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. Laser scanning confocal microscopy (LSCM) is capable of producing high-contrast, high-resolution images of biological and material specimens with optical sectioning capability. The improved image contrast and depth sectioning in LSCM is enabled by a physical pinhole placed in front of the image plane, which allows in-focus portion of light to be measured, while rejecting stray light from out-of-focus background [1]. LSCM can be implemented either in reflection or transmission modes. In the reflection or epi-detection mode, the light collected from the specimen is typically de-scanned to pass through