Abstract
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.