Propagation of spoof surface plasmon on metallic square lattice: bending and splitting of self-collimated beams

Strongly suppressed diffuse scattering in periodic graphene metamaterials

As an emerging two-dimensional material, graphene offers an alternative material platform for exploring new metamaterial phenomena and device functionalities. In this work, we examine diffuse scattering properties in graphene metamaterials. We take periodic graphene nanoribbons as a representative example and show that diffuse reflection in graphene metamaterials as dominated by diffraction orders is restricted to wavelengths less than that of first-order Rayleigh anomaly, and is enhanced by plasmonic resonances in graphene nanoribbons, as similar to metamaterials made of noble metals. However, the overall magnitude of diffuse reflection in graphene metamaterial is less than 10^-2 due to the large period to nanoribbon size ratio and ultra-thin thickness of the graphene sheet, which suppress the grating effect from the structural periodicity. Our numerical results indicate that, in contrast to the cases of metallic metamaterials, diffuse scattering plays a negligible role in spectral characterization of graphene metamaterials in cases with large resonance wavelength to graphene feature size ratio, which corresponds to typical chemical vapor deposition (CVD)-grown graphene with relatively small Fermi energy. These results shed light on fundamental properties of graphene nanostructures and are helpful in designing graphene metamaterials for applications in infrared sensing, camouflaging, and photodetection, etc.

High-sensitivity quasi-periodic photonic crystal biosensor based on multiple defective modes

The sensitivities of the octagonal quasi-periodic photonic crystal (QPC) defective modes are theoretically studied. The octagonal QPC biosensors are composed of silicon columns arranged in a liquid background. By designing a defect structure, a variety of localized modes with different spatial symmetries and field profiles are obtained, and a maximum refractive index sensitivity 800 nm/RIU is achieved around 1500 nm transmission peak when the central rod's size equals 100 nm, and the corresponding detection limit reaches 0.00042. The liquid can flow freely among the rods through the entire structure, so it is convenient to monitor the concentration of protein in the liquid environment dynamically. The influence of the protein's thickness to the shift of the resonant wavelength is also studied, where a minimum protein's thickness of less than 10 nm can be detected by optimizing the central column's size to be 400 nm, and the spatial field profiles of different resonant modes are analyzed to explain the corresponding sensitivities.

Active Control of the Spoof Plasmon Propagation in Time Varying and Non-reciprocal Metamaterial

We present an efficient concept based on time varying and non reciprocal metamaterials to achieve an active control of the spoof plasmon (SP) propagation at sub-wavelength scale. An experimental demonstration of non-reciprocal guiding device based on split ring resonator is proposed as an application of this concept in the microwave regime. We show that this device is able to blue-shift the propagated SP waves and to achieve an active steering of these SPs at sub-wavelength scale by controlling the modulation frequency of the time varying metamaterial. This approach could be extended plainly to infrared and optical regimes by considering suitable technologies.

Strong beaming of microwave surface waves with complementary split-ring-resonator arrays

A thin copper sheet, populated by an array of complementary split ring resonators, presents strong surface wave beaming in orthogonal directions at two distinct frequencies. This simple array is significantly thinner than existing single frequency beaming surfaces. The observed beaming frequencies are associated with the two lowest resonance modes of the split rings, and the beams are subwavelength in width and approximately non-diverging. The beaming is analysed through comparison of near-field scans of the surface-normal electric fields with numerical simulations.

Spoof Plasmonics: From Metamaterial Concept to Topological Description

Advances in metamaterials have offered the opportunity of engineering electromagnetic properties beyond the limits of natural materials. A typical example is "spoof" surface plasmon polaritons (SPPs), which mimic features of SPPs without penetrating into metal, but only with periodic corrugations on metal surfaces. They hold considerable promise in device applications from microwaves to the far infrared, where real SPP modes do not exist. The original spoof SPP concept is derived from the description of corrugated surfaces by a metamaterial that hosts an effective plasma frequency. Later, studies have attempted to describe spoof SPP modes with the band structure by strictly solving Maxwell's equations, which can possess band gaps from polaritonic anticrossing principle or Bragg interference. More recently, as inspired by the development of topological framework in condensed matter physics, the topological description of spoof SPPs is used to propose topologically protected waveguiding phenomena. Here, the developments of spoof SPPs from both practical and fundamental perspectives are reviewed.

Wide-angle broadband absorption in tapered patch antennas

Strip array is a classical antenna structure, which provides an effective way to generate and explore new material properties and device functionalities. In this paper, we demonstrate wide-angle broadband absorption in patch antennas made of tapered strip arrays in the metal-insulator-metal geometry. By superimposing multiple resonances associated with the tapered width of the strips, near-perfect absorption is designed and realized over a wide bandwidth from 29.2 THz to 38 THz with efficiency exceeding 80% in the mid-infrared region. The strong absorption band is insensitive to incident angles up to 75°. The angle-independent absorption is attributed to the unique mechanism of coupling between relevant magnetic resonances and free-space incident light. Our tapered patch antenna design offers the advantage of simplicity, and therefore flexibility in engineering natural materials for strong omnidirectional absorption with a variable and wide bandwidth, which could be of interest in applications such as bolometric sensing, camouflaging, and spectral filtering.

Spatially variant periodic structures in electromagnetics

Spatial transforms are a popular technique for designing periodic structures that are macroscopically inhomogeneous. The structures are often required to be anisotropic, provide a magnetic response, and to have extreme values for the constitutive parameters in Maxwell's equations. Metamaterials and photonic crystals are capable of providing these, although sometimes only approximately. The problem still remains about how to generate the geometry of the final lattice when it is functionally graded, or spatially varied. This paper describes a simple numerical technique to spatially vary any periodic structure while minimizing deformations to the unit cells that would weaken or destroy the electromagnetic properties. New developments in this algorithm are disclosed that increase efficiency, improve the quality of the lattices and provide the ability to design aplanatic metasurfaces. The ability to spatially vary a lattice in this manner enables new design paradigms that are not possible using spatial transforms, three of which are discussed here. First, spatially variant self-collimating photonic crystals are shown to flow unguided waves around very tight bends using ordinary materials with low refractive index. Second, multi-mode waveguides in spatially variant band gap materials are shown to guide waves around bends without mixing power between the modes. Third, spatially variant anisotropic materials are shown to sculpt the near-field around electric components. This can be used to improve electromagnetic compatibility between components in close proximity.

Ultrathin 90-degree sharp bends for spoof surface plasmon polaritons

At low frequencies outside the plasmonic range, strongly confined surface waves can be achieved on periodically structured metal surfaces, thereby allowing for the design of compact electromagnetic guiding devices. Here, we propose an approach to realize highly efficient transmission of spoof surface plasmons around 90-degree sharp bends on ultrathin metallic films in the microwave regime. We demonstrate that by judiciously engineering the structure, the dispersion relation can be designed to reduce the scattering. Furthermore, the reflection can be suppressed by proper structural decoration at the bending corner. A one-dimensional scattering theory is employed to understand and verify the transmission properties of our waveguide bend structure. Our design scheme is not restricted to the specific structure we propose here but can be applied to other guiding components built up on two dimensional metal surfaces.