Multi-dimensional differential imaging with FE-CARS microscopy

Focus-engineered coherent anti-Stokes Raman scattering (FE-CARS) microscopy is used to visualize microscopic samples with differential imaging contrast in both longitudinal and lateral directions. Multi-dimensional contrast is achieved by applying a one-dimensional π-phase step in the transverse beam profile of the Stokes beam. The observed longitudinal differential contrast results from fortuitous phase-matching at axial interfaces and is highly sensitive to the resonant part of the third-order susceptibility. Based on phase-sensitive recordings of the vibrational spectrum of dimethyl sulfoxide in the CH3-stretching range, it is shown that FE-CARS uniquely probes the spectral phase of the Raman bands of the sample.