= Biexciton
A <biexciton> is a two-<Electron>, two-<electron hole> excitation. If $E_X$ and $E_{XX}$ are total excitation <energies> relative to the empty emitter, the cascade $XX\to X\to0$ emits photons of <energies> $E_{XX}-E_X$ and $E_X$. Define its binding energy by $B_{XX}=2E_X-E_{XX}$. The first cascade photon then has <energy> $E_X-B_{XX}$. 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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