We analyze the evolution of a quantum Brownian particle starting from an initial state that contains correlations between this system and its environment. Using a path-integral approach, we obtain a master equation for the reduced density matrix of the system finding relatively simple expressions for its time-dependent coefficients. We examine the evolution of delocalized initial states ͑Schrödinger cats͒ investigating the effectiveness of the decoherence process. Analytic results are obtained for an Ohmic environment ͑Drude's model͒ at zero temperature.

In a variety of mesoscopically disordered systems, high levels of optical excitation resulting from pulsed laser irradiation can establish optical coherence within separate particles or locally ordered domains, leading to second harmonic emission whose temporal signature characterizes the decay of the excited state population. Examples of such systems will include colloids, cell and membrane suspensions, and many plastics, glasses and other modern materials. With pulsed excitation of sufficient intensity to elicit the onset of saturation, second harmonic emission on the throughput of a subsequent probe beam exhibits a characteristic decay and recovery. Detailed calculations show that such features arise not only in systems whose optical response involves two discrete levels, but also in systems of considerably greater electronic complexity. Deconvolution of the temporal trace of the harmonic signal serves as an independent means of monitoring the decay of the excited state. The extent of recovery in the harmonic signal also serves to register the extent of local coherence, and hence in many systems the localization of structural order. Finally, the principles introduced in the theory are shown to be applicable to other types of system such as certain photochromic materials.

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

Individual stable atoms and molecules are seldom found far apart unless they are gases. Most matter exists in a condensed phase whose cohesion and integrity are due to fundamental interactions among the particles. The balance between dispersion forces of attraction and short-range electron-cloud repulsions—as in the Lennard-Jones potential—supports the formation of stable equilibrium structures. Remarkably, off-resonant laser radiation can induce additional, optomechanical forces between particles. A publication by Michael Burns and his team1 verified this effect experimentally for a simple system of two identical, spherical particles

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