Bipolar optical force on a dipolar nanoparticle: Thermally induced switching between optical pulling and optical pushing

The ability to readily switch between optical pulling and pushing forces on micro-objects is both important and fascinating for arbitrary optical manipulations. The bipolar optical force (namely, optical pulling versus optical pushing) on an object can be achieved generally by elaborately tailoring illuminating optical fields and/or surrounding media. Here, we propose an approach to achieve the temperature-reconfigured bipolar optical force on a dipolar nanoparticle. The basic idea is demonstrated on a particle immersed in a Bessel beam and made of material exhibiting a temperature-induced reversible transition, such as vanadium dioxide. Numerical results based on the decomposed force expressions indicate that the optical pulling force originates from the recoil force dominated by the coupling of electric and magnetic dipoles simultaneously excited on the particle, while the optical pushing force comes mostly from the interception force, which surpasses the recoil force when the excitation of the magnetic dipole is suppressed. Our dynamical simulations show that the nanoparticle can be transversely trapped stably in the vicinity of maximum light intensity while longitudinally switchable between the pulling against and pushing along light propagation. The bipolar characteristic does not require any change of the manipulating light or ambient material, opening interesting possibilities in flexible optical manipulations.