Abstract
We investigated theoretically and numerically the optical pulling and pushing forces acting on silicon (Si) nanospheres (NSs) with strong coherent interaction between electric and magnetic resonances. We examined the optical pulling and pushing forces exerted on Si NSs by two interfering waves and revealed the underlying physical mechanism from the viewpoint of electric- and magnetic-dipole manipulation. As compared with a polystyrene (PS) NS, it was found that the optical pulling force for a Si NS with the same size is enlarged by nearly two orders of magnitude. In addition to the optical pulling force appearing at the long-wavelength side of the magnetic dipole resonance, very large optical pushing force is observed at the magnetic quadrupole resonance. The correlation between the optical pulling/pushing force and the directional scattering characterized by the ratio of the forward to backward scattering was revealed. More interestingly, it was found that the high-order electric and magnetic resonances in large Si NSs play an important role in producing optical pulling force which can be generated by not only s-polarized wave but also p-polarized one. Our finding indicates that the strong coherent interaction between the electric and magnetic resonances existing in nanoparticles with large refractive indices can be exploited to manipulate the optical force acting on them and the correlation between the optical force and the directional scattering can be used as guidance. The engineering and manipulation of optical forces will find potential applications in the trapping, transport and sorting of nanoparticles.
Highlights
If the light wavelength is chosen in this spectral range, it is generally sufficient to consider only the EDs and MDs with coherent interaction when discussing the optical force acting on such Si NSs
We have investigated theoretically and numerically the optical pulling and pushing forces acting on Si NSs which possess strong coherent interaction between electric and magnetic resonances
The physical mechanism responsible for the generation of optical pulling force under the irradiation of two interfering waves is elucidated based on the manipulation of the electric and magnetic dipoles induced by the incident light with different polarizations
Summary
Silicon (Si) nanospheres (NSs) with diameters ranging from 100 to 250 nm, which exhibit distinct electric and magnetic dipole (ED and MD) resonances in the visible to near inferred spectral range, have attracted tremendous interest because they are considered as the most promising building blocks for metamaterials operating at optical frequencies where artificial atoms made of metals fail to work due to large Ohmic loss [1,2,3,4,5,6,7,8,9,10,11,12,13]. Optical pulling force, which makes particles move against the propagation direction of light in the absence of intensity gradient, has stimulated the interest of researchers working in this field because it is a counter-intuitive phenomenon since the discovery of optical radiation pressure It was discussed theoretically by Chen et al by using a Bessel beam and demonstrated experimentally by Brzobohatý et al by using two interfering waves [36,37]. For nanoparticles with large refractive indices such as Si, germanium (Ge) and gallium arsenide (GaAs) NSs, it is expected that optical pulling force can be achieved at a much smaller size because of the significant directional scattering resulted from the strong coherent interaction between EDs and MDs. The optical force generated by single plane wave irradiation has been systematically investigated for Ge NSs [41]. The orientations of the ED and MD induced by the two interfering waves with different polarizations and the resulting directional scattering and optical force are illustrated
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