We present spectral interferometric four-wave mixing (FWM) microscopy with a nearly shot-noise limited sensitivity and with the capability of separating FWM signals from fluorescence signals. We analyze the requirements for obtaining the shot-noise limited sensitivity and experimentally achieve the sensitivity that is only 4-dB lower than the shot-noise limit. Moreover, we show that only FWM signals can be extracted through the Fourier filtering even when the FWM spectrum is overlapped and overwhelmed by the fluorescence spectrum. We demonstrate simultaneous acquisition of FWM and two-photon excited fluorescence images of fluorescent monodispersed polystyrene microspheres.

We propose that four-wave mixing (FWM) microscopy can be applied to three-dimensional mapping of refractive index (RI) structure inside transparent samples. We derive an analytical relationship between the RI and the intensity of the FWM signal that is due to nonresonant optical nonlinearity. By using the relationship, the RI profile can be directly and quantitatively obtained from the intensity distribution of the FWM signal. We experimentally demonstrate the RI profiling of a phase grating fabricated in a non-alkali glass.

We present a simple and highly sensitive optical detection method based on two-beam interferometry for application to coherent nonlinear optical microscopy (CNOM). The theoretical sensitivity of this method is higher than that of conventional spectral interferometry (SI). We experimentally applied this technique to stimulated parametric emission (SPE) microscopy and achieved a high sensitivity that is only 4 or 5 dB lower than that of a theoretical shot noise limit. In order to validate the practical applicability of this technique, we demonstrated a noise reduction experiment in the observation of a plant cell with an SPE microscope.

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The ability to perform high-resolution unstained live imaging is very important to in vivo study of cell structures and functions. Stimulated parametric emission (SPE) microscopy is a nonlinear-optical microscopy based on ultra-fast electronic nonlinear-optical responses. For the first time, we have successfully applied this technique to archive three-dimensional (3D) images of unstained sub-cellular structures, such as, microtubules, nuclei, nucleoli, etc. in live cells. Observation of a complete cell division confirms the ability of SPE microscopy for long time-scale imaging.