Vibrational phase contrast microscopy by use of coherent anti-Stokes Raman scattering

In biological samples the resonant coherent anti-Stokes Raman scattering signal of less abundant constituents can be overwhelmed by the nonresonant background, preventing detection of those molecules. We demonstrate a method to obtain the phase of the oscillators in the focal volume that allows discrimination of those hidden molecules. The phase is measured with respect to the local excitation fields using a cascaded phase-preserving chain. It is measured point-by-point and takes into account refractive index changes in the sample, phase curvature over the field-of-view, and interferometric instabilities. The detection of the phase of the vibrational motion can be regarded as a vibrational extension of the linear (refractive index) phase contrast microscopy introduced by Zernike around 1933.