Charge Carrier Cooling Bottleneck Opens Up Nonexcitonic Gain Mechanisms in Colloidal CdSe Quantum Wells

Ultrathin two-dimensional (2D) materials have received much attention in the past years for a wide variety of photonic applications because of their pronounced room temperature excitonic features, leading to unique properties in terms of light-matter interaction. However, only a few studies focus on light amplification and the complex photophysics at high excitation density. The beneficial nature of strong excitonic effects on optical gain remain hence unquantified, and despite the increased binding energies of the excitonic species, it remains unclear what the involvement of 2D excitons would be in optical gain. Here, we use colloidal CdSe nanoplatelets as a model system for colloidal 2D materials and show, using a quantitative and combinatory approach to ultrafast spectroscopy, that several excitation density-dependent optical gain regimes exist. At low density, optical gain originates from excitonic molecules delivering large material gains up to 20 000 cm(-1) with an Auger limited lifetime of a few hundred picoseconds. At increasing pair density, we observe a persistence of this excitonic gain regime and the unexpected coexistence of blue-shifted and significantly enhanced optical gain up to 105 cm(-1). We show that this peculiar situation originates from a carrier cooling bottleneck at high density that limits further exciton formation from unbound charge carriers. The insulating (multi-)exciton gas is found to coexist with the conductive phase, indicating the absence of a full Mott transition. Our results shed a new light on the photophysics of excitons in strongly excited 2D materials and pave the way for the development of more efficient (broadband) optical gain media and/or high exciton density applications.