton absorption lines bleach at very low plasma densities

Band gap reduction scales as the square root of free carrier density

Exciton oscillator strength vanishes at the band edge crossing point

Abstract

We use two-color pump-probe spectroscopy to study Rydberg excitons in Cu$_2$O in the presence of free carriers injected by above-band-gap excitation. Already at plasma densities $\rho_\text{eh}$ below one hundredth electron-hole pair per \textmu m$^{3}$, the Rydberg exciton absorption lines are bleached while their energies remain constant, until they finally disappear, starting from the highest observed principal quantum number $n_\text{max}$. As confirmed by calculations, the band gap is reduced by many-particle effects caused by free carriers scaling as $\rho_\text{eh}^{1/2}$. An exciton line looses oscillator strength when the band edge approaches the exciton energy vanishing completely at the crossing point. We quantitatively describe this plasma blockade by introducing an effective Bohr radius that determines the energy distance to the shifted band edge. In combination with the negligible associated decoherence this opens the possibility to control the Rydberg exciton absorption through the plasma-induced band gap modulation.