CARS is a nonlinear scattering process. The interaction of two (or three) different lasers (pump-, probe- and Stokes–laser) with the molecules creates a coherent signal which will be resonantly amplified when the wavelength difference between the two lasers (pump and Stokes) matches a Raman–mode (see figure). The CARS signal is very intense compared to a normal spontaneous Raman signal (10⁴ – 10⁶ more intensive). Being coherent it is directed into space making the collection of almost 100% of the signal possible. The blue shift of the signal relative to the excitation lasers makes the filtering of the CARS signal from disturbing interferences (induced single photon fluorescence, elastic scattered light) relatively easy.

Due to the intense signal generated in a CARS process the method suits perfectly to acquire data in very short times. Combining it with a laser scanning microscope allows to record in a matter of seconds full images based on vibrational (chemical) contrast.