Raman Optical Activity Using Twisted Photons

Raman optical activity underpins a powerful vibrational spectroscopic technique for obtaining detailed structural information about chiral molecular species. The effect centers on the discriminatory interplay between the handedness of material chirality with that of circularly polarized light. Twisted light possessing an optical orbital angular momentum carries helical phase fronts that screw either clockwise or anticlockwise and, thus, possess a handedness that is completely distinct from the polarization. Here a novel form of Raman optical activity that is sensitive to the handedness of the incident twisted photons through a spin-orbit interaction of light is identified, representing a new chiroptical spectroscopic technique.

Figure 1

Different forms of handedness; circularly polarized light, twisted light with azimuthal phase, and a pair of nonsuperimposable molecular enantiomers.

Figure 2

Time-ordered Feynman graphs for calculating the $E1$$E2$ contribution to the quantum amplitude for scattering of a twisted photon of mode $(k,\eta ,\ell ,p)$. Time progresses vertically, and the blue-dashed horizontal lines demarcate the boundaries between initial, intermediate, and final system states. The $E1E1$ contribution requires only one pair of graphs, with the $E2$ vertices replaced by $E1$ interactions.

Figure 3

Scattering geometry for an incident LG vortex photon described by the unit vectors $(\widehat{\mathit{x}},\widehat{\mathit{y}},\widehat{\mathit{z}})$ scattered by a molecule (purple sphere) at a scattering angle $\theta$. The unit vectors $({\widehat{\mathit{x}}}^{\mathit{\prime }},{\widehat{\mathit{y}}}^{\mathit{\prime }},{\widehat{\mathit{z}}}^{\mathit{\prime }})$ describe the scattered photon (wavy line).