Fig. 1.

Principle of Raman scattering mechanisms of (a) spontaneous Raman and (b) narrowband CARS shown by Jablonski diagram (energy level diagram). Arrows represent photons (longer length denotes higher photon energy), $V$ is the vibrational level, ${\mathrm{\Omega }}_{\mathrm{R}}$ is the resonant vibrational frequency of a given vibrational mode. For CARS, the laser source consists of two time-locked pulse trains of different wavelengths (pump + stokes) such that the difference in photon energy corresponds to the vibrational mode to be imaged. The anti-Stokes photon is detected indicating the strength of the signal at a given vibrational mode.