All-optical control of exciton flow in a colloidal quantum well complex

Excitonics: all optical control A convenient all-optical way to control exciton flow between two different colloidal quantum wells (CQWs) at room temperature has been demonstrated by scientists in Singapore. Junhong Yu and coworkers from Nanyang Technological University have shown how the combination of stimulated emission and Forster resonance energy transfer (FRET) can strongly manipulate the flow of excitons between closely spaced donor CdSe core-only CQWs and acceptor CdS/CdSe/Cds core-shell CQWs. A mixed solution of these CQWs in the molar ratio of 4:1 donor to acceptor was dried onto the surface of a hollow quartz capillary tube and excited by sub-nanosecond laser pulses. Experimental data at different excitation fluences clearly shows that stimulated emission can modulate the population of excited donors and unexcited acceptors which in turn affects the FRET process between the donors and acceptors, leading to exciton flow control. Excitonics, an alternative to romising for processing information since semiconductor electronics is rapidly approaching the end of Moore's law. Currently, the development of excitonic devices, where exciton flow is controlled, is mainly focused on electric-field modulation or exciton polaritons in high-Q cavities. Here, we show an all-optical strategy to manipulate the exciton flow in a binary colloidal quantum well complex through mediation of the Fo''rster resonance energy transfer (FRET) by stimulated emission. In the spontaneous emission regime, FRET naturally occurs between a donor and an acceptor. In contrast, upon stronger excitation, the ultrafast consumption of excitons by stimulated emission effectively engineers the excitonic flow from the donors to the acceptors. Specifically, the acceptors' stimulated emission significantly accelerates the exciton flow, while the donors' stimulated emission almost stops this process. On this basis, a FRET-coupled rate equation model is derived to understand the controllable exciton flow using the density of the excited donors and the unexcited acceptors. The results will provide an effective all-optical route for realizing excitonic devices under room temperature operation.