Coherent anti-Stokes Raman Scattering microscopy

Duncan and coworkers reported coherent anti-Stokes Raman Scattering (CARS) imaging of D2O in onion cells using a dye laser system in 1982. Three laser fields at the pump, Stokes and probe frequencies interact with a medium to generate a new field at the anti-Stokes frequency in CARS. The CARS signal can be significantly enhanced when the beating frequency is in resonance with a molecular vibration. Other coherent Stokes Raman processes occur simultaneously, including coherent Stokes Raman scattering (CSRS), stimulated Raman gain (SRG), stimulated Raman loss (SRL), and Raman-induced Kerr effect (RIKE). The pump and Stokes beams in CARS microscopy are tightly focused into a sample and a CARS image is generated by scanning either the sample or the laser beams. This microscopy permits nondestructive molecular imaging without any labeling. The large and polarization-dependent CARS signal from the lamellar myelin membrane opens up a new avenue for the study of multiple sclerosis.