A biexciton is a two-Electron, two-electron hole excitation. If and are total excitation energies relative to the empty emitter, the cascade emits photons of energies and . Define its binding energy by . The first cascade photon then has energy . Positive binding shifts that line below the single-exciton line; confinement can also produce negative binding, shifting it above. The cascade does not emit a single photon of twice the exciton energy.
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A biexciton is a quantum mechanical state that consists of two excitons. An exciton is a bound state of an electron and a hole (the absence of an electron) in a semiconductor or insulator. When an electron in a semiconductor absorbs energy (such as from a photon), it can be excited from the valence band to the conduction band, leaving behind a hole in the valence band. The electron and hole can then interact through electrostatic attraction, forming an exciton.