Physical Review

B60 2363-74


Electromagnetic Forces in Photonic Crystals

M.I. Antonoyiannakis and J.B. Pendry


We have developed a general methodology for numerical computations of electromagnetic (EM) fields and forces in matter, based on solving the macroscopic Maxwell's equations in real space and adopting the Maxwell Stress Tensor formalism. Our approach can be applied to both dielectric and metallic systems of frequency-dependent dielectric function; as well as to objects of any size and geometrical properties in principle. We are particularly interested in calculating forces on nanostructures.

Our findings confirm that a body reacts to the EM field by minimising its energy, i.e. it is attracted (repelled) by regions of lower (higher) EM energy. When travelling waves (of real wavevector) are involved, forces can be additionally understood in terms of momentum exchange between the body and its environment. However when evanescent waves (of complex wavevector) dominate, the forces are complicated, often become attractive and cannot be explained by means of real momentum being exchanged.

We have studied the EM forces induced by a laser beam on a crystal of dielectric spheres of GaP. We observe effects due to the lattice structure, as well as due to the single scattering from each sphere. In the former case the two main features are a maximum momentum exchange (and largest forces) when the frequency lies within a band gap; and a multitude of force orientations when the Bragg conditions for multiple outgoing waves are met. In the latter case the radiation couples to the EM eigenmodes of isolated spheres (Mie resonances) and very sharp attractive and repulsive forces occur. Depending on the intensity of the incident radiation these forces can overcome all other interactions present (gravitational, thermal and Van der Waals) and may provide the main mechanism for formation of stable structures in colloidal systems.

This paper is available from the Condensed Matter e-Print Archive, as a PDF file or as a PostScript file.

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