Optical trapping of gain-assisted plasmonic nano-shells: Theorical study of the optical forces in a pumped regime below the emission threshold

We study theoretically the opto-mechanics of a metallic nano-shell with a gain-enriched dielectric core in stationary Optical Tweezers. In order to avoid the counterproductive effects of scattering forces we choose a two counter-propagating beams configuration. The application of an external pump enhances the plasmonic resonance of the nano-shell thus affecting the optical forces acting on the particle even at pump powers below the emission threshold. We show that the trapping strength can be largely improved without the necessity to increase the trapping beam power. We support the theoretical analysis with Brownian dynamics simulations that show how particle position locking is achieved at high gains in exended optical trapping potentials. Finally, for wavelengths blue-detuned with respect to the plasmon-enhanced resonance, we observe particle channeling by the standing wave antinodes due to gradient force reversal.