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

Optically induced potential energy landscapes

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

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

Optical binding in nanoparticle assembly: Potential energy landscapes

Optical binding is an optomechanical effect exhibited by systems of micro- and nanoparticles, suitably irradiated with off-resonance laser light. Physically distinct from standing-wave and other forms of holographic optical traps, the phenomenon arises as a result of an interparticle coupling with individual radiation modes, leading to optically induced modifications to Casmir-Polder interactions. To better understand how this mechanism leads to the observed assemblies and formation of patterns in nanoparticles, we develop a theory in terms of optically induced energy landscapes exhibiting the three-dimensional form of the potential energy field. It is shown in detail that the positioning and magnitude of local energy maxima and minima depend on the configuration of each particle pair, with regards to the polarization and wave vector of the laser light. The analysis reveals how the positioning of local minima determines the energetically most favorable locations for the addition of a third particle to each equilibrium pair. It is also demonstrated how the result of such an addition subtly modifies the energy landscape that will, in turn, determine the optimum location for further particle additions. As such, this development represents a rigorous and general formulation of the theory, paving the way toward full comprehension of nanoparticle assembly based on optical binding

Electro-optic response in isotropic media

A formal proof has recently been given to show that an electro-optic response of chiral molecules in isotropic media is forbidden by time reversal invariance. A claim to the contrary by Beljonne et al. for the occurrence of such an effect in second order, when dephasing terms are included, is examined critically using a quantum electrodynamics (QED) formulation

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