Autoconfocal transmission microscopy based on two-photon-induced photocurrent of Si photodiodes

We describe a simple, self-aligned confocal transmission microscopy technique based on two-photon-induced photocurrents of silicon photodiodes. Silicon detectors produce photocurrents in quadratic dependence on incident intensity under the pulsed illumination of light with wavelengths longer than 1.2 μm. We exploit this nonlinear process to reject out-of-focus background and perform depth-sectioning microscopic imaging. We demonstrate a comparable background rejection capability of the technique to linear confocal detection and present three-dimensional imaging in biological specimens.