Excitons

Abstract

Optical band-to-band absorption can produce an electron and a hole in close proximity which attract each other via Coulomb interaction and can form a hydrogen-like bond state, the exciton. The spectrum of free Wannier-Mott excitons in bulk crystals is described by a Rydberg series with an effective Rydberg constant given by the reduced effective mass and the dielectric constant. A small dielectric constant and large effective mass yield a localized Frenkel exciton resembling an excited atomic state. Excitons increase the absorption slightly below the band edge significantly. The interaction of photons and excitons creates a mixed state, the exciton-polariton, with photon-like and exciton-like dispersion branches. An exciton can bind another exciton or carriers to form molecules or higher associates of excitons. Free charged excitons (trions) and biexcitons have a small binding energy with respect to the exciton state. The binding energy of all excitonic quasiparticles is significantly enhanced in low-dimensional semiconductors. Basic features of confined excitons with strongest transitions between electron and hole states of equal principal quantum numbers remain similar. The three-dimensional confinement of quantum dots allows for forming stable antibinding exciton associates. Excitonic states of quantum dots are a prominent physical basis for the realization of photonic qubits. The exciton emission is deployed for producing single photons on demand with qubits encoded into the polarization or another degree of freedom. Pairs of entangled photons are obtained from the biexciton-exciton cascade. Photons used as flying qubits enable inherently secure data transmission; semiconductor-based quantum optics is a highly active field of research and development.