Twisted Raman: Orbital Angular Momentum Control of Raman Optical Activity

Raman optical activity (ROA) conventionally detects differences in Raman scattering associated with opposite circular-polarization states σ, providing a sensitive probe of molecular chirality. Here we show that a different handedness of light can drive ROA: the wavefront handedness encoded in the topological charge l of a focused optical vortex. Instead of using photons carrying spin angular momentum σℏ, the response is controlled by photons carrying orbital angular momentum lℏ, with the sign of l defining the optical handedness. We develop a nonparaxial theory of ROA with Laguerre-Gaussian beams and show that the Maxwell-consistent focused field allows a linearly polarized vortex beam to produce a topological-charge-dependent ROA signal from an isotropic ensemble of independent, randomly oriented chiral molecules, as in a gas or dilute solution. The response contains both electric-dipole-magnetic-dipole and electric-dipole-electric-quadrupole interference terms, establishing optical orbital angular momentum as a control parameter for chiral Raman spectroscopy.