Sub-Diffraction Resolution CARS Microscopy via Rabi Modulation: a Theoretical Investigation

Label-free imaging modalities, such as coherent anti-Stokes Raman spectroscopy (CARS) microscopy, have demonstrated high contrast levels and video rate image acquisition times. CARS is a nonlinear process where two lasers coherently populate a vibrational state due to a two-photon resonant interaction. A probe laser coherently depopulates the excited state, generating light at the anti-Stokes frequency, which is collected and imaged. We show that the spatial resolution of CARS can be enhanced by modulating the ground state coherence. The resulting modulation of the CARS emission intensity introduces side bands at the Rabi frequency, to the CARS spectra. The additional laser when focused to diffraction limited Gaussian shaped intensity profile will generate a distribution of Rabi side bands that depend on the radial distance from the center of the focus. By accurately measuring the amplitudes of the side bands as the laser focus is scanned, a sub-diffraction resolution image can be constructed.