Practice question

Two-photon microscopy excites fluorophores through simultaneous absorption of two near-infrared photons, each providing half energy needed for transition. Longer wavelengths near 700-1000 nanometers scatter less, penetrate deeper, and generate fluorescence exclusively at focal plane where photon density reaches threshold, intrinsically providing optical sectioning without pinhole. Since out-of-focus fluorophores remain unexcited, photobleaching and phototoxicity diminish markedly. Compared to single-photon confocal setups using visible lasers, this nonlinear excitation preserves viability during prolonged live imaging of thick specimens, brain slices, and developing embryos while retaining subcellular detail.