State filling in InAs quantum-dot laser structures

We have measured the passive modal absorption, modal gain, and spontaneous emission spectra of a quantum-dot system where the inhomogeneous broadening is sufficiently small so that the ground- and excited-state transitions can be spectrally resolved. Absorption by ground- and excited-state transitions is in the ratio 1:1.88 which is close to the ratio of 2 expected for dots with similar dimensions in two directions. The absorption cross section per dot is measured to be 1.1 × 10^-14 cm². Optical gain from the ground-state saturates with current at a maximum value of one third of that predicted from the measured absorption if the system is fully inverted. The measured population inversion factor spectrum shows that the carrier distributions cannot be described only by a single global Fermi distribution and that the system is not in overall equilibrium. However, using parameters obtained by fitting the absorption spectrum, we find that for these particular samples the inversion factor spectrum can be described by a possible model where the ground- and excited-state occupancies are each described by a Fermi distribution but with different quasi-Fermi energy separations. We speculate that photon mediation within the homogeneous linewidth could be one possible process which establishes quasi-equilibrium within each of the ground- and excited-state inhomogeneous distributions.