Sculpting optical energy landscapes for multi-particle nanoscale assembly

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

Role of magnetic and diamagnetic interactions in molecular optics and scattering

This paper aims to explicitly clarify the role and interpretation of diamagnetic interactions between molecules and light in quantum electrodynamics. In contrast to their electric and magnetic counterparts, the diamagnetic couplings between light and matter have received relatively little interest in the field of molecular optics. This intriguing disregard of an interaction term is puzzling. The diamagnetic couplings possess unique physical properties that warrant their inclusion in any multiphoton process, and the lack of gauge invariance for paramagnetic and diamagnetic susceptibilities necessitates their inclusion. Their role and importance within nonrelativistic molecular quantum electrodynamics in the Coulomb gauge is illuminated, and it is highlighted how for any multiphoton process their inclusion should be implicit. As an indicative example of the theory presented, the diamagnetic contributions to both forward and nonforward Rayleigh scattering are derived and put into context alongside the electric and magnetic molecular responses. The work represents clarification of diamagnetic couplings in molecular quantum electrodynamics, which subsequently should proffer the study of diamagnetic interactions in molecular optics due to their unique physical attributes and necessary inclusion in multiphoton processes

Optical spin-orbit interactions in molecular scattering of twisted light

The unique role that can electric quadrupole transition moments play in chiroptical interactions has recently been established with twisted light beams. Manifesting a spin-orbit interaction in paraxial light, the engagement of electric quadrupoles in electronic transitions is highlighted in optical phenomena such as absorption, in both chiral and achiral media. However, spin-orbit interactions of light are also well-known in scattering from small particles, and recent exploratory experimental work suggests a chiroptical interaction of this nature in the scattering of optical vortex beams. Using a quantum electrodynamical formulation, such a sensitivity to the handedness of a vortex beam is accounted for in molecular scattering processes

Discriminatory effects in the optical binding of chiral nanoparticles

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

Passive laser irradiation as a tool for optical catalysis

The mechanisms of absorption, emission, and scattering of photons form the foundations of optical interactions between light and matter. In the vast majority of such interactions there is a significant interplay and energy exchange between the radiation field and the material components. In absorption for example, modes of the field are depopulated by photons whose energy is at resonance with a molecular transition producing excited material states. In all such optical phenomena, the initial state of the radiation field differs in mode occupation to its final state. However, certain optical processes can involve off-resonance laser beams that are unchanged on interaction with the material: the output light, after interaction, is identical to the laser input. Such off-resonance interactions include forward Rayleigh scattering, responsible for the wellknown gradient force in optical trapping, and the laser-induced intermolecular interaction commonly termed optical binding; in both processes, an intense beam delivers its effect without suffering change. It is possible for beams detuned from resonance to provide not only techniques for optomechanical and optical manipulation, but also to passively influence other important and functional interactions such as absorption from a resonant beam, or energy transfer. Such effects can be grouped under the banner of ‘optical catalysis’, since they can significantly influence resonant processes. Furthermore, off-resonance photonics affords a potential to impact on chemical interactions, as in the passive modification of rotational constants and phase transitions. To date, apart from optical manipulation, the potential applicability of passive photonics, particularly in the realm of chemical physics and materials science, has received little attention. Here we open up this field, highlighting the distinct and novel role that off-resonance laser beams and the ensuing photonics can play

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

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

A quantum electrodynamical approach to chirality and photonics: nonlinear optics, structured light, and optical forces

In this thesis, the theory of quantum electrodynamics (QED) is utilised to study the interaction between radiation and matter. In particular, the non-relativistic limit of the theory is employed, describing the optical processes and coupling between molecules and light. The predictive power of this theory is highlighted from the outset, whereby it is shown that a new form of quantum uncertainty, with its origins in delocalised photon emission and absorption, exists in non-linear optics: a non-localised mechanism for the processes of both spontaneous parametric down-conversion and second harmonic generation is established by accounting for virtual photon propagation. The subsequent chapter brings forth the often ignored diamagnetic couplings to optical processes. Their interesting and unique physical properties are shown to manifest themselves in scattering and two-photon absorption processes, and an argument for their inclusion in any multiphoton optical process is outlined. Next, the question of whether the sign of the topological charge (handedness) of a beam possessing optical orbital angular momentum (structured light) engages in chiroptical processes with chiral molecules is resolved. It is shown that through the engagement of the electric quadrupole molecular moment, discriminatory effects with regards to the direction of the vortex twist are anticipated with anisotropic matter. Finally, the laser-induced intermolecular forces that exist between molecules present within an intense laser beam are focused on. Specifically, it is shown that there exists a discriminatory force between chiral molecules when subjected to circularly polarised light

Kramers-Heisenberg dispersion formula for scattering of twisted light

An extremely active research topic of modern optics is studying how light can be engineered to possess forms of structure such as a twisting or a helical phase and the ensuing optical orbital angular momentum (OAM) and its interactions with matter. In such circumstances, the plane-wave description no longer suffices and both paraxial and nonparaxial solutions to the wave equation are desired. Within the framework of molecular QED theory, a general formulation is developed for the scattering of twisted light beams by molecular systems through the Kramers-Heisenberg dispersion formula and ensuing scattering cross section, which takes account of the effects of the phase and intensity structure of twisted light, revealing scattering effects not exhibited by unstructured, plane-wave light. The theory is applicable to linear scattering as well as to nonlinear optical effects for both chiral and nonchiral species, and explicit results are derived for Rayleigh and Raman scattering (including second-order contributions), Rayleigh and Raman optical activity, and their circular-vortex differential scattering analogs. These processes necessitate the inclusion of magnetic-dipole and electric-quadrupole coupling terms, as well as the usual leading electric-dipole interaction term. It is seen that the coupling of electric quadrupole moments to structured light affords a unique sensitivity to the phase properties of the beam, most importantly, its optical OAM, and its inclusion permits the contribution to the scattering cross section proportional to the square of the mixed electric dipole-quadrupole polarizability to be evaluated for which interesting features result. These include its discriminatory behavior arising from circularly polarized input radiation and its dependence on the topological charge, which can also serve to enhance scattering. Also presented are results for a contribution of identical order proportional to the pure electric-dipole and quadrupole polarizabilities