Recent theoretical work on circular dichroic effects, for absorption processes in chiral materials, has reopened questions over the possibility that the interactions of vortex beams may display a sensitivity to material handedness. The interest in such a phenomenon arises from the fact that any engagement of optical phase gradients, in quadrupole-allowed electronic transitions, will represent a distinctive form of engagement with chiral matter. This is an issue that numerous careful experiments have so far failed to fully resolve, with some of them giving a clear null result, yet others giving positive indications. A definitive outcome from any such investigation would represent a touchstone for a broader, yet more challenging question: is there any mechanism by means of which twisted light, which conveys both orbital and spin angular momentum, can exhibit coupling between the two? It emerges that such a possibility can be identified, but the constraints upon its manifestation are severe. This presentation sets out the principles and the conclusions to which they lead, informing the pathway for ongoing experimentation.
The issue of whether the optical orbital angular momentum of light can play any significant role in chiroptical interactions has seen a resurgence of interest in the past few years. Revising preliminary expectations, it has been shown both theoretically and experimentally that the topological charge can indeed play a decisive role in some chiroptical interactions, with the rates of these optical phenomena proving sensitive to the sign of the vortex charge ℓ. Using quantum electrodynamics, it is now revealed how the inclusion of molecular electric-quadrupole transition moments in both chiral and achiral anisotropic media produces such an effect. Specifically, for single-photon absorption it transpires that both the orbital and spin angular momentum must be engaged through a circularly polarized vortex beam. The chiroptical effect is identified as a manifestation spin-orbit interaction in light.
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.
Issues of a fundamental quantum origin exert a significant effect on the output mode structures in optically parametric processes. An assumption that each frequency conversion event occurs in an infinitesimal volume produces uncertainty in the output wave-vector, but a rigorous, photon-based theory can provide for a finite conversion volume. It identifies the electrodynamic mechanisms operating within the corresponding region of space and time, on an optical wavelength and cycle timescale. Based on quantum electrodynamics, this theory identifies specific material parameters that determine the extent and measure of delocalized frequency conversion, and its equations deliver information on the output mode structures. The results also indicate that a system of optimally sized nanoparticles can display a substantially enhanced efficiency of frequency conversion.
Stacked multi-layer films have a range of well-known applications as optical elements. The various types of theory
commonly used to describe optical propagation through such structures rarely take account of the quantum nature of light,
though phenomena such as Anderson localization can be proven to occur under suitable conditions. In recent and ongoing
work based on quantum electrodynamics, it has been shown possible to rigorously reformulate, in photonic terms, the
fundamental mechanisms that are involved in reflection and optical transmission through stacked nanolayers. Accounting
for sum-over-pathway features in the quantum mechanical description, this theory treats the sequential interactions of
photons with material boundaries in terms of individual scattering events. The study entertains an arbitrary number of
reflections in systems comprising two or three internally reflective surfaces. Analytical results are secured, without
recourse to FTDT (finite-difference time-domain) software or any other finite-element approximations. Quantum
interference effects can be readily identified. The new results, which cast the optical characteristics of such structures in
terms of simple, constituent-determined properties, are illustrated by model calculations.
The laser-induced intermolecular force that exists between two or more particles subjected to a moderately intense laser beam is termed ‘optical binding’. Completely distinct from the single-particle forces that give rise to optical trapping, the phenomenon of optical binding is a manifestation of the coupling between optically induced dipole moments in neutral particles. In conjunction with optical trapping, the optomechanical forces in optical binding afford means for the manipulation and fabrication of optically bound matter. The Casimir-Polder potential that is intrinsic to all matter can be overridden by the optical binding force in cases where the laser beam is of sufficient intensity. Chiral discrimination can arise when the laser input has a circular polarization, if the particles are themselves chiral. Then, it emerges that the interaction between particles with a particular handedness is responsive to the left- or right-handedness of the light. The present analysis, which expands upon previous studies of chiral discrimination in optical binding, identifies a novel mechanism that others have previously overlooked, signifying that the discriminatory effect is much more prominent than originally thought. The new theory leads to results for freely-tumbling chiral particles subjected to circularly polarized light. Rigorous conditions are established for the energy shifts to be non-zero and display discriminatory effects with respect to the handedness of the incident beam. Detailed calculations indicate that the energy shift is larger than those previously reported by three orders of magnitude.
To understand the forces and dynamics of two or more neutral particles trapped within an optical beam, careful consideration of the influence of inter-particle forces is required. The well-known, field-independent intrinsic force is known to derive from the Casimir-Polder interaction. However, the magnitude of this force may be over-ridden by the effect known as optical binding, in cases when the laser beam is of sufficient intensity. This binding interaction is completely independent of optomechanical effects relating to optical tweezers, and involves a stimulated (pairwise) forward-scattering process. Unlike the Casimir-Polder coupling, optical binding is not always an attractive force when both particles are in their ground state. Associated with optical binding are potential energy surfaces, which reveal intricate patterns of local minima – sets of positions in which one of the particles will sit at equilibrium (with the other notionally set at the origin). These optical energy landscapes, which can be illustrated by use of contour diagrams, have mostly been considered for systems in which spherical particles are optically bound. The effect of different particle shapes, for example tube-like structures, can also be explored. Moreover, although the theory of conventional optical binding generally assumes situations in which both particles reside in their ground states, new results arise when individual particles are excited to a higher electronic state. Although, in the experimentally most convenient structural configuration (for tumbling spherical particles), pairwise optical binding vanishes in the short-range region, novel effects can arise as a result of non-zero optical binding between three neighbouring particles.