Optically induced inter-particle forces: from the bonding of dimers to optical electrostriction in molecular solids

The quantum electrodynamical formulation of the Casimir-Polder interaction invites a consideration of optically induced inter-particle forces, which arise in the same level of perturbation theory. For the observation of such effects, quantitative assessments of the coupling mechanism suggest levels of intensity that are now routinely available from pulsed lasers. In this paper, a theoretical analysis of the principal mechanism is followed by a development for two specific systems, chosen to illustrate the broad significance and the range of systems in which such effects might be manifest. The first system is a van der Waals dimer, a loosely bound and essentially isolated molecular pair in which a small optically induced shift in the equilibrium bond length proves to be well within the limits of measurement by microwave rotational spectroscopy. The second system illustrates the opposite extreme, where an optically induced modification of the forces between densely packed molecules in the condensed phase is shown to produce anisotropic patterns of laser-induced compression and expansion, an effect termed optical electrostriction. Again, such an effect should be readily measurable, although the necessary conditions are such that a number of secondary effects might also be likely to arise. The experimental challenge of precluding those secondary effects in the condensed phase and unequivocally identifying optically induced intermolecular forces is discussed. Possible applications are also entertained, including optical actuators for nanoscale electromechanical systems