Kayn A. Forbes

Bio: Kayn A. Forbes is an academic researcher from University of East Anglia. The author has contributed to research in topics: Optical vortex & Paraxial approximation. The author has an hindex of 11, co-authored 37 publications receiving 398 citations.

Optical orbital angular momentum: twisted light and chirality.

Abstract: The question of how the orbital angular momentum of structured light might engage with chiral matter is a topic of resurgent interest. By taking account of electric quadrupole transition moments, it is shown that the handedness of the beam can indeed be exhibited in local chiral effects, being dependent on the sign of the topological charge. In the specific case of absorption, a significant interplay of wavefront structure and polarization is resolved, and clear differences in behavior are identified for systems possessing a degree of orientational order and for those that are randomly oriented.

Orbital angular momentum of twisted light: chirality and optical activity

Abstract: Optical activity is conventionally understood as a natural difference in the optical responses of chiral materials with opposite handedness. It stems from the quantised spin angular momentum ±ħ per photon, with the ± representing either left- or right-handed circular polarisations. Less well known, until recently, was the possibility that matter might also respond in a similar, discriminatory way to the handedness of twisted light, or ‘optical vortices’, whose orbital angular momentum (OAM) is quantised as ℓℏ per photon, where ℓ is the topological charge whose sign determines a wavefront twist to the left or right. Initial studies focusing on whether, in spectroscopic applications, chiral matter might respond differently to the vortex handedness of +ℓ and −ℓ beams, failed to identify any viable mechanism. However, in the last few years, theory and experiment have both supplied ample evidence that, under certain conditions, such forms of interaction do exist—and as a result, the field of chirality and optical OAM is beginning to flourish at a pace. This topical review presents a survey of this new field, working up from a description of those initial studies to the cutting-edge experiments now taking place. Analysing the fundamental mechanisms provides for a revision of previous precepts, broadening their scope in the light of recent advances in understanding, and highlighting a vibrant synergy between the fields of optical activity and twisted light.

Raman Optical Activity Using Twisted Photons

Abstract: 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.

Spin-orbit interactions and chiroptical effects engaging orbital angular momentum of twisted light in chiral and achiral media

Abstract: There is recurrent interest in the orbital angular momentum (OAM) conveyed by optical vortices, which are structured beams with a helically twisted wave front. Particular significance is attached to the issue of how material interactions with light conveying OAM might prove sensitive to the handedness and degree of twist in the optical wave front. As a result of recent experimental and theoretical studies, the supposition that beams with OAM might enable spectroscopic discrimination between oppositely handed forms of matter has become a renewed focus of attention. Some of the tantalizing conclusions that are beginning to emerge from this research have, however, not yet established a definitive basis for a supporting mechanism. To resolve this problem requires the development of theory to support a faithful representation, and a thorough understanding, of the fundamental molecule-photon physics at play in such optical processes - even for processes as basic as absorption. The present analysis establishes mechanisms at play that entail an unconventional manifestation of optical spin-orbit interactions, engaging transition electric-quadrupole moments. Powerful symmetry principles prove to render distinctively different criteria governing the exhibition of two-dimensional (2D) and 3D chirality. These results elucidate the operation of such effects, identifying their responsibility for discriminatory optical interactions of various forms in both chiral and achiral media.

Optical binding of nanoparticles

Abstract: Abstract Optical binding is a laser-induced inter-particle force that exists between two or more particles subjected to off-resonant light. It is one of the key tools in optical manipulation of particles. Distinct from the single-particle forces which operate in optical trapping and tweezing, it enables the light-induced self-assembly of non-contact multi-particle arrays and structures. Whilst optical binding at the microscale between microparticles is well-established, it is only within the last few years that the experimental difficulties of observing nanoscale optical binding between nanoparticles have been overcome. This hurdle surmounted, there has been a sudden proliferation in observations of nanoscale optical binding, where the corresponding theoretical understanding and predictions of the underlying nanophotonics have become ever more important. This article covers these new developments, giving an overview of the emergent field of nanoscale optical binding.

The Quantum Theory of Light

Abstract: (b) Write out the equations for the time dependence of ρ11, ρ22, ρ12 and ρ21 assuming that a light field interaction V = -μ12Ee iωt + c.c. couples only levels |1> and |2>, and that the excited levels exhibit spontaneous decay. (8 marks) (c) Under steady-state conditions, find the ratio of populations in states |2> and |3>. (3 marks) (d) Find the slowly varying amplitude ̃ ρ 12 of the polarization ρ12 = ̃ ρ 12e iωt . (6 marks) (e) In the limiting case that no decay is possible from intermediate level |3>, what is the ground state population ρ11(∞)? (2 marks) 2. (15 marks total) In a 2-level atom system subjected to a strong field, dressed states are created in the form |D1(n)> = sin θ |1,n> + cos θ |2,n-1> |D2(n)> = cos θ |1,n> sin θ |2,n-1>

Electromagnetic chirality: from fundamentals to nontraditional chiroptical phenomena

Abstract: Chirality arises universally across many different fields. Recent advancements in artificial nanomaterials have demonstrated chiroptical responses that far exceed those found in natural materials. Chiroptical phenomena are complicated processes that involve transitions between states with opposite parities, and solid interpretations of these observations are yet to be clearly provided. In this review, we present a comprehensive overview of the theoretical aspects of chirality in light, nanostructures, and nanosystems and their chiroptical interactions. Descriptions of observed chiroptical phenomena based on these fundamentals are intensively discussed. We start with the strong intrinsic and extrinsic chirality in plasmonic nanoparticle systems, followed by enantioselective sensing and optical manipulation, and then conclude with orbital angular momentum-dependent responses. This review will be helpful for understanding the mechanisms behind chiroptical phenomena based on underlying chiral properties and useful for interpreting chiroptical systems for further studies. Strengthening the theoretical understanding of chirality is necessary for developing applications based on its phenomena. Junsuk Rho of Korea’s Pohang University of Science and Technology (POSTECH) reviewed with colleagues the theoretical aspects of chirality, a symmetry property that describes mirror-image objects or systems that cannot be superimposed. Chiral materials have attracted much attention due to their interesting interactions. Scientists are familiar with how geometrically chiral objects and systems interact with light. However, such ‘chiroptical effects’ can also be found in achiral systems, Rho and his colleagues explain. Also, globally achiral light can be locally chiral near nanostructures. Scientists need to extend their concepts and theoretical understandings of chiroptical systems in order to be able to further develop applications based on their phenomena, such as in metamaterials, sensing, spintronics and stereochemistry.