Excitonexciton

Excitonexciton is a term sometimes used to describe the interaction between two excitons, and in many contexts it refers to the biexciton, a bound state of two excitons. An exciton is a bound electron–hole pair in a semiconductor or insulator, while a biexciton consists of two electrons and two holes bound together by Coulomb forces to form a discrete, quasi-particle complex.

Formation and binding energy: Biexcitons form under intense optical excitation or in nanostructures where exciton density is high. The binding energy (Eb) varies with material and dimensionality: in bulk semiconductors it is typically a few millielectronvolts; in quantum wells it can reach several meV; in two-dimensional materials such as monolayer transition metal dichalcogenides, biexciton binding energies can be tens of meV.

Observables and dynamics: Biexciton signatures appear as emission lines at energies below the exciton line, due to recombination of constituents within the bound state. Formation and decay occur on ultrafast timescales, ranging from sub-picoseconds to several picoseconds for formation, with longer decay in some materials. Experimental methods include photoluminescence, pump–probe spectroscopy, and two-photon excitation studies.

Relevance and applications: Understanding exciton–exciton interactions is important for the physics of many-body effects in semiconductors and for the design of high-density light sources. Biexcitons are also explored as potential sources of entangled photon pairs via biexciton–exciton cascades in quantum dots and certain two-dimensional materials. Nonetheless, strong exciton–exciton interactions at high density can lead to nonradiative losses such as exciton-exciton annihilation, impacting device performance.