Observation of Broadband Optical Gain in Quasi-2D CdSe Nanoplatelets

Solution processable nanomaterials for photonic applications, in particular light emission and lasing, have received much attention in the past decade(s). A demonstration of ultralow continuous wave optical gain and lasing using CdSe platelets, quasi-2D materials in a colloidal dispersion, was the most recent milestone in this field. Until now, the optical gain in these quasi-2D systems was though to originate from the biexciton-to-exciton transition, much as is the case for 0D colloidal QDs. The net gain should therefore occur at biexciton carrier densties and moreover only develop in a narrow region below the lowest energy exciton feature. Here, we show that quasi-2D colloidal CdSe platelets are instead able to deliver broadband optical gain and that this gain is caused by the formation of a high temperature, unbound electron-hole gas, a first demonstration for a solution processable material. Moreover, the control over the plasma temperature indicates that the material gain can be tuned and even enhanced in different spectral regions. These results shed a new light on the role of excitons in solution processable inorganic gain media and pave the way for the development of more efficient gain media.

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