Detection and imaging of single-molecules has become routine over the last 3 decades, with a wide range of applications, particularly in biology, but also in physics and chemistry. To date such imaging relies almost fully on detection of fluorescence as the red-shifted photons are easily counted and the background is virtually zero. Yet fluorescence imaging has its limits: first, only fluorescent (or labelled) samples can be perceived; second fluorescence is prone to bleaching; and finally fluorescence, spontaneous emission, is a slow, occurring on a timescale of nanoseconds. As a result one misses out on all coherences, vibrational dynamics, and ultrafast energy or charge transfer. At ICFO, we have developed a complementary approach, based on the detection of stimulated emission, with several advantages. All molecules can be stimulated to emit a photon, also those that do not fluoresce. Most importantly the stimulation is ultrafast, in fact instantaneous, such that bleaching is avoided, while femto-picosecond dynamics of excited states is traced. The downside is that the stimulating laser beam also produces a significant amount of background light. We have overcome this challenge by using synchronized pump and probe pulses from the same broadband laser, ultrafast modulation, balanced and phase sensitive detection. In a first application, we show stimulated emission imaging of individual colloidal quantum dots 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 disentangle the ultrafast charge trajectories in the excited state and determine the populations that experience stimulated emission, spontaneous emission, and excited-state absorption processes. Next we would like to extend our stimulated microscopy of single quantum systems to molecules and biomolecular complexes.
Reference
Piatkowski L, Accanto N, Calbris G, Christodoulou S, Moreels I & van Hulst NF 2019, ‘Ultrafast Stimulated Emission Microscopy of Single Nanocrystals’, Science 366, 1240-1243.
Highlighted by Ian Osborne, “Tracking excitations” Science 366 p1210 (2019), and Belle Dumé, “Nanomaterials: Stimulated emission detection images single nanocrystals at high speed”, Physics World 12 Dec 2019.
