CARS stands for Coherent Anti-Stokes Raman Scattering. It is a method of using light in the visible and near-infrared region to probe molecular resonances that would normally be in the mid- to far-infrared region. This is useful because liquid water is largely opaque to these wavelengths, so they can not be readily probed except to very shallow depths.

CARS is a nonlinear technique that utilizes two overlapped laser beams, a "pump" beam and a "Stokes" beam, the frequencies of which are separated by the resonance frequency of interest. When illuminated by these beams of sufficiently high peak intensity, a resonant molecule will emit an anti-Stokes photon, which can be measured to find the density of the molecular species.

We are producing a new type of instrument which will use what we call Nonlinear Interferometric Vibrational Imaging (NIVI). Rather than just generating a single CARS signal in the sample and detecting the photons, we will actually create two separate CARS signals from the same pump and Stokes signal in two different samples. One of these is a "reference" sample with the molecule in it, while the other is the tissue to be measured. Two anti-Stokes signals will be generated, and interfered together. By detecting this interference, we can infer the presence and 3D location of a molecule in tissue.

This is a diagram of the laser system used in the NIVI project: