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Ultrafast stimulated emission microscopy of single nanocrystals

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Science  06 Dec 2019:
Vol. 366, Issue 6470, pp. 1240-1243
DOI: 10.1126/science.aay1821

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Tracking excitations

Illumination can be used to excite a sample from its ground state to a number of excited states. Typically, however, the details of the excitation dynamics are hidden from view because they decay so fast. Piatkowski et al. combined pump-probe transient absorption and two-pulse photoluminescence correlation spectroscopy, allowing them to assess stimulated emission and ground-state bleaching contributions to the transient absorption signal. This approach provides a window on the excitation dynamics within single nanocrystals and should also be useful for ultrafast nanocharacterization of complex samples.

Science, this issue p. 1240

Abstract

Single-molecule detection is a powerful method used to distinguish different species and follow time trajectories within the ensemble average. However, such detection capability requires efficient emitters and is prone to photobleaching, and the slow, nanosecond spontaneous emission process only reports on the lowest excited state. We demonstrate direct detection of stimulated emission from individual colloidal nanocrystals at room temperature while simultaneously recording the depleted spontaneous emission, enabling us to trace the carrier population through the entire photocycle. By capturing the femtosecond evolution of the stimulated emission signal, together with the nanosecond fluorescence, we can disentangle the ultrafast charge trajectories in the excited state and determine the populations that experience stimulated emission, spontaneous emission, and excited-state absorption processes.

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