# Fano-enhanced pulling and pushing optical force on active plasmonic nanoparticles

## TL;DR

This paper demonstrates tunable optical pulling and pushing forces on plasmonic nanoparticles enhanced by Fano resonance, revealing new mechanisms for manipulating light-matter interactions in active nanostructures.

## Contribution

It introduces a method to achieve large optical forces using Fano resonance in active plasmonic nanostructures with gain and loss compensation.

## Key findings

- Giant pulling forces are achieved at quadrupole Fano resonance.
- Optical gain compensates metal losses, enabling strong mode coupling.
- The Lorentz force analysis clarifies the origin of negative forces.

## Abstract

We demonstrate tunable pulling and pushing optical forces on plasmonic nanostructures around Fano resonance. The plasmonic nanostructure containing a spherical core with optical gain and a metallic shell shows much larger optical pulling force than a pure gain sphere. One can obtain large field enhancement and giant pulling force at the emerged quadrupole mode. The introduction of optical pump compensate the dissipative loss from metal shell, thus enable the strong coupling between a narrow quadrupole mode and a board dipole mode, giving rise to Fano resonance. The giant negative forces origin from the reversal of electric field at Fano resonance, which lead to pulling force on bound currents and charges. Meanwhile, the separation of the Lorentz force helps to reveal the nature of the pulling forces in gain system. We have shown that by applying the Lorentz force density formula, it is possible to obtain the correct value of the force inside our complex inhomogenous structure made up of dispersive and lossy metamaterial irrespective of the electromagnetic momentum density. Our results provide a practical way to manipulate nanoparticles and give deep insight into light-matter interaction.

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Source: https://tomesphere.com/paper/1704.06477