Session D62: Nanostructures and Metamaterials III

Topological structures of and for light

Topology is an invaluable tool to elucidate the properties of physical systems. Typically, it refers to instances where an entity has so-called topological protection, which means that the entity cannot disappear if small changes are made to the system. In this lecture I will address two illustrations of topological protection in light fields.
The first relates to optical singularities, which can have a topological charge. I will experimentally show that the charge of both phase- and polarization singularities plays a role in how they are distributed in space and also in the way they are created and annihilated. The second relates to the behavior of light in photonic nanostructures of which the properties have a non-trivial topology. We experimentally investigated the propagation of edge states between topologically non-trivial photonic crystals. I will show how the propagation direction is related to the far-field optical spin of the modes. Time allowing I will address the heterogeneity of the spatial distribution of the near-field optical spin of the mode, which is of importance to quantum optical application of these states.   

Topological phases of electromagnetic waves in superlattices of negative- and positive- epsilon materials

Recently, a new topological invariant is proposed to classify electromagnetic waves in an isotropic medium with positive or negative permittivity and permeability, and various interface modes such as surface plasmons are interpreted as topological boundary modes [1]. We expect similar topological phases of electromagnetic waves even for metamaterials.
To this end, we consider the electromagnetic waves in a 1D multilayer consisting positive- and negative- epsilon layers, studied in [2]. Depending on the ratio of layer thicknesses and the values of the dielectric constants of the constituent layers, the spectra changes, and the dispersion opens a gap at the origin depending on the spatial average of the dielectric constant. We associate this with a change in the topological number proposed in [1]. In addition, we find that interface states appear at an interface between different topological phases as expected.

[1] K.Y. Bliokh, D. Leykam, M. Lein, F. Nori, Nat. Commun. 10, 580 (2019); [2] A.P. Vinogradov, A.V. Dorofeenko, I.A. Nechepurenko, Metamaterials 4, 181 (2010).
  

Diverged photonic spin hall effect in Weyl Photonic metamaterial

Weyl semimetal is gaining increasing attention due to the unprecedented physical observables including the chiral magnetoresistance effect in condensed physics matter. In optics however, observables besides the detection of Weyl node and the Fermi arcs are still in lack. In this work, we discover that when circularly polarized beams are reflected by a Weyl medium, a handedness dependent spin hall effect can be realized due to the topological properties of the scattering matrix arises from the Weyl node. The scattering properties of the Weyl node could dramatically influence the trajectory of the light in real space resulting in a handedness dependent Imbert-Fedorov shift. More importantly, the shift is shown to be diverging in approximate to the Weyl node. Our discovery creates a whole new and readily available platform realizing diverged spin hall effect of optical Weyl medium.   

Optically tunable topological photonic structures

Photonic topological insulators offer the possibility to eliminate backscattering losses and improve the efficiency of optical communication systems. We combine the properties of a planar silicon photonic crystal and the concept of topological protection to demonstrate dynamically controlled transmission in a topological photonic crystal that exhibits the valley Hall effect.   

Non-Hermitian landau damping and bulk Fermi arc in superlattice-gate-tuned graphene plasmons

Landau damping, in which a plasmon emits an electron-hole pair, has been considered as a hindering factor for low-loss nanophotonic applications of graphene surface plasmon-polaritons (GSP), and is usually avoided by Pauli-blocking at high doping. In contrast to this simple view against landau damping, here we unravel its ironic utility and show how significantly this loss mechanism can extend the existing framework of graphene nanophotonics. Given a properly designed spatial pattern, landau damping becomes a key element for exotic non-Hermitian band structure engineering of GSPs, without damaging their quality factor severely. We propose that periodic damping textures can be assigned by field-effect gating through a combination of two electrodes; a metagate, thin and periodically punctured, provides a superlattice photonic crystal structure to GSPs, and a backgate, placed far below the metagate, controls local carrier densities on graphene selectively on the regions above holes in the metagate. This setting allows us to use the backgate voltage as a switch for the bulk Fermi arc, a topological signature of an exceptional point pair in the band structure. Our work thus opens up a new regime of graphene plasmonics involving open systems and non-Hermitian topological physics.   

SAW interdigitated transducers as topological mechanical metamaterial

A lattice model is developed to describe the mechanical displacement of and current through each electrode of a surface acoustic wave (SAW) interdigitated tranducer (IDT). Each electrode of an IDT is treated as a mass connected mechanically to its neighbors with a spring and electrically with a capacitor. Simulations for the electrical admittance of a typical SAW IDT are performed and compared with the results of an accurate FEM simulation. The utility of this lattice model is demonstrated by simulating the admittance of an IDT structure known as a hiccup resonator, which has a mode in the center of the band gap. It is shown here that this mode is a topologically protected edge state described by the 1D Su-Schrieffer-Heeger (SSH) model. Hiccup resonators have been used in commercial products for decades, and as such it may considered the first mass-produced topological mechanical metamaterial.   

A polarization-actuated plasmonic circulator

We show both by simulation and experiment, a plasmonic structure functions as a nanoscopic optical circulator when the input laser beam is properly polarized. Moreover, depending on the helicity of the spin angular momentum carried by the light, the circulation direction of surface plasmons could be correspondingly controlled.   

Integration of single layer MoSe2 with photonic rings for electrically switchable routing of light

Semiconducting transition metal dichalcogenide monolayers exhibit pronounced optical signatures in the visible and near-infrared regimes due to strong excitonic resonances, which can be modulated significantly through electrostatic doping. When interfaced with such tunable dielectric environments, nanophotonic devices inherit gate-dependent functionalities. Here, we demonstrate an electro-optical switch composed of a titanium dioxide ring resonator fabricated on monolayer molybdenum diselenide. Gate-varying measurements reveal electrically tunable cavity transmission, suggesting future application in on-chip photonic circuitry.   

Bound States in the Continuum in Elastic Metamaterials

Diffraction of elastic waves is considered for a system consisting of two parallel arrays of thin (subwavelength) cylinders that are arranged periodically. The embedding media supports waves with all polarizations, one longitudinal and two transverse, having different dispersion relations. An interaction with defects mixes the in-plane longitudinal and transverse modes. It is shown that the system supports bound states in the continuum (BSC) that have no specific polarization, that is, there are standing waves localized in the scattering structure whose wave numbers lies in the first open diffraction channels for both longitudinal and transverse modes. BSCs are shown to exists only for specific distances between the arrays and for specific values of the wave vector component along the array. An analytic solution is obtained for BSCs containing coupled elastic waves with different dispersion relations. For distances between the parallel arrays much larger than the wavelength, the existence of BSCs is explained by a destructive interference similar to the interference in a Fabri-Perot interferometer.   

Strong nonreciprocal transmission self-induced by nonlinear PT-symmetric metamaterials

Self-induced nonreciprocity based on optical nonlinear effects is a more appealing technique compared to other ways to achieve nonreciprocal response due to the absence of any kind of bias based on magnets, currents, or time modulation. We demonstrate strong self-induced nonreciprocal transmission by using a new compact nonlinear parity-time (PT) symmetric system based on epsilon-near-zero (ENZ) metamaterials photonically doped with gain and loss defects [B. Jin and C. Argyropoulos, “Nonreciprocal Transmission in Nonlinear PT-Symmetric Metamaterials Using Epsilon-near-Zero Media Doped with Defects,” Adv. Opt. Mat., p. 1901083 (2019)]. The strong self-induced nonreciprocal transmission arises from the extreme asymmetric field distribution achieved upon excitation from opposite incident directions combined with the boosted nonlinear effects at the nanoscale. The transmittance from one direction is kept exactly unity while the transmittance from the other direction is decreased to very low values, achieving very high optical isolation. The presented work can have a plethora of applications, such as nonreciprocal ultrathin coatings for the protection of sources or other sensitive equipment from external pulsed signals, circulators, and isolators without the use of bulk magnets.   

High-efficient harmonic generation via Plasmonic Tamm states

Plasmonic Tamm states in nanoscaled plasmonic waveguides can increase the electric and magnetic field intensity separately by three orders of magnitude via the spatial confinement and phase-matching. Theoretical or numerical models about plasmonic Tamm states have been proposed in metal-insulator-metal plasmonic waveguides and graphene plasmonic crystals. In this work, we experimentally realized plasmonic Tamm states by combining a nanoantenna with a plasmonic Tamm structure. The field enhancement was characterized by our home-made multi-photon microscope, in which highly efficient second and third harmonic generations were both observed. Our work demonstrates a promising potential for electromagnetic nanofocusing and nanosensing.