We review two approaches to achieving sub-diffraction-limited resolution coherent anti-Stokes Raman scattering (CARS) microscopy (Beeker et al., Opt. Express, 2009, 17, 22632 and Beeker et al., J. Herek, Phys. Rev. A, 2010, 81, 012507). We performed a numerical investigation, based on the density matrix model, of the CARS emission process and identified two modified CARS experiments that lead to sub-diffraction-limited resolution images. At the heart of both processes is the spatial manipulation of the coherence between the ground state and the vibrational state being probed by the CARS process via a control state and a control laser that is resonant with the ground state to control state transition. We find two possible regimes of operation: in the first regime, the control and vibrational states are coupled via incoherent processes so that the populations of the two states reach equilibrium very quickly compared to the relevant coherence times. Under these conditions, pre-populating the control state provides a saturable suppression of the coherence between the ground state and the vibrational state, suppressing CARS emission. By using a donut mode to pre-populate the control state, CARS is suppressed everywhere but the central node, allowing sub-diffraction-limited resolution imaging. In the second regime, the control state has a rather long coherence lifetime, and the resonant laser drives Rabi oscillations that periodically deplete the ground state. As a result, the CARS emission process is amplitude-modulated, which appear as sidebands on the CARS spectrum. By a process of spectral resolution and trilateration, sub-diffraction-limited resolution images can be obtained.
- Coherent anti-Stokes Raman scattering
- Density matrix