exciton: a standalone construct

Yakov Frenkel introduced the exciton in 1931 (On the transformation of light into heat in solids. I) to explain how a crystal can absorb light without immediately freeing a charge: the absorbed energy travels as a neutral, mobile excitation in which an electron and the positively charged vacancy it leaves behind, called a hole, stay bound to each other by their electrical attraction. Gregory Wannier refined the picture in 1937 by treating the electron and hole as a loosely bound pair spread over many atoms, so the term now spans the tightly bound Frenkel exciton, localised on a single molecule or site, and the large-radius Wannier exciton found in conventional semiconductors. An exciton is therefore a quasiparticle, a composite excitation that moves and carries energy through a solid as if it were a single particle while carrying no net charge, and it holds the optical energy until the electron and hole recombine or the pair splits into free carriers. It is the standard construct for describing how insulators and semiconductors absorb and emit light at energies below the amount needed to create free charges, and its binding energy sets the threshold that separates a bound pair from an ionised electron and hole.[1][2][3]

Unit

A single quasiparticle excitation, a bound electron-hole pair, characterised by its binding energy and radius

Recovery means

Not applicable

Measured by

Identified spectroscopically by absorption or emission peaks below the band gap and by the binding energy separating bound pairs from free carriers