Frequency-upconverted stimulated emission by simultaneous five-photon absorption

Since the invention of the laser in 1960, multiphoton effects have become useful in techniques for real applications as well as conceptual predictions. Here, we report the first experimental observation of frequency-upconverted stimulated emission from a novel fluorophore through simultaneous five-photon absorption. Compared to lower-order nonlinear absorption, the fifth-order dependence on input light intensity of the five-photon absorption process will provide much stronger spatial confinement, allowing the achievement of a much higher contrast in imaging. Stimulated emission has also been achieved by the absorption of two to four photons under near-infrared laser excitation, making this gain medium a promising multiphoton imaging probe with attractive features, including the absence of autofluorescence from biological samples, large penetration depth, and improved sensitivity and resolution. Through simultaneous five-photon absorption, scientists observe efficient frequency-upconverted stimulated emission from the mid- or near-infrared to the visible region in a novel fluorophore. The fifth-order dependence on the input light intensity provides much stronger spatial confinement than lower-order nonlinear absorption, thus offering much higher contrast for imaging.