Laser scanning dark-field coherent anti-Stokes Raman scattering (DF-CARS): a numerical study

We present and model a dark-field illumination scheme for coherent anti-Stokes Raman scattering (DF-CARS) that highlights the interfaces of an object with chemical sensitivity. The proposed DF-CARS scheme uses dedicated arrangements of the pump k_p1, Stokes k_S and probe k_p2 beams' k-wave-vectors to address the sample's interfaces along the x, y or z axis. The arrangements of the incident k-wave-vectors are derived from the Ewald sphere representation of the outgoing anti-Stokes radiation and the effective CARS excitation wave-vector k_eff = k_p1 + k_p2 - k_S under the intention to avoid probing the object frequency K(0,0,0), i.e., the contribution of a homogeneous sample (dark-field configuration). We suggest a possible experimental realization using simple masks placed in the back pupil of the excitation microscope objective lens. Applying a full vectorial model, the proposed experimental implementation is numerically investigated on grounds of the Debye-Wolff integral and dynadic Green function to confirm the predicted chemical interface contrast.