Chapter 1 - Molecular Quantum Electrodynamics of Radiation-Induced Intermolecular Forces

Abstract

Recent advances in laser technology have resulted in new phenomena involving radiation-induced interparticle forces being observed such as the cooling, trapping and manipulation of atoms and molecules, and their optical binding. The study of the last mentioned of these processes from a theoretical standpoint is the subject of the present article. This is carried out within the framework of nonrelativistic quantum electrodynamics, whose background is first explicated. Because light modifies the intermolecular potential, a brief summary of the calculation of the retarded van der Waals dispersion energy between a pair of molecules is given. This is followed by the evaluation of the radiation-induced energy shift using two different physical viewpoints and calculational techniques. One involves fourth-order, diagrammatic, time-dependent perturbation theory, whereas the other entails the coupling of electric dipole moments at each center induced by the externally applied radiation field. Identical results are obtained for interaction energies for two identified mechanisms—dynamic and static. The contribution to the optical binding force arising from coupling of chiral molecules is also derived in order to ascertain whether discriminatory effects occur. This is done through the fluctuating moment method. Applications of the theory are then made to spherical and cylindrical nanoparticles by investigating the topology of the potential energy surface. Regions where forces and torques are maximized, minimized, and vanish are identified, as well as conditions under which particle array formation is favored. The effect of broadband, or coherent, or Laguerre–Gaussian radiation, in addition to a second counter-propagating beam, on the energy shift is also examined.