Optical Control of Intermolecular Forces

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The use of laser light to manipulate small particles and cells / commonly known as 'optical tweezers' / is a well established research tool. It is increasingly prominent, finding applications in diverse laboratories. Tweezer methods generally operate by exerting a force that attracts particles to regions of high intensity, usually the centre of the laser beam. This mechanism, and others that are employed in laser cooling and trapping of atoms, utilise forces that operate directly on individual particles of matter. Recently a wave of excitement has been created by a discovery of something quite different: optically induced forces that operate between particles, over nanoscale dimensions. Such inter-particle forces offer a number of highly distinctive features which can be exploited for the controlled optical manipulation of matter. Through such interactions, new opportunities for creating optically ordered matter have already been demonstrated both theoretically and experimentally, leading to the introduction of terms such as 'optical binding' and 'optical matter' in recent literature. Whereas the forces that determine normal physical and chemical behaviour owe their origin to the intrinsic properties of individual molecules, these optically induced forces operate very differently / and best of all, they are experimentally controllable. This is an area of science that is now advancing with extreme rapidity, its enormous potential having been first predicted in an influential paper on the future of chemistry, published fifteen years ago. Last year the applicant and co-workers at the University of East Anglia developed a comprehensive theory of optically induced inter-particle forces, identifying a number of effects not previously determined, and showing that such forces can be either attractive or repulsive according to given conditions. Calculations based on the new theory were performed for a variety of systems, identifying and illustrating the scope for applications. The first results on carbon nanotubes indicated possibilities for optically modifying the structure of nanotube films, while subsequent applications identified new forms of optical patterning and clustering. Now it is proposed to develop theory for application to more complex systems, to pave the way for innovative technical applications. The first task will be to construct a detailed representation of how these inter-particle forces operate within a variety of systems including atomic clusters, colloids and liquid crystals; the formation and stability of optically ordered microarrays will also be examined. Focusing next on surface and near-surface effects, methods are to be sought for optically modifying the character of surface adsorption, molecular interactions on surfaces, and the optical ordering of nanoparticles in suspension. Fully characterising such effects should unlock significant materials applications. Attention will also be given to the special effects anticipated when particles are irradiated by the complex optical fields associated with structured light, such as the exotic laser beams known as 'optical vortices'. Other systems of interest are the optomechanical response to the patterned light field that results from the interference between laser beams. It has been shown that holographic methods can simultaneously trap and steer hundreds of suspended particles, yet the detailed role of optical binding in such applications has not yet been studied; the results of this investigation will be keenly awaited. Other issues to be examined are the possibility of optically modifying surface treatments, and processes of thin film production.