Wanli Lu

A simple design of an artificial electromagnetic black hole

We conduct a rigorous study on the properties of an artificial electromagnetic black hole for transverse magnetic modes. A multilayered structure of such a black hole is then proposed as a reduced variety for easy experimental implementations. An actual design of composite materials based on the effective medium theory is given with only five kinds of real isotropic materials. The finite element method confirms the functionality of such a simple design.

Transformation media based super focusing antenna

We propose a new kind of focusing antenna with a large effective diameter based on transformation optics. The device contains a traditional parabolic antenna embedded in a dielectric core and coated by a negative index shell. Numerical simulations are performed to illustrate its advantages.

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.

Optical torque on small chiral particles in generic optical fields

We derive an analytical expression of the optical torque (OT) on chiral particles in generic monochromatic optical fields within the dipole approximation. Besides the extinction terms owing to the interaction between the dipoles excited on the particle and the incident field, our expression includes also some long missing terms that are understood as recoil due to the radiation of the dipoles excited on the particle. The recoil terms are shown to have a significant contribution to the OT in many situations. Inspired by our expression, we further proved that the net OT vanishes for a lossless isotropic chiral spherical particle of any size illuminated by arbitrary monochromatic optical fields. Finally, we trace the origin of OT on a small chiral sphere immersed in a zeroth-order vector Bessel beam, taking advantage of our analytical expression. It is found that the azimuthal OT perpendicular to the beam’s propagation direction comes from the transfer of the spin angular momentum of the incident field to the particle, while the longitudinal OT along the illumination direction originates from the particle chirality, which generates longitudinal angular momentum on the optical beam and thus makes the particle subject to a longitudinal OT by recoil. Thus the longitudinal OT tends to rotate the chiral particle with opposite helicities in opposite directions, while the transverse OT shows little dependence on the handedness of particle chirality. Our results may help in understanding OT as an extra handle for particle manipulations in optical tweezers.

Selectively transporting small chiral particles with circularly polarized Airy beams

Based on the full wave simulation, we demonstrate that a circularly polarized vector Airy beam can selectively transport small chiral particles along a curved trajectory via the chirality-tailored optical forces. The transverse optical forces can draw the chiral particles with different particle chirality towards or away from the intensity maxima of the beam, leading to the selective trapping in the transverse plane. The transversely trapped chiral particles are then accelerated along a curved trajectory of the Airy beam by the chirality-tailored longitudinal scattering force, rendering an alternative way to sort and/or transport chiral particles with specified helicity. Finally, the underlying physics of the chirality induced transverse trap and de-trap phenomena are examined by the analytical theory within the dipole approximation.

Rigorous full-wave calculation of optical forces on dielectric and metallic microparticles immersed in a vector Airy beam

Based on the generalized Lorenz-Mie theory and the Maxwell stress tensor approach we present the first rigorous full-wave solution of the optical forces acting on spherical microparticles immersed in a two-dimensional vector Airy beam beyond the paraxial approximation. The critical aspect lies in evaluating efficiently and accurately the partial wave expansion coefficients of the incident Airy beam, which are achieved by using the vector angular spectrum representation for a variety of polarizations. The optical field distributions are then simulated to show the self-accelerating and self-healing effects of the Airy beam. The dielectric and gold microparticles are shown to be trapped within the main lobe or the nearby side-lobes mostly by the transverse gradient optical force while driven forward along the parabolic trajectory of the Airy beam by the longitudinal scattering force. It is thus demonstrated theoretically that the vector Airy beam has the capability of precisely transporting both dielectric and metallic microparticles along the prespecified curved paths.