Efficient method for the calculation of the optical force of a single nanoparticle based on the quasinormal mode expansion.

An efficient method for the calculation of the optical force of a single nanoparticle is proposed based on the expansion of quasinormal modes (QNMs), which are eigensolutions of source-free Maxwell's equations with complex eigenfrequencies. In this method, the optical force is calculated by integrating the Maxwell stress tensor (MST) over a closed surface encompassing the nanoparticle. The electromagnetic (EM) field required for evaluating the MST is computed by a rigorous modal analysis, in which the EM field is expanded onto a small set of QNMs. Once the QNMs of the nanoparticle are solved, their excitation coefficients are obtained analytically. This means that additional full-wave computations are not required if the nanoparticle's location and the wavelength or distribution of the excitation field vary. Comparisons with full-wave numerical calculations of optical force evidence the high efficiency and accuracy of our formalism.