Mechanisms for optical binding

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 physical measurements

Multiple light scattering and optomechanical forces

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. © 2010 Society of Photo-Optical Instrumentation Engineers

Geometric configurations and perturbative mechanisms in 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 electrodynamic mechanisms of optical binding

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

Laguerre-Gaussian wave propagation in parabolic media

We report a new set of Laguerre-Gaussian wave-packets that propagate with periodical self-focusing and finite beam width in weakly guiding inhomogeneous media. These wave-packets are solutions to the paraxial form of the wave equation for a medium with parabolic refractive index. The beam width is defined as a solution of the Ermakov equation associated to the harmonic oscillator, so its amplitude is modulated by the strength of the medium inhomogeneity. The conventional Laguerre-Gaussian modes, available for homogenous media, are recovered as a particular case.Comment: 11 pages, 5 figure

Perspective: Quantum Hamiltonians for optical interactions

The multipolar Hamiltonian of quantum electrodynamics is extensively employed in chemical and optical physics to treat rigorously the interaction of electromagnetic fields with matter. It is also widely used to evaluate intermolecular interactions. The multipolar version of the Hamiltonian is commonly obtained by carrying out a unitary transformation of the Coulomb gauge Hamiltonian that goes by the name of Power-Zienau-Woolley (PZW). Not only does the formulation provide excellent agreement with experiment, and versatility in its predictive ability, but also superior physical insight. Recently, the foundations and validity of the PZW Hamiltonian have been questioned, raising a concern over issues of gauge transformation and invariance, and whether observable quantities obtained from unitarily equivalent Hamiltonians are identical. Here, an in-depth analysis of theoretical foundations clarifies the issues and enables misconceptions to be identified. Claims of non-physicality are refuted: the PZW transformation and ensuing Hamiltonian are shown to rest on solid physical principles and secure theoretical ground