State Filling and Stimulated Emission by Colloidal InP/ZnSe Core/Shell Quantum Dots

Colloidal InP-based quantum dots (QDs) have been widely studied for luminescent color conversion or electroluminescence, yet the nature of the emitting state remains a matter of debate and reports on stimulated emission by these materials are nearly absent. Here, we investigate the properties of photo-excited InP/ZnSe QDs using femtosecond transient absorption spectroscopy. We show that the evolution of the band-edge bleach with increasing exciton number can be interpreted as state filling of the conduction- and valence-band edge states by delocalized electrons and holes. In line with this interpretation, net stimulated emission is observed once the average exciton number exceeds 1. We account for this lower-than-expected gain threshold and for the spectral properties of the gain band by reckoning that the Stokes shift between band-edge absorption and emission is the dominant spectral shift. The underlying exciton-phonon coupling leads to a stimulated emission mechanism, where also single excitons could lead to net optical gain. To fully profit from this advantageous stimulated emission scheme, we argue that InP/ZnSe QDs with more narrow emission lines are needed and spurious trapping of electron-hole pairs at high exciton numbers must be suppressed, for example by better controlling the composition of the core/shell interface.