Rotating-polarization coherent anti-Stokes Raman spectroscopy, (RP-CARS) is a particular implementation of the coherent anti-Stokes Raman spectroscopy (CARS). RP-CARS takes advantage of polarization-dependent selection rules in order to gain information about molecule orientation anisotropy and direction within the optical point spread function.
Coherent anti-Stokes Raman spectroscopy (CARS) is a non-
linear process in which the energy difference of a pair of incoming photons
matches the energy of the vibrational mode of a molecular bond of interest.
This phonon population is coherently probed by a third photon and anti-
Stokes radiation is emitted.[1]
Polarization-dependent artifacts
In presence of molecular orientation anisotropy in the sample, CARS images often display artefacts due to polarization-dependent selection rules that affects the measured intensity with respect of the alignment between the polarization plane of the incident light and the main orientation plane of the molecular bonds.[2]
This is due because the four-wave mixing process is more efficient when the polarization plane of the incident light is aligned with the main orientation plane of the molecular vibrations.
RP-CARS
RP-CARS takes advantage of the polarization-dependent selection rules to detect the local microscopic orientation of the chemical bonds under investigation. By means of RP-CARS it is possible to visualize the
degree of orientation anisotropy of selected molecular bonds and to detect their average orientation direction.[3]
It is possible by continuously rotating the orientation of the polarization plane of the incident light with a rotating waveplate and then, sequentially, for each image pixel, analysing the orientation dependence of the CARS signal intensity. This allows measuring for each pixel the average-orientation plane of the molecular bonds of interest and the degree of this spatial anisotropy in the point-spread-function volume.[4]
Applications
Possible biomedical-oriented applications of this technique are related to the study of the myelin and myelopathies. Myelin is a highly ordered structure, in which many lipid-
enriched, densely compacted phospholipid bilayers are spirally rolled up around the
cylindrical axons. The linear acyl chains of the phospholipid molecules present a
perpendicular orientation with respect to the myelin surface. Therefore, in a myelinated nerve
fiber, a large number of molecular bonds are ordered around a radial axis of symmetry. Such a
strong molecular anisotropy and azimuthal symmetry make RP-CARS a suitable tool to investigate
neural white matter.[4]
