Rotating-polarization coherent anti-Stokes Raman spectroscopy

Rotating-polarization coherent anti-Stokes Raman spectroscopy, as known as RP-CARS, is a particular implementation of the Coherent anti-Stokes Raman spectroscopy microscopy (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.

CARS process

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.

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.[1] 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.[2] 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.[3]

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.[3]

References

  1. ↑ Bélanger, E., et al. "Quantitative myelin imaging with coherent anti-Stokes Raman scattering microscopy: alleviating the excitation polarization dependence with circularly polarized laser beams." Opt. Express 17.21 (2009): 18419-18432.
  2. ↑ Giuseppe de Vito, Angelo Bifone, and Vincenzo Piazza. "Rotating-polarization CARS microscopy: combining chemical and molecular orientation sensitivity." Optics express 20.28 (2012): 29369-29377.
  3. 1 2 Giuseppe de Vito and Vincenzo Piazza, “Fast signal analysis in Rotating-Polarization CARS microscopy,” Optical Data Processing and Storage, , (2014)

See also

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