Here m 0 is the electron mass in vacuum, c is the speed of light, n is the refraction index, and ω ω i is the frequency of emitted light. Typical values of τ τ i are about nanoseconds, and these short radiative lifetimes favor the radiative recombination of excitons over the non-radiative one. When quantum yield of radiative emission is high, the process can be considered as resonance fluorescence.