Efficient method for the calculation of the optical force of multiple nanoparticles based on the coupling theory of quasinormal modes.

An efficient method is proposed for the calculation of the optical force of multiple nanoparticles. 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 with the coupling theory of quasinormal modes (QNMs), in which the EM field is expanded onto a small set of QNMs of each nanoparticle. Once these dominant modes, which are eigensolutions of source-free Maxwell equations with complex eigenfrequencies, are known, any variation of the interparticle distance, illumination polarization, or wavelength can be treated analytically. Comparisons with the full-wave numerical method demonstrate the accuracy and efficiency of the formalism. With the formalism, force maps are calculated at remarkable computation speed, providing a promising simulation tool for applications such as plasmon tweezer and photoinduced force microscopy.