Nonlinear chiral molecular photonics using twisted light: hyper-Rayleigh and hyper-Raman optical activity

Abstract

Chiroptical and optical activity effects involve differential interactions between matter and light. Generally this involves chiral molecules absorbing or scattering right- and left-handed circularly polarized photons at different rates due to the chiroptical interplay of molecular and optical chirality. Laser light which propagates with a helical phase and twisted wavefront possesses optical orbital angular momentum. These optical vortices can twist either clockwise or anticlockwise, and as such they exhibit an optical handedness or chirality completely distinct from that of circular polarization. It has recently been established that the linear optical effects of single-photon absorption and scattering can exhibit optical activity and chiroptical interactions with respect to the optical vortex handedness. Here a fundamental mechanism of optical activity for twisted light is exhibited in nonlinear processes, with specific emphasis on hyper-Rayleigh and hyper-Raman scattering. In comparison to unstructured or plane-wave light, it is shown that using twisted photons produces novel scattering mechanisms dependent on parameters unique to optical vortex beams. Specifically, the scattered intensity for both hyper-Rayleigh and hyper-Raman optical activity is dependent on the sign and magnitude of the OAM of the incident twisted photons, as well as the transverse position of the chiral scatterer. Moreover, symmetry analysis reveals that, unlike the recently discovered linear optical activity effects with optical vortices, nonlinear scattering of twisted light by chiral molecules leads to a modification of scattering through uniquely weighted individual hyperpolarizability contributions.

Optical vortex dichroism in chiral particles

“…Whilst previous chiroptical absorption mechanisms have been discovered for vortex light in oriented media, the importance of the result here is that it persists in fluids consisting of chiral particles, which has an acute importance in the field of optical activity and spectroscopies utilized in chemical and biochemical systems which are invariably in the liquid phase. There have been both a number of circular-vortex differential [19,47,48] and vortex differential [40,[49][50][51][52] effects reported, though no vortex dichroism (absorption) effects in isotropic chiral molecular matter of the nature here has been reported thus far to the best of our knowledge. The underlying principles of this work can easily be extended to other types of optical activity, such as optical rotation or Rayleigh and Raman optical activity (scattering effects).…”

Principles of fundamental symmetry in optical vortex interactions

“…It is interesting to observe that a prominent role has also been found for electric quadrupole interactions in the chiroptical discrimination exhibited in optical harmonic scattering from fluid suspensions of chiral nanoparticles. [50][51][52][53][54][55] One area for further exploration and extension of the principles elicited here is the growing topic of spatio-temporal optical vortices, [56][57][58][59][60] while other intriguing possibilities may arise in the chiral interactions of vector vortex beams with transverse-variable-polarization. [61][62][63][64][65][66][67] It will be interesting to see if novel features and enhanced chiral propensities will also emerge in these new connections.…”

Fundamental symmetry origins in the chiral interactions of optical vortices

“…The method required to determine these quantities from the cylindrical coordinate expressions that naturally arise in structured beam interactions, duly accounting for molecular orientational averaging, was first shown in a 2019 paper, 80 and its methods have been deployed in several subsequent works by the same authors. The same irreducible tensor properties of the molecule determine the outcome in each case—compare, for example, the results for hyper‐Raman scattering in an optical vortex, with those that arise in conventional beams 94,95 . However, the key difference in experiments using vortex radiation is that they enable a separation of invariants associated with magnetic dipole and electric quadrupole interactions, since only the latter are l ‐dependent.…”