Vortex beams and optical activity of sucrose

Complex Light and Optical Forces XII

2018

The extent to which structured light might conceivably resolve the handedness of chiral matter is a topic of resurgent interest, and it is one that has been a challenge since the earliest days of optical vortex studies. It has not even been certain whether or not the orbital angular momentum of light could interact in such a way -though it has been established that electric quadrupole interactions enable twisted light to engage with local electronic transitions. Crucially, certain recent experiments have provided tantalizing evidence to support the existence of a chiral effect that is sensitive to handedness, against initial expectations. By detailed electrodynamic calculation, a new study has now fully identified the mechanism, and also provided an in-depth analysis of the role of electric quadrupole transition moments as they engage with the phase gradient of beams with a twisted wavefront. Focusing on single photon absorption, it emerges that the orbital angular momentum associated with the vorticity of a structured beam can indeed be exhibited in chiral effects, provided the material itself is not only chiral, but also has some structural order -which essentially limits the effect to chiral solids, poled liquid crystals, and oriented arrays of chiral nanoparticles. Circular polarization is still required, and the extent of circular dichroism proves to vary around the beam, being locally determined by the absorber orientation with reference to the beam axis. In agreement with earlier studies, and consistent with symmetry principles, the new analysis verifies that any dependence on wavefront vorticity vanishes in a freely mobile fluid. The reformulation of theory now paves the way for an extension to other kinds of chiroptical phenomena in orientationally ordered systems.

Section: Discussionmentioning

confidence: 75%

Chiroptical interactions between twisted light and chiral media

Forbes¹,

Complex Light and Optical Forces XII

2018

“…In the latter case, some rate contributions with a dependence on topological charge persist, in the case of vortex light-though only when associated with circular polarizations to support an optical chirality density-as established in equation (15). A weak dependence of exactly this kind has been observed in experimental studies [131,132].…”

Section: -M1 and E1mentioning

confidence: 79%

Quantum formulation for nanoscale optical and material chirality: symmetry issues, space and time parity, and observables

2018

J. Opt.

To properly represent the interplay and coupling of optical and material chirality at the photonmolecule or photon-nanoparticle level invites a recognition of quantum facets in the fundamental aspects and mechanisms of light-matter interaction. It is therefore appropriate to cast theory in a general quantum form, one that is applicable to both linear and nonlinear optics as well as various forms of chiroptical interaction including chiral optomechanics. Such a framework, fully accounting for both radiation and matter in quantum terms, facilitates the scrutiny and identification of key issues concerning spatial and temporal parity, scale, dissipation and measurement. Furthermore it fully provides for describing the interactions of structured or twisted light beams with a vortex character, and it leads to the complete identification of symmetry conditions for materials to provide for chiral discrimination. Quantum considerations also lend a distinctive perspective to the very different senses in which other aspects of chirality are recognized in metamaterials. Duly attending to the symmetry principles governing allowed or disallowed forms of chiral discrimination supports an objective appraisal of the experimental possibilities and developing applications.

“…To date, there have been few studies of the spectroscopic application of twisted light, with work in the nascent area generally concerned with answering fundamental questions such as whether optical OAM could be involved in atomic electronic transitions [82,83]. Interestingly, a few exploratory studies on spectroscopic applications of structured light possessing OAM and optical activity have indicated that such a chiroptical effect is possible [84,85], and the same group has recently published work where recognizable chiroptical effects occur in the transmission of vortex beams though mouse brain tissue [86]. From the analysis undertaken in this paper, it is clear that the application of twisted light in spectroscopy, particularly chiral spectroscopy, offers much additional scope for potentially unique applications in molecular systems, exploiting the more complicated and intricate structures of the OAM-bearing laser beam.…”

Section: Discussion and Summarymentioning

confidence: 99%

Enhanced optical activity using the orbital angular momentum of structured light

Forbes

2019

Phys. Rev. Research

Recent molecular photonics studies have highlighted the significant role that phase-structured light possessing orbital angular momentum (OAM) can have when interacting with matter. These studies discovered chiroptical effects sensitive to both the magnitude and sign of the optical OAM in both the absorption and scattering of twisted photons by molecules and nanoparticles. Specifically, it has been shown how a structured beam engaging with electric-quadrupole transitions in the material allows a unique sensitivity to the helical-phase structure of twisted light. In this paper we highlight experimental methodologies and systems suitable to observe and quantify the chiroptical processes of Rayleigh and Raman optical activity, and the newly discovered circular-vortex differential scattering effect with structured light-including the importance of off-axis beam alignment, input beam intensity structure, multipolar moments, and scattering-angle dependencies. It is shown that with a judicious choice of experimental setup, chiroptical effects that scale with the topological charge or OAM of the input beam enable optical activity signals to be enhanced and significantly exceed those based solely on circularly polarized, unstructured light. The new technique thus offers a highly useful and important spectroscopic application of structured light. The more detailed role that perfect optical vortices with high OAM will play in such optical activity effects is now highlighted, to show where there is substantial scope for experimental application, specifically in vibrational optical activity and chiral spectroscopy.