Molecular symmetries by coherent Raman scattering

Summary form only given. Obtaining information on the matter organization on the micrometer scale still remains a challenge in physics, chemistry or biology. Non-linear vibrational processes such as Coherent Anti-Stokes Raman Scattering (CARS) are powerful tools for 3D imaging of chemical properties without sample preparation. The CARS process is a stimulated Raman process based on the interaction of 2 photons, called pump at frequency ω
_p
, and Stokes at frequency ω
_s
. When the frequency difference corresponds to a vibrational resonance (Ω
_r
= ω
_p
-ω
_s
), an anti-Stokes photon is generated at the frequency ω
_as
= 2ω
_p
-ω
_s
. In this case, the measured intensity specifically reveals the presence of the chemical bonds, much more efficiently than the spontaneous Raman, thus enabling rapid imaging. It is well known that the symmetry properties of matter have a strong influence on its physical properties. Pioneers of nonlinear optics have shown that the nonlinear susceptibility tensor, involved in the optical response, express molecular symmetries at different level. To read the susceptibility tensor elements, polarization-resolved schemes have been proposed decades ago but requires long acquisition times and time consuming post-processing. The use of irreducible formalism, instead of Cartesian formalism, can overcome this complexity. The irreducible formalism allows to describe a tensor in a limited number of irreducible components which remains unchanged under rotation and define a proper basis. Based on irreducible tensor formalism, we introduce a microscopy technique that is able to retrieve the individual symmetry orders of molecular organization in a simple acquisition. The symmetry-resolved CARS (SR-CARS) signal not only depends on the presence of molecular bonds, but also on their organization within the focal volume. By switching between combinations of left- and right-handed circular polarization states, it is possible to directly image symmetry contribution of the sample.