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The standard method for manipulating small particles is the optical tweezer, which relies on the strong optical gradient found at the focus of highly focused beams. However, this is not easily scalable for control of several particles simultaneously. Our recent work explores the possibility of achieving optical forces for manipulation of particles without using focused illumination, instead relying on near-field interactions when particles near a surface are illuminated with a simple plane wave, to provide repulsive and lateral forces, which can be controlled via the illuminating light’s polarisation, enabling fast modulation speeds. The use of plane waves would allow massively-parallel control, movement and sorting of vast numbers of particles simultaneously. A crucial requirement is active levitation because electrically polarised particles are known to strongly attract towards most uncharged surfaces naturally, hindering manipulation. Previous concepts for overcoming this obstacle have involved epsilon-near-zero metamaterials or multi-layered structures that introduce a degree of difficulty into the fabrication.
In this work, we show that particles with an optical magnetic dipole resonance naturally repel from a plasmonic surface, such as any metal below its plasma frequency. Optical magnetic resonances have already been suggested as a means for novel nanophotonic applications, including Huygens metasurfaces and plasmon-assisted tractor beams. The repulsion is driven by the optical gradients found in the particle’s backscattering from the surface. We have conducted numerical proof-of-principle simulations that show this repulsion occurring for a realistic core-shell particle (designed to emit pure magnetic dipole scattering) near a bulk gold surface, showing clear evidence that the repulsion is driven by the particle’s magnetic resonance. We therefore propose a straightforward experiment that can demonstrate active levitation of a nanoparticle without structured light. We anticipate new nanomechanical devices deriving from this principle.
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