Optical chirality from dark-field illumination of planar plasmonic nanostructures

Dark-field illumination is shown to make planar chiral nanoparticle arrangements exhibit circular dichroism in extinction analogous to true chiral scatterers. Circular dichrosim is experimentally observed at the maximum scattering of single oligomers consisting rotationally symmetric arrangements of gold nanorods, with strong agreement to numerical simulation. A dipole model is developed to show that this effect is caused by a difference in the geometric projection of a nanorod onto the handed orientation of electric fields created by a circularly polarized dark-field that is normally incident on a glass substrate. Owing to this geometric origin, the wavelength of the peak chiral response is also experimentally shown to shift depending on the separation between nanoparticles. All presented oligomers have physical dimensions less than the operating wavelength, and the applicable extension to closely packed planar arrays of oligomers is demonstrated to amplify the magnitude of circular dichroism. The realization of strong chirality in these oligomers demonstrates a new path to engineer optical chirality from planar devices using dark-field illumination

Scattering matrix for chiral harmonic generation and frequency mixing in nonlinear metasurfaces

We generalize the concept of optical scattering matrix ($S$-matrix) to characterize harmonic generation and frequency mixing in planar metasurfaces in the limit of undepleted pump approximation. We show that the symmetry properties of such nonlinear $S$-matrix are determined by the microscopic and macroscopic symmetries of the metasurface. We demonstrate that for description of degenerate frequency mixing processes such as optical harmonic generation, the multidimensional $S$-matrix can be replaced with a reduced two-dimensional $S$-matrix. We show that for metasurfaces possessing specific point group symmetries, the selection rules determining the transformation of the reduced nonlinear $S$-matrix are simplified substantially and can be expressed in a compact form. We apply the developed approach to analyse chiral harmonic generation in nonlinear metasurfaces with various symmetries including rotational, in-plane mirror, and out-of-plane mirror symmetries. For each of those symmetries, we confirm the results of the developed analysis by full-wave numerical calculations. We believe our results provide a new paradigm for engineering nonlinear optical properties of metasurfaces which may find applications in active and nonlinear optics, biosensing, and quantum information processing.Comment: 16 pages, 7 figures, 3 tables, 46 reference

Evolution of topological edge modes from honeycomb photonic crystals to triangular-lattice photonic crystals

The presence of edge modes at the interface of two perturbed honeycomb photonic crystals with C6 symmetry is often attributed to the different signs of Berry curvature at the K and K′ valleys. In contrast to the electronic counterpart, the Chern number defined in photonic valley Hall effect is not a quantized quantity but can be tuned to a finite value including zero simply by changing geometrical perturbations. Here, we argue that the edge modes in photonic valley Hall effect can exist even when Berry curvature vanishes. We numerically demonstrate the presence of the zero-Berry-curvature edge modes in triangular-lattice photonic crystal slab structures in which C3 symmetry is maintained but the inversion symmetry is broken. We investigate the evolution of the Berry curvature from the honeycomb-lattice slab structure to the triangular-lattice photonic crystal slab and show that the triangular-lattice photonic crystals still support edge modes in a very wide photonic band gap. We find that the edge modes with zero Berry curvature can propagate with extremely low bending loss along the interface formed by the triangular-lattice photonic crystals

Directional coupling of surface plasmon polaritons at complementary split-ring resonators

AbstractWe propose a complementary split-ring resonator (CSRR) for a directional coupling of surface plasmon polaritons. An air-slot split-ring in a gold film is investigated using the finite-difference time-domain method. The normally incident light couples to either a monopole or a dipole SPP depending on the polarization of light. Adjusting the angle of the linear polarization of the incident light enables a one-way propagation of SPPs on the gold film. Theoretical analysis based on the propagation of cylindrical waves from the SPP point source is provided with Hankel function. The propagated power in one direction is obtained to be 30 times higher than the opposite direction with a coupling efficiency of 18.2% from the simulation for an array of the CSRRs. This approach to the directional coupling of SPPs will be advantageous for miniaturizing photonic and plasmonic circuits and devices.</jats:p