Stimulated emission (SE) can now be used to detect the signal-to-noise ratio (SNR) of fluorescence and convert it to a coherent signal. A technique developed by a team from National Yang-Ming University’s Institute of Biophotonics Engineering induces the electronic transition of an excited fluorophore optical signal into SE before its energy dissipates through other radiative or nonradiative decay processes. This makes the SE signals dependent on the population of the excited states and the intensity of the stimulation beam. The signals scale linearly with both excitation and stimulation beams, which give it an overall quadratic power dependence. Scientists can now use stimulated emission to detect the signal-to-noise of fluorescence. Courtesy of National Yang-Ming University. In this study, the researchers used destructive interference, generated by a Michelson interferometer, to reduce the high direct current (DC) background signal from the stimulation beam, thus increasing the SNR. This method reduces the direct current signal but not the SE signal. To improve the detection limit, the team reduced the stimulation beam power from 2 mW to 0.2 mW. In the past, the researchers established SE-based detection in long-working-distance settings by implementing the method of pump-probe with lock-in detection. This scheme can easily be used for other applications, they said, including the acquisition and reconstruction of fluorescence lifetime images by introducing an electronic delay between the pump and probe beams. More recently, SE has been harnessed to acquire fluorescence lifetime data, improve the spatial resolution of micrographs, interrogate dark fluorophores, and detect fluorophores over a long working distance. Now in development is the technique's potential for high-resolution 3-D biological imaging by optical coherence tomography, using fluorescence as a viable and unique contrast. The work was funded by the National Science Council of Taiwan. It is published in Biomedical Optics & Medical Imaging. For more information, visit www.ym.edu.tw.