A correlation between the carrier plasma temperature and optical gain in quasi-2D CdSe platelets

Recently, a report by Grim et al.1 highlighted the peculiar carrier dynamics in CdSe platelets as they were able to develop amplified spontaneous emission and lasing action under continuous wave (CW) conditions. An exciting prospect, yet remarkable as various literature reports , e.g. by Pelton et al.2, indicated the need for high exciton (X) densities of up to 50 X per platelet in order to observe any bleach of the band edge under pulsed excitation. Using transient absorption spectroscopy, we show that optical gain upon off-resonance (3.1 eV) pumping indeed only develops at exciton densities of up to 80 electron-hole (eh) pairs per platelet, a density suprisingly close to the Mott density. Reducing the excess energy per carrier by pumping the light-hole transition resonantly, we observe a decrease in the gain threshold to 20 eh-pairs. The observation of room-temperature lasing in the low density regime , the case of Grim et al. , can be matched with these observations if we assume a eh-plasma gain controlled by the temperature of the plasma itself. The latter can remain quite high due to surprisingly slow cooling. Remarkably, this scenario implies that the formation and existence of well-bound excitons is counterproductive for gain development as is evidenced by our data