Rotating-polarization coherent anti-Stokes Raman spectroscopy
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.
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.[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]
References
- de Vito, Giuseppe; Bifone, Angelo; Vincenzo, Piazza (2012). "Rotating-polarization CARS microscopy: combining chemical and molecular orientation sensitivity". Optics Express. OSA Publishing. 20 (28): 29369–29377. doi:10.1364/OE.20.029369.
- Bélanger, E.; Bégin, S.; Laffray, S.; De Koninck, Y.; Vallée, R.; Côté, D. (2009). "Quantitative myelin imaging with coherent anti-Stokes Raman scattering microscopy: alleviating the excitation polarization dependence with circularly polarized laser beams". Optics Express. 17 (21): 18419. Bibcode:2009OExpr..1718419B. doi:10.1364/OE.17.018419. ISSN 1094-4087.
- de Vito, Giuseppe; Bifone, Angelo; Piazza, Vincenzo (2012). "Rotating-polarization CARS microscopy: combining chemical and molecular orientation sensitivity". Optics Express. 20 (28): 29369. Bibcode:2012OExpr..2029369D. doi:10.1364/OE.20.029369. ISSN 1094-4087.
- de Vito, Giuseppe; Piazza, Vincenzo (2014). "Fast signal analysis in Rotating-Polarization CARS microscopy". Optical Data Processing and Storage. 1 (1). doi:10.2478/odps-2014-0001. ISSN 2084-8862.
