The majority of mechanisms that can be deployed for optical micromanipulation are not especially amenable for
extension into the nanoscale. At the molecular level, the rich variety of schemes that have been proposed to achieve
mechanical effect using light commonly exploit specific chemical structures; familiar examples are compounds that can
fold by cis-trans isomerization, or the mechanically interlocked architectures of rotaxanes. However, such systems are
synthetically highly challenging, and few of them can realistically form the basis for a true molecular motor. Developing
the basis for a very different strategy based on programmed electronic excitation, this paper explores the possibility of
producing controlled mechanical motion through optically induced modifications of intermolecular force fields, not
involving the limitations associated with using photochemical change, nor the high intensities required to produce and
manipulate optical binding forces between molecules. Calculations reveal that significant, rapidly responsive effects can
be achieved in relatively simple systems. By the use of suitable laser pulse sequences, the possibilities include the
generation of continuous rotary motion, the ultimate aim of molecular motor design.
Optical binding is a phenomenon that is exhibited by micro-and nano-particulate systems, suitably irradiated with offresonance
laser light. Recent quantum electrodynamical studies have shown that the optomechanical effect owes its
origin to a radiative intervention with the Casimir-Polder dispersion force. The potential energy surfaces for optically
induced inter-particle coupling reveal unexpected features of considerable intricacy, and when several particles are
present, the effect can result in the formation of geometrically varied non-contact assemblies. In general, previous studies
have been restricted to considering only the dynamic electromagnetic coupling between particles, where the latter are
considered to be non-polar and centrosymmetric. However, when optical binding between non-polar particles takes
place, other forms of interaction need to be entertained - more especially so, since any presence of a permanent dipole
moment necessarily also admits a non-zero hyperpolarizability. Consequently, amongst the static contributions to the
interaction between any pair of particles, a coupling between the electric dipole of one and the hyperpolarizability of the
other must also be considered. In this paper we study these static contributions to the overall optical binding, comparing
their effect with other inter-particle interactions, particularly the prominent electric dipole-dipole coupling. The results
suggest that static coupling between polar particles can significantly modify the observed optical binding.
Optical binding is a phenomenon that is exhibited by micro-and nano-particle systems, suitably irradiated with offresonance
laser light. Recent quantum electrodynamical studies on optically induced inter-particle potential energy
surfaces have revealed unexpected features of considerable intricacy. When several particles are present, multi-particle
binding effects can commonly result in the formation of a variety of geometrical assemblies. The exploitation of these
features presents a host of opportunities for the optical fabrication of nanoscale structures, based on the fine control of
attractive and repulsive forces, and the torques that operate on particle pairs. This paper reports the results of a
preliminary analysis of the structures formed by optically driven self-assembly, and the three-dimensional symmetry of
energetically favored forms. In systems where permanent dipole moments are present, optical binding may also be
influenced by a static interaction mechanism. The possible influence of such effects on assembly formation is also
explored, and consideration is given to the possible departures from such symmetry which might then be anticipated.
The term 'optical binding' conveniently encapsulates a variety of phenomena whereby light can exert a modifying
influence on inter-particle forces. The mutual attraction that the 'binding' description suggests is not universal; both
attractive and repulsive forces, as well as torques can be generated, according to the particle morphology and optical
geometry. Generally, such forces and torques propel particles towards local sites of potential energy minimum, forming
the stable structures that have been observed in numerous experimental studies. The underlying mechanisms by means
of which such effects are produced have admitted various theoretical interpretations. The most widely invoked
explanations include collective scattering, dynamically induced dipole coupling, optically-dressed Casimir-Polder
interactions, and virtual photon coupling. By appeal to the framework that led to the first predictions of the effect, based
on quantum electrodynamics, it can be demonstrated that many of these apparently distinct representations reflects a
different facet of the same fundamental mechanism, leading in each case to the same equations of motion. Further
analysis, based on the same framework, also reveals the potential operation of another mechanism, associated with
dipolar response to local dc fields that result from optical rectification. This secondary mechanism can engender shifts
in the positions of the potential energy minima for optical binding. The effects of multi-particle interactions can be
addressed in a theoretical representation that is especially well suited for modeling applications, including the
generation of potential energy landscapes.
When off-resonant light travels through a transparent medium, light scattering is the primary optical process to occur. Multiple-particle events are relatively rare in optically dilute systems: scattering generally takes place at individual atomic or molecular centers. Several well-known phenomena result from such single-center interactions, including Rayleigh and Raman scattering, and the optomechanical forces responsible for optical tweezers. Other, less familiar effects may arise in circumstances where throughput radiation is able to simultaneously engage with two or more scattering sites in close, nanoscale, proximity. Exhibiting the distinctive near-field electromagnetic character, inter-particle interactions such as optical binding and a variety of inelastic bimolecular processes can then occur. Although the theory for each two-center process is well established, the connectivity of their mechanisms has not received sufficient attention. To address this deficiency, and to consider the issues that ensue, it is expedient to represent the various forms of multi-particle light scattering in terms of transitions between different radiation states. The corresponding quantum amplitudes, registering the evolution of photon trajectories through the material system, can be calculated using the tools of quantum electrodynamics. Each of the potential outcomes for multi-particle scattering generates a set of amplitudes corresponding to different orderings of the constituent photon-matter interactions. Performing the necessary sums over quantum pathways between radiation states is expedited by a state-sequence development, this formalism also enabling the identification of intermediate states held in common by different paths. The results reveal the origin and consequences of linear momentum conservation, and they also offer new insights into the behavior of light between closely neighboring scattering events.
The phenomenon of optical binding is now experimentally very well established. With a recognition of the facility to
collect and organize particles held in an optical trap, the related term 'optical matter' has also been gaining currency,
highlighting possibilities for a significant interplay between optically induced inter-particle forces and other interactions
such as chemical bonding and dispersion forces. Optical binding itself has a variety of interpretations. With some of
these explanations being more prominent than others, and their applicability to some extent depending on the nature of
the particles involved, a listing of these has to include the following: collective scattering, laser-dressed Casimir forces,
virtual photon coupling, optically induced dipole resonance, and plasmon resonance coupling. It is the purpose of this
paper to review and to establish the extent of fundamental linkages between these theoretical descriptions, recognizing
the value that each has in relating the phenomenon of optical binding to the broader context of other, closely related
The characteristic near-field behavior of electromagnetic fields is open to a variety of interpretations. In a classical sense
the term 'near-field' can be taken to signify a region, sufficiently close to some primary or secondary source, that the
onset of retardation features is insignificant; a quantum theoretic explanation might focus more on the large momentum
uncertainty that operates at small distances. Together, both near-field and wave-zone (radiative) features are fully
accommodated in a retarded resonance propagation tensor, within which each component individually represents one
asymptotic limit - alongside a third term that is distinctly operative at distances comparable to the optical wavelength.
The propagation tensor takes different forms according to the level of multipole involved in the signal production and
detection. In this presentation the nature and symmetry properties of the retarded propagation tensor are explored with
reference to various forms of electric interaction, and it is shown how a suitable arrangement of optical beams can lead to
the complete cancellation of near-fields. The conditions for such behavior are fully determined and some important
optical trapping applications are discussed.
Optical binding is a phenomenon that is exhibited by micro-and nano-particles systems, suitably irradiated with off-resonance
laser light. When several particles are present, the effect commonly results in the formation of particle
assemblies. In the optically induced potential energy surfaces responsible for such assembly formation, the location and
intensity of local energy maxima and minima depend on the particle configurations with respect to the input beam
polarization and Poynting vector. This paper reports the results of recent quantum electrodynamical studies on the
energy landscapes for systems of three and more particles; the analysis of local minima allows determination of the
energetically most favorable positions, and it shows how the addition of further particles subtly modifies each energy
landscape. The analysis includes the identification and characterization of potential points of stability, as well as the
forces and torques that the particles experience as a consequence of the throughput electromagnetic radiation. As such,
the development of theory represents a rigorous and general formulation paving the way towards a fuller comprehension
of nanoparticle assembly based on optical binding.
Recent quantum electrodynamical studies on optically induced inter-particle potential energy surfaces have revealed
unexpected features of considerable intricacy. The exploitation of these features presents a host of opportunities for the
optical fabrication of nanoscale structures, based on the fine control of a variety of attractive and repulsive forces, and
the torques that operate on particle pairs. Here we report an extension of these studies, exploring the first detailed
potential energy surfaces for a system of three particles irradiated by a polarized laser beam. Such a system is the key
prototype for developing generic models of multi-particle complexity. The analysis identifies and characterizes potential
points of stability, as well as forces and torques that particles experience as a consequence of the electromagnetic fields,
generated by optical perturbations. Promising results are exhibited for the optical fabrication of assemblies of molecules,
nanoparticles, microparticles, and colloidal multi-particle arrays. The comprehension of mechanism that is emerging
should help determine the fine principles of multi-particle optical assembly.
Optical binding can be understood as a laser perturbation of intermolecular forces. Applying state-of-the-art QED
theory, it is shown how light can move, twist and create ordered arrays from molecules and nanoparticles. The
dependence on laser intensity, geometry and polarization are explored, and intricate potential energy landscapes are
exhibited. A detailed exploration of the available degrees of geometric freedom reveals unexpected patterns of local
force and torque. Numerous positions of local potential minimum and maximum can be located, and mapped on contour
diagrams. Islands of stability and other structures are then identified.
Multi-dimensional potential energy surfaces are associated with optical binding. A detailed exploration of the available degrees of geometric freedom reveals unexpected turning points, producing intricate patterns of local force and torque. Although optical pair interactions outweigh Casimir-Polder coupling even over short distances, the forces are not always attractive. Numerous local potential minimum and maximum can be located, and mapped on contour diagrams. Islands of stability appear, and structures conducive to the formation of rings. The results, based on quantum electrodynamics, apply to optically trapped molecules, nanoparticles, microparticles and colloids.
The physics of slow-light propagation in atomic Lambda systems is described by the theory of integrable systems, which allows the existence of solitons. Slow-light solitons are stable polarization structures that propagate through the atomic medium at a controllable speed. They represent generalizations of the experimentally demonstrated slow-light pulses in atomic media where one light polarization dominates the other, the probe, and controls its group velocity. In the general case, the overall intensity controls the speed of the entire polarization structure. For zero detuning between light and atoms, even more general shape-preserving pulses exist. Quantum fluctuations of slow-light pulses can be stored in atomic media. In the case of solitons, these are fluctuations of the soliton parameters.
We examine novel features that might emerge from the interaction of Laguerre-Gaussian beams with liquid crystals. We study the response of nematic liquid crystal media to the throughput of twisted laser light. Specific attention is focused on the spatial evolution of the director orientation angle.
In recent years, twisted laser beams and optical vortices have attracted considerable interest, in terms of both their fundamental quantum properties and also their potential technical applications. Here we examine what novel features might emerge from the interaction of such beams with chiral matter. In this connection we assess the possible scope for exploiting similarities between the angular momentum properties of circularly polarised light and optical vortices - both with regard to their mechanical torque and also the associated spectroscopic selection rules. Twisted beams have generally been studied only in their interactions with achiral matter, with the theory largely developed for electric dipole coupling. In chiral systems, the low symmetry enables many optical transitions to be allowed under the selection rules for both electric and magnetic multipoles, and the entanglement of spin and orbital photon angular momentum requires careful extrication. Specific issues to be addressed are: what new features, if any, can be anticipated when such beams are used to interrogate a chiral system, and whether in such cases enantiomeric specificity can be expected. To this end we develop theory that goes to a higher order of multipole expansion, also engaging magnetic dipole and electric quadrupole transitions. Finally, we study the response of nematic liquid crystal media to the throughput of twisted laser light. Specific attention is focused on the spatial evolution of the director orientation angle.
In the formal development of optical response theory in terms of susceptibilities, proper representation of the optical frequency dependence necessitates modeling both the discrete linewidth and the finite signal enhancement associated with the onset of resonance. Such dispersion behavior is generally accommodated by damping factors, featured in both resonant and non-resonant susceptibility terms. For the resonant terms, the sign of such damping corrections is unequivocal; however the correct choice of sign for non-resonant terms has become a matter of debate, heightened by the discovery that entirely opposite conventions are applied in mainstream literature on Raman scattering and nonlinear optics. Where the two conventions are applied to electro-optical processes in fluids there are significant and potentially verifiable differences between the associated results. Through a full thorough quantum electrodynamical treatment the universal correctness of one convention can be ascertained and flaws in the counter-convention identified. Resolution of the central issue requires consideration of a number of fundamental questions concerning the nature of dissipation in quantum mechanical systems. It is concluded that optical susceptibilities formulated with correct signing of the damping corrections must fulfill several fundamental tests: satisfaction of a new sum rule; invariance of the associated quantum amplitudes under time-reversal symmetry, and a resilience to canonical transformation.
The propensity of conventional optical beams to convey angular momentum is very well known. As a spin-1 elementary particle any photon can assume a polarisation state with a well defined 'spin' angular momentum of plus or minus 1 in the direction of propagation, corresponding to a circular polarisation of either left or right helicity. The mechanical effects of photonic angular momentum are manifest in a variety of phenomena operating at both the atomic and macroscopic scale. Photon angular momentum also exercises a key role in atomic spectroscopy and a host of other fundamental optical phenomena.
The aim of this work is to study the interaction between matter and Laguerre-Gaussian beams, and others of related structure in which a helical wavefront confers an endowment with 'orbital' angular momentum. Although the principles and methods of production of these twisted beams are already quite well understood, the detailed study of the interactions is a novel subject. We explore changes in selection rules transfer of linear and angular momentum in the context of nonlinear processes, especially harmonic and sum-frequency generation.
At high levels of optical excitation, local coherence in particles or ordered domains within mesoscopically disordered materials can lead to second harmonic emission whose temporal signature characterizes the decay kinetics of the excited state population. Examples of such systems include colloids, cell and membrane suspensions, and many plastics, glasses and other modern materials. The effect is prominent in frequency regions where the second order optical nonlinearity is dominated by transitions involving one particular electronic excited state, and where a two- level model closely models the optical response. With ultrafast pulsed excitation of sufficient intensity to elicit the onset of saturation, second harmonic emission on throughput of a subsequent probe beam exhibits a characteristic decay and recovery. Detailed calculations show that such features also arise in systems whose optical response involves more than two levels.