Intensity Suppression of Harmonic Generation Signals Based on Ground State Depletion

Optical nonlinearity can be applied to various fields. Nonlinearity such as multiphoton fluorescence and harmonic generation has been widely used for biomedical microscopy. Through the properties of the excited signals, the structures and molecular composition of bio-tissues can be revealed. Due to the extremely high intensity required for nonlinear signal excitation, nonlinear microscopy can provide optical sectioning power so that it is suitable for thick-tissue applications. To further improve the resolution, stimulated emission depletion microscopy (STED microscopy) has been applied to two-photon fluorescence microscopy. By suppressing the fluorescence through stimulated emission, the excited fluorescence point spread function (PSF) can be effectively reduced so that the resolution can be increased to higher than diffraction limit. However, because no real state transition is involved in production of harmonic generation signals, the harmonic generation signals cannot be suppressed through stimulated emission. Other ways has to be discovered to achieve the signal suppression.Although the production of harmonic generation signals has no real-state involved, in the previousresearches, the harmonic generation signals have been proved to be enhanced through absorption when the materials contain real levels which match the signal-photon or multiphoton absorption. Making use of the positive correlation between absorption and harmonic generation intensity, suppression of the harmonic generation can be achieved by absorption suppression. In this project, research of suppressing harmonic generation signals through ground-state depletion will be carried 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 be suppressed through ground-state depletion and the suppression of third harmonic generation can thus be achieved. From simulations and experiments, this project aims to demonstrate this idea and apply this concept to third harmonic generation microscopy to improve the spatial resolution.