Coherent anti-Stokes Raman (CARS) Microscopy part 1: Introduction Coherent anti-Stokes Raman Scattering (CARS) microscopy is a high speed chemically-specific microscopy with high sensitivity, which builds on the power of Raman spectroscopy. In the traditional Raman process, some laser light is shifted in frequency when incident on a sample of interest. This frequency shift is equal to the vibrational frequency of the atomic bond, υVIB , so samples can be chemically characterized by measuring these frequencies. Each atomic bond has many vibrational modes – including stretching and bending. The benefits of Raman spectroscopy are that any type of material can be investigated with the technique. Complex molecules with minor differences can be differentiated thanks to the wealth of information in the spectrum. The downside of Raman spectroscopy is that a spectrum can take up to a minute to acquire. Raman process: light scatters with either no change in frequency (Rayleigh) or an increase (anti-Stokes) or decrease (Stokes). In the CARS process, two lasers excite the atomic bond. The laser frequencies are chosen such that the difference between them is equal to the vibrational frequency of the atomic bond : υVIB = υP – υS. This resonant process is 4 or 5 orders of magnitude more efficient than normal Raman, so CARS images can be taken at video rates. CARS process : This four-wave mixing process requires three photons to be absorbed, and one emitted (anti-Stokes). ►(part 2): Equipment and methods ◄back to Optical microscopy / spectroscopy research