Selective suppression of CARS signal with two competing stimulated Raman scattering processes.

Coherent anti-Stokes Raman scattering (CARS) spectroscopy and microscopy have been used in studying the structure and dynamics of a wide range of chemical and biological systems. However, the spatial resolution of CARS microscopy is still limited by the diffraction barrier, and hence a suitable scheme to selectively switch off the CARS imaging signal is essential for super-resolution CARS microscopy. Here, we present theoretical descriptions about three different ways to selectively suppress the pump-Stokes-pump two-beam CARS signal by employing three-beam double stimulated Raman scattering (SRS) schemes. Using a semiclassical theory for the interaction of radiation with the Raman-active molecule, we obtain coupled differential equations for the intensities of the pump, Stokes, depletion, and the generated CARS signal fields. We find approximate solutions of these coupled differential equations. They are then used to show that the pump-Stokes-pump CARS signal can be selectively suppressed by increasing the added depletion beam intensity, when the three injected beam frequencies are tuned in such a way that they can induce two SRS processes simultaneously. To show that these switching-off methods can be used to develop super-resolution CARS imaging techniques, we numerically calculate the full-width-at-half-maximum of the CARS imaging point spread function assuming that the spatial profiles of the pump and Stokes beams are Gaussian functions and that the spatial profile of the depletion beam is doughnut-shaped. We anticipate that the proposed selective CARS suppression schemes will be of use in developing super-resolution, label-free CARS microscopy.