Proceedings of nanoGe Fall Meeting 2018 (NFM18)
DOI: https://doi.org/10.29363/nanoge.nfm.2018.069
Publication date: 6th July 2018
Two-dimensional (2D) materials have received much attention in the past years for a wide variety of photonic applications due to their pronounced excitonic features leading to unique properties in terms of light emission. However, only a few studies focus on the use of these materials for light amplification or net optical gain development and the ensuing high carrier density photo-physics. The beneficial nature of the strong excitonic effects on optical gain remain hence unquantified and , despite the large binding energies, it remains unclear what the involvement of is at the concomitant high carrier densities. Here, we use colloidal 2D CdSe nanoplatelets as a model system and show, using a quantitative and combinatory approach to ultrafast spectroscopy, that several distinct and carrier density-dependent optical gain regimes exist for these materials. At low density, optical gain is found to originate from excitonic molecules delivering large material gains up to 20.000 cm-1, yet with an Auger limited lifetime of few hundred picoseconds. At increasing pair density, we observe a surprising transition to a combined regime of blue-shifted and disruptively large optical gain, combined with the typical exciton mediated gain. We show that this peculiar situation originates from a carrier cooling bottleneck at high density. Surprisingly, the insulating (multi-)exciton gas is found to co-exist with the conductive phase in a density regime nearly one order of magnitude beyond the expected Mott transition. The ensuing exciton ground state absorption even counter-acts the development of net optical gain in certain spectral regions. Our results shed a new light on the disruptive photo-physics of high binding energy excitons in strongly excited 2D materials and pave the way for the development of more efficient broadband optical gain media and/or high density excitonic devices such as polariton lasers.
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