Huajin Chen

Fano Resonance-Induced Negative Optical Scattering Force on Plasmonic Nanoparticles

We demonstrate theoretically that Fano resonance can induce a negative optical scattering force acting on plasmonic nanoparticles in the visible light spectrum when an appropriate manipulating laser beam is adopted. Under the illumination of a zeroth-order Bessel beam, the plasmonic nanoparticle at its Fano resonance exhibits a much stronger forward scattering than backward scattering and consequently leads to a net longitudinal backward optical scattering force, termed Fano resonance-induced negative optical scattering force. The extinction spectra obtained based on the Mie theory show that the Fano resonance arises from the interference of simultaneously excited multipoles, which can be either a broad electric dipole mode and a narrow electric quadrupole mode, or a quadrupole and an octupole mode mediated by the broad electric dipole. Such Fano resonance-induced negative optical scattering force is demonstrated to occur for core-shell, homogeneous, and hollow metallic particles and can therefore be expected to be universal for many other nanostructures exhibiting Fano resonance, adding considerably to the flexibility of optical micromanipulation on the plasmonic nanoparticles. More interestingly, the flexible tunability of the Fano resonance by particle morphology opens up the possibility of tailoring the optical scattering force accordingly, offering an additional degree of freedom to optical selection and sorting of plasmonic nanoparticles.

Lateral optical force on paired chiral nanoparticles in linearly polarized plane waves.

We demonstrate that a lateral optical force (LOF) can be induced on paired chiral nanoparticles with opposite handedness under the illumination of a linearly polarized plane wave. The LOFs on both chiral particles are equal and thus can move the pair sideways, with the direction depending on the separation between two particles, as well as the handedness of particle chirality. Analytical theory reveals that the LOF comes largely from the optical potential gradient established by the multiple scattering of light between the paired particles with asymmetric chirality. In addition, it is weakly dependent on the material loss of a particle, a feature of gradient force, while heavily dependent on the magnitude and handedness of particle chirality. The effect is expected to find applications in sorting and separating chiral dimers of different handedness.

Manipulating Unidirectional Edge States Via Magnetic Plasmonic Gradient Metasurfaces

We show that a strongly enhanced coupling of spatially propagating electromagnetic waves to self-guiding unidirectional edge states (UESs) can be achieved by engineering a magnetic plasmonic gradient metasurface (GMS) made of an array of ferrite rods. The conversion efficiency of the incident photons into self-guiding UESs exhibits a transition from zero on an ordinary periodic surface to nearly 80 % on a surface incorporating a GMS. The underlying physics lies in that the magnetic plasmonic GMS enables a direct excitation of the edge states due to the band-folding or momentum compensation effect, which are in turn transformed into the self-guiding UESs on the ordinary periodic surface. The excitation of the UESs can also be revealed by considering the partial wave scattering amplitudes of the constituent rods on the surface, which manifests a change from a standing wave in the region subject to an external illumination to a self-guiding wave propagating and confined on the surface, a signature of UESs. The magnetic plasmonic GMS can also be used to implement the unidirectional phase control of the UES and the nonreciprocal Goos-Hänchen shift as a consequence of the time-reversal-symmetry breaking nature of the system and the strong coupling of the incident wave. In addition, the unidirectional features are shown to be flexibly controlled by either tailoring the gradient or tuning the external magnetic field, adding considerably to the performance of the magnetic plasmonic GMS systems.

Continuously tuning effective refractive index based on thermally controllable magnetic metamaterials.

By employing a thermally active magnetic material, we theoretically design a kind of electromagnetic metamaterial with intrinsic magnetic response, termed magnetic metamaterial (MM). The retrieved effective electric permittivity ε(eff) and magnetic permeability μ(eff) exhibit a nearly continuous transition from double negative to double zero, and then to double positive by controlling the temperature, indicating a flexible tunability of the effective refractive index. The beam splitting, collimation, focusing, and total reflection are achieved at different typical temperatures. Most importantly, with the MM implemented under a gradient temperature, a gradient negative-zero-positive index metamaterial (NZPIM) can possibly be realized, thus providing a new platform to study wave features in NZPIM.

Giant omnidirectional radiation enhancement via radially anisotropic zero-index metamaterial

We demonstrate a remarkable enhancement of isotropic radiation via radially anisotropic zero-index metamaterial (RAZIM). The radiation power can be enhanced by an order of magnitude when a line source and a dielectric particle is enclosed by a RAZIM shell. Based on the extended Mie theory, we illustrate that the basic physics of this isotropic radiation enhancement lies in the confinement of higher order anisotropic modes by the RAZIM shell. The confinement results in some high field regions within the RAZIM shell and thus enables strong scattering from the dielectric particle therein, giving rise to a giant amplification of isotropic radiation out of the system. The influence of the loss inherent in the RAZIM shell is also examined. It is found that the attenuation of omnidirectional power enhancement due to the loss in the RAZIM can be compensated by gain particles.

Unidirectionally molding electromagnetic waves with magnetic metamaterials and metasurfaces

We have investigated a kind of metamaterials consisting of an array of ferrite rods, which possess intrinsic magnetic response, thus termed magnetic metamaterials (MMs). It is demonstrated that a Gaussian beam striking the interface of an MM slab exhibits nonreciprocal reflection and absorption due to the excitation of the nonreciprocal magnetic surface plasmon. By combing two MM slabs of opposite magnetization together, a one-way waveguide robust against position and size disorders as well as the inhomogeneity of an external magnetic field can be constructed as corroborated from both theoretical and experimental points of view.