Superresolution Harmonic Generation Microscopy Based on Ground State Depletion(1/2)

Optical nonlinearity can be applied to various fields. Nonlinearity such as multiphoton fluorescenceand harmonic generation has been widely used for biomedical microscopy. Through the propertiesof the excited signals, the structures and molecular composition of bio-tissues can be revealed. Dueto the extremely high intensity required for nonlinear signal excitation, nonlinear microscopy canprovide optical sectioning power so that it is suitable for thick-tissue applications. To furtherimprove the resolution, stimulated emission depletion microscopy (STED microscopy) has beenapplied to two-photon fluorescence microscopy. By suppressing the fluorescence through stimulatedemission, the excited fluorescence point spread function (PSF) can be effectively reduced so that theresolution can be increased to higher than diffraction limit. However, because no real state transitionis involved in production of harmonic generation signals, the harmonic generation signals cannot besuppressed through stimulated emission. Other ways has to be discovered to achieve the signalsuppression. Although the production of harmonic generation signals has no real-state involved, inthe previous researches, the harmonic generation signals have been proved to be enhanced throughabsorption when the materials contain real levels which match the signal-photon or multiphotonabsorption. Making use of the positive correlation between absorption and harmonic generationintensity, suppression of the harmonic generation can be achieved by absorption suppression. In thisproject, research of suppressing harmonic generation signals through ground-state depletion will becarried out. By choosing proper excitation wavelength and nonlinear material,absorption-enhancement third harmonic generation signals should be observed from the materials.In addition, if the material contains dart-state or triplet-state, the absorption of the material can besuppressed through ground-state depletion and the suppression of third harmonic generation canthus be achieved. From simulations and experiments, this project aims to demonstrate this idea andapply this concept to third harmonic generation microscopy to improve the spatial resolution.