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

Colloidal InP-based quantum dots (QDs) are 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, the properties of photo-excited InP/ZnSe QDs are investigated using femtosecond transient absorption spectroscopy. It is shown 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. The authors account for this lower-than-expected gain threshold and 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 can lead to net optical gain. To fully profit from this advantageous stimulated emission scheme, it is argued 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.