Rydberg Excitons in the Presence of an Ultralow-Density Electron-Hole Plasma

We study the Rydberg exciton absorption of Cu2O in the presence of free carriers injected by above-bandgap illumination. Already at plasma densities rho(EH) below one hundredth electron-hole pair per mu m(3), exciton lines are bleached, starting from the highest observed principal quantum number, while their energies remain constant. Simultaneously, the band gap decreases by correlation effects with the plasma. An exciton line loses oscillator strength when the band gap approaches its energy, vanishing completely at the crossing point. Adapting a plasma-physics description, we describe the observations by an effective Bohr radius that increases with rising plasma density, reflecting the Coulomb interaction screening by the plasma.