Negative force on free carriers in positive index nanoparticles

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