Single Molecule Imaging Technique For Single Nanocrystals

A new single molecule imaging technique that does not rely on fluorescent emitters could have many applications in nanotechnology, photonics, and photovoltaics. At room temperature, the technique detects stimulated emissions from single quantum dots. Because of its speed, it is possible to track charge-carrier populations throughout the absorption and emission cycle.

In biology, single molecule imaging techniques are widely used. They have so far relied solely on detecting spontaneous fluorescence from the sample being imaged. Researchers typically excite the sample at wavelengths where it absorbs light and then detect redshifted (lower energy) fluorescence signals in these fluorescence-based techniques. This makes it relatively simple to remove the excitation beam’s background light and detect only the fluorescence.

Techniques for detecting stimulated emission (SE) have several advantages. SE is present in all molecules, including those that do not fluoresce. SE also avoids bleaching because the molecule spends little time in the excited state and is much faster because the light is emitted on femtosecond timescales, implying that SE can provide information on excited state dynamics. The disadvantage is that the laser beam that drives the stimulated emission emits much background light. Researchers have solved this problem by imaging individual colloidal nanocrystals (single molecule imaging technique), or quantum dots, with ultrashort laser pulses. (QDs). The team demonstrated that using laser pulses could force individual QDs to emit light via a SE process rather than waiting for them to do so spontaneously.

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