Yamanaka, M., Kawano, S., Fujita, K., Smith, N. I. and Kawata, S.
Beyond the diffraction-limit biological imaging by saturated excitation microscopy. Journal of Biomedical Optics 13: 050507 (2008).

We demonstrate high-resolution fluorescence imaging in biological samples by saturated excitation (SAX) microscopy. In this technique, we saturate the population of fluorescence molecules at the excited state with high excitation intensity to induce strong nonlinear fluorescence responses in the center of laser focus, which contributes the improvement of the spatial resolution in three dimensions. Using SAX microscopy, we observed stained microtubules in HeLa cells with improved spatial resolution. We also measured the relation of the fluorescence and excitation intensity with several kinds of fluorescence dyes and, in the results, confirmed that SAX microscopy has the potential to observe any kind of fluorescence samples in current usage.