Josephson junction infrared single-photon detector.

Josephson junctions are superconducting devices used as high-sensitivity magnetometers and voltage amplifiers as well as the basis of high-performance cryogenic computers and superconducting quantum computers. Although device performance can be degraded by the generation of quasiparticles formed from broken Cooper pairs, this phenomenon also opens opportunities to sensitively detect electromagnetic radiation. We demonstrate single near-infrared photon detection by coupling photons to the localized surface plasmons of a graphene-based Josephson junction. Using the photon-induced switching statistics of the current-biased device, we reveal the critical role of quasiparticles generated by the absorbed photon in the detection mechanism. The photon sensitivity will enable a high-speed, low-power optical interconnect for future superconducting computing architectures.

Cylindrical cloaking at oblique incidence with optimized finite multilayer parameters.

We propose multilayer cylindrical invisibility cloaks that are optimized for oblique incidences through a combination of analytic formalism of scattering and genetic optimization. We show that by using only four layers of homogeneous and anisotropic metamaterials without large values of constitutive parameters, the scattering for oblique incidences can be reduced by 2 orders. Although the optimization is done at a single incident angle, the cloak provides reduced scattering over a large range of incident angles.

Optical force on a cylindrical cloak under arbitrary wave illumination.

The optical force distribution in the cylindrical cloak under arbitrary incident waves is presented. We show that on the inner surface of the cloak both the induced surface currents and polarization charges interact with the waves and give opposite radiation pressure onto the inner surface. The Lorentz force in the cloak can contribute to change the trajectory of the rays, while in some cases it may only reflect the rays having a tendency to decrease the total energy it carries. The force is symmetric and in balance. Therefore the total momentum transfer from the waves to the cylindrical cloak is zero.

Lateral shift makes a ground-plane cloak detectable.

We examine the effectiveness of the ground-plane invisibility cloak generated from quasiconformal mapping of electromagnetic space. This cloak without anisotropy will generally lead to a lateral shift of the scattered wave, whose value is comparable to the height of the cloaked object, making the object detectable. This can be explained by the fact that the corresponding virtual space is thinner and wider than it should be. Ray tracing on a concrete model shows that, for a bump with a maximum height of 0.2 units to be hidden, the lateral shift of a ray with 45° incidence is around 0.15 units.

Optical delay of a signal through a dispersive invisibility cloak.

We present a full-wave analysis method on the transmission of a Gaussian light pulse through a spherical invisibility cloak with causal dispersions. The spatial energy distribution of the Gaussian light pulse is distorted after the transmission. A volcano-shaped spatial time-delay distribution of the transmitted light pulse is demonstrated as a concrete example in our physical model. Both the time-delay and the energy transport depend on the polarization of light waves. This study helps to provide a complete picture of energy propagation through an invisibility cloak.

Guiding waves through an invisible tunnel.

Transformation optics provides a promising way to guide waves in the open space. It is shown that a small waveguide coated with transformation medium will behave as a big virtual tunnel connecting two waveguide ports separated faraway. The waves are transmitted and guided smoothly in the open space through this 'invisible tunnel'. The transformation medium is obtained by squeezing the space between the two ports into a small region. Numerical simulations are performed to illustrate this idea.

Spherical cloaking using nonlinear transformations for improved segmentation into concentric isotropic coatings.

Two novel classes of spherical invisibility cloaks based on nonlinear transformation have been studied. The cloaking characteristics are presented by segmenting the nonlinear transformation based spherical cloak into concentric isotropic homogeneous coatings. Detailed investigations of the optimal discretization (e.g., thickness control of each layer, nonlinear factor, etc.) are presented for both linear and nonlinear spherical cloaks and their effects on invisibility performance are also discussed. The cloaking properties and our choice of optimal segmentation are verified by the numerical simulation of not only near-field electric-field distribution but also the far-field radar cross section (RCS).

Electromagnetic detection of a perfect invisibility cloak.

A perfect invisibility cloak is commonly believed to be undetectable from electromagnetic (EM) detection because it is equivalent to a curved but empty EM space created from coordinate transformation. Based on the intrinsic asymmetry of coordinate transformation applied to motions of photons and charges, we propose a method to detect this curved EM space by shooting a fast-moving charged particle through it. A broadband radiation generated in this process makes a cloak visible. Our method is the only known EM mechanism so far to detect an ideal perfect cloak (curved EM space) within its working band.

Experimental verification of reversed Cherenkov radiation in left-handed metamaterial.

By using a phased electromagnetic dipole array to model a moving charged particle, we experimentally verified a reversed Cherenkov radiation in the left-handed media in the frequency range from 8.1 to 9.5 GHz. Our results demonstrate the feasibility of new types of particle detectors and radiation generators.

Limitations of high-order transformation and incident angle on simplified invisibility cloaks.

We studied the scattering from the simplified cylindrical cloaks analytically at both normal and oblique incidences. We found that these simplified cylindrical cloaks may produce a larger scattering at nonnormal incidences than that from an object without any cloak, making this object more "visible". Even at normal incidence, the high-order transformation with impedance matched at the outer boundary can produce stronger scattering than the linear simplified one without matched impedance. This is due to the inefficiency of guided waves close to the inner boundary. Therefore, a square root transformation can improve scattering by guiding waves away from the inner boundary.

Manipulating the directivity of antennas with metamaterial.

In this paper we use spatially variant metamaterial substrate to manipulate the directivity of antennas. We show theoretically that by embedding a dipole at different locations inside this substrate, the emitted rays can be directed to different orientations as required. As a result, spatial multiplexing can be realized by carefully selecting proper parameters of this substrate. It can also be observed that the electric field received in this antenna system is enhanced when it is used for reception. Simulations based on finite element method are used to validate our theoretical analysis, showing a controllable high directive property. In order to simplify the physical realization process, we propose the reduced parameters for practical design and also study it with numerical simulations.

Cherenkov radiation in anisotropic double-negative metamaterials.

We theoretically study reversed Cherenkov radiation (CR) in anisotropic double-negative metamaterials (DNMs) in general, and particularly in detail for one of the most practical cases, i.e., CR in a waveguide partially filled with anisotropic DNMs. The theory presented here provides a theoretical basis for possible experiments and potential applications. As an example, we discuss the physical properties of CR and the potential applications such as particle detectors and high-power sources.

Numerical determination of frequency behavior in cloaking structures based on L-C distributed networks with TLM method.

The increasing interest in metamaterials with negative refractive index has been prompted by a variety of promising optical and microwave applications. Often, the resulting electromagnetic problems to be solve are not analytically derivable; therefore, numerical modeling must be employed and the Transmission Line Modeling (TLM) method constitutes a possible choice. After having greatly simplified the existing TLM techniques for the modeling of metamaterials, we propose in this paper to carry out a frequency study of cloaking structure.

Time domain simulation of electromagnetic cloaking structures with TLM method.

The increasing interest in invisible cloaks has been prompted in part by the availability of powerful computational resources which permit numerical studies of such a phenomenon. These are usually carried out with commercial software. We report here a full time domain simulation of cloaking structures with the Transmission Line Modeling (TLM) method. We first develop a new condensed TLM node to model metamaterials in two dimensional situations; various results are then presented, with special emphasis on what is not easily achievable using commercial software.

Surface plasmon mode analysis of nanoscale metallic rectangular waveguide.

A detailed study of guided modes in a nanoscale metallic rectangular waveguide is presented by using the effective dielectric constant approach. The guided modes, including both traditional waveguide mode and surface plasmon mode, are investigated for the silver rectangular waveguide. The mode evolution in narrow waveguide is also discussed with the emphasis on the dependence of mode dispersion with waveguide height. Finally, the red-shift of the cutoff wavelength of the fundamental mode is observed when the waveguide height decreases, contrary to the behavior of regular metallic waveguide with PEC boundary. The comprehensive analysis can provide some guideline in the design of subwavelength optical devices based on the dispersion characteristics of metallic rectangular bore.

Multi-frequency resonator based on dual-band S-shaped left-handed material.

In this paper, we experimentally realize a one-dimensional RHM (Right-handed Material)-LHM (Left-handed Material) multi-frequency resonator that consists of a dual-negative-band LHM and air arranged in an X-band waveguide. Multi-resonant frequencies are observed within two left-handed bands of the LHM. The effects of the loss and the hyperbolic dispersion relation of LHM layer are discussed. The incorporation of such a LHM into the resonator design allows more flexibility to realize multi-resonance.

Imaging properties of finite-size left-handed material slabs.

Finite-size left-handed material (LHM) slabs are studied both analytically and numerically. The analytical method is based on Huygens' principles using truncated current sheets that cover only the apertures of the slabs. It is shown that the main effects on the images' spectra due to the size of the slabs can be predicted by the proposed analytical method, which can, therefore, be used as a fast alternative to the numerical simulations. Furthermore, the property of negative-energy streams at the image plane is explained. This unique property is found to be due to the interactions between propagating and evanescent waves and can only occur with LHM slabs, both finite size and infinite.

Design and measurement of a four-port device using metamaterials.

A band separating device that uses an anisotropic metamaterial exhibiting negative refraction is designed and measured. The metamaterial has frequency dispersion in one component of the permeability tensor. A beam is incident onto the metamaterial and undergoes frequency dependent reflection and refraction directing different bandwidths towards three distinct measurement ports. Design issues are discussed, and measurement results are presented.

Limitation of FDTD in simulation of a perfect lens imaging system.

The ability of the Finite-Difference Time-Domain method to model a perfect lens made of a slab of homogeneous left-handed material (LHM) is investigated. It is shown that because of the frequency dispersive nature of the medium and the time discretization, an inherent mismatch in the constitutive parameters exists between the slab and its surrounding medium. This mismatch in the real part of the permittivity and permeability is found to have the same order of magnitude as the losses typically used in numerical simulations. Hence, when the LHM slab is lossless, this mismatch is shown to be the main factor contributing to the image resolution loss of the slab.

Retrieval of the effective constitutive parameters of bianisotropic metamaterials.

We propose a method to retrieve the effective constitutive parameters of a slab of bianisotropic metamaterial composed of split-ring resonators from the measurement of the S parameters. Analytical inversion equations are derived for homogeneous loss-less bianisotropic media, and a numerical retrieval approach is presented for the case of lossy bianisotropic media. The method is verified both analytically and numerically, and it is shown that the results for various split-ring resonator metamaterials qualitatively corroborate the conclusions found in published papers. The proposed retrieval method can be used as a valuable tool for the study of anisotropic and bianisotropic properties of metamaterials.