Near-field light emission from dark states excitonic occupations

We theoretically analyze the carrier capture and distribution among the available energy levels of a symmetric semiconductor quantum dot under continuous-wave excitation resonant with the barrier energy levels. At low temperature, all the dot level occupations but one monotonically decrease with energy. The uncovered exception, corresponding to the second dark energy level, displays a carrier density exceeding that of the lowest level by more than a factor two. The root cause is not radiative recombination before relaxation, but instead, carrier trapping due to the symmetry-induced suppression of radiative recombination. Such a behavior can be observed by collection-mode near-field optical microscopy