Negative force on free carriers in positive index nanoparticles

Abstract

We theoretically demonstrate the reversal of optical forces on free charge carriers in positive refractive index nanostructures. Though optical momentum in positive refractive index materials is necessarily parallel to the local energy flow, reversal of optical momentum transfer can be accomplished by exploiting the geometry and size of subwavelength particles. Using the Mie scattering theory and separation of optical momentum transfers to the bound and free charges and currents, we have shown that metal nanoparticles can exhibit strong momentum transfer to free carriers opposite to the direction of incident electromagnetic waves. This can be explained for small particles in terms of a reversal of Poynting power inside the material resulting in a negative net force on free carriers in small particles. Two-dimensional simulations further illuminate this point by demonstrating the effect of incident wave polarization.