Recent high-resolution absorption spectroscopy on excited excitons in cuprous oxide [Nature 514, 343 (2014)] has revealed significant deviations of their spectrum from that of the ideal hydrogenlike series. Here we show that the complex band dispersion of the crystal, determining the kinetic energies of electrons and holes, strongly affects the exciton binding energies. Specifically, we show that the nonparabolicity of the band dispersion is the main cause of the deviation from the hydrogen series. Experimental data collected from high-resolution absorption spectroscopy in electric fields validate the assignment of the deviation to the nonparabolicity of the band dispersion.

We study the Rydberg exciton absorption of Cu_{2}O in the presence of free carriers injected by above-band-gap illumination. Already at plasma densities ρ_{EH} below one hundredth electron-hole pair per μm^{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.

Coherent optical control of individual particles has been demonstrated both for atoms and semiconductor quantum dots. Here we demonstrate the emergence of quantum coherent effects in semiconductor Rydberg excitons in bulk Cu2O. Due to the spectral proximity between two adjacent Rydberg exciton states, a singlefrequency laser may pump both resonances with little dissipation from the detuning. As a consequence, additional resonances appear in the absorption spectrum that correspond to dressed states consisting of two Rydberg exciton levels coupled to the excitonic vacuum, forming a V-type three-level system, but driven only by one laser light source. We show that the level of pure dephasing in this system is extremely low. These observations are a crucial step towards coherently controlled quantum technologies in a bulk semiconductor.

Quantum-optical research on semiconductor single-photon sources puts special emphasis on the measurement of the second-order correlation function g (2) (τ), arguing that g (2) (0)<1/2 implies the source field represents a good single-photon light source. We analyze the gain of information from g (2) (0) with respect to single photons. Any quantum state, for which the second-order correlation function falls below 1/2, has a nonzero projection on the single-photon Fock state. The amplitude p of this projection is arbitrary, independent of g (2) (0). However, one can extract a lower bound on the single-to-multi-photon-projection ratio. A vacuum contribution in the quantum state of light artificially increases the value of g (2) (0), cloaking actual single-photon projection. Thus, we propose an effective second-order correlation function˜( ) ( ) g 0 2 OPEN ACCESS RECEIVED

Recent high-resolution absorption spectroscopy on highly excited excitons in cuprous oxide (Kazimierczuk et al 2014 Nature 514 343–347) have revealed significant deviations of their spectrum from the ideal hydrogen-like series. In atomic physics, the influence of the ionic core and the resulting modifications of the Coulomb interaction are accounted for by the introduction of a quantum defect. Here we translate this concept to the realm of semiconductor physics and show how the complex band dispersion of a crystal is mirrored in a set of empirical parameters similar to the quantum defect in atoms. Experimental data collected from high-resolution absorption spectroscopy in electric fields allow us to compare results for multiple angular momentum states of the yellow and even the green exciton series of . The agreement between theory and experiment validates our assignment of the quantum defect to the nonparabolicity of the band dispersion.