Biexcitons

A biexciton is a bound state consisting of two excitons, i.e., two electrons and two holes held together by Coulomb interactions in a semiconductor. An exciton is a bound electron–hole pair, so a biexciton can be viewed as a four-particle complex whose total energy lies below the energy of two isolated excitons. The binding energy of a biexciton is the difference between twice the single-exciton energy and the biexciton energy.

Biexcitons form most readily in systems with strong Coulomb interactions and reduced dielectric screening, such as low-dimensional semiconductors. They are more easily observed in quantum wells, quantum wires, and especially two-dimensional materials like transition metal dichalcogenides, where confinement and reduced screening enhance exciton and biexciton binding. In quantum dots, biexcitons correspond to two electron–hole pairs occupying discrete levels, with emission governed by radiative recombination pathways.

Experimental signatures of biexcitons appear primarily in optical spectroscopy. In photoluminescence, a biexciton manifests as a peak at an energy lower than the single-exciton line by the biexciton binding energy. Temperature, excitation density, and polarization can influence the line shape and intensity. In some systems, biexciton–exciton cascades can produce correlated or entangled photon pairs, motivating interest for quantum light sources. Observations in two-dimensional materials such as MoS2, WS2, WSe2, and MoSe2 have highlighted large binding energies compared with bulk semiconductors.

Theoretically, biexcitons are treated as a four-body problem using effective mass models and dielectric screening. Methods include variational calculations, quantum Monte Carlo, and solutions to extended Bethe–Salpeter-type equations. Challenges arise from complex many-body interactions, phonons, and disorder, which influence binding strength and stability.