Plasmonic modes, microscopic optical force densities, and curvature induced electrostatic forces of nanoparticle systems

Plasmonic resonances of nanoparticles have drawn lots of attentions due to their interesting and useful properties such as strong field enhancements. These systems are typically studied using either classical electrodynamics or fully quantum theory. Each approach can handle some aspects of plasmonic systems accurately and efficiently, while having its own limitation. The self-consistent hydrodynamics model (SC-HDM) has the advantage that it can incorporate the quantum effect of the electron gas into classical electrodynamics in a consistent way. We use the method to study the plasmonic response of polygonal rods under the influence of an external electromagnetic wave, and we pay particular attention to the size and shape of the particle and the effect of charging. Since our method could give the correct total optical forces, we further demonstrate that the microscopic force density of the electron gas can be defined numerically and calculated within SC-HDM that includes quantum, non-local and retardation effects. We demonstrate this technique by calculating the microscopic optical force density distributions and the optical binding force induced by external light on nanoplasmonic dimers. We discover that an uneven distribution of optical force density can lead to a spinning torque acting on individual particles.Finally, we show that there is a new kind of corrugation-induced force that acts between electrically neutral plasmonic surfaces. Absent in flat surfaces, such a force owns its existence entirely to geometric curvature, and originates from the kinetic energy associated with the electron density which tends to make the profile of the electron density smoother than that of the ionic background and hence induces curvature-induced electrostatic local charges. Such a force cannot be found using standard classical electromagnetic approaches, and we use SC-HDM as well as first principles density functional calculations to explore the character of such forces. We found that the force can be attractive or repulsive, depending on the details of the nano-corrugation, and its magnitude is comparable to light induced forces acting on plasmonic nano-objects.