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The theory of electron emission by metallic photocathodes under the exposure of long wavelength lasers will be studied. Photon energy in long wavelength lasers is less than the work function of the photocathode's material and can only emit electrons via tunneling through the potential barrier. The optical resonance effects (e.g., plasmonic resonances) will be studied as an improvement to the performance of photocathodes. This paper is intended to provide self-sufficient materials to design optical resonant surfaces (e.g., metasurfaces) for electron emission applications. [ABSTRACT FROM AUTHOR]

We propose an alternative class of magneto-optical metamaterials offering enhanced angle of rotation in polarization compared to pure magneto-optical materials. In this approach, the permittivity tensor of a magneto-optical material is tailored by embedded wire meshes. We show that the angle of rotation in the magneto-optical metamaterial can be enhanced up to 9 times compared to pure magneto-optical material alone, while the polarization extinction ratio remains below -20dB over more than 2 THz bandwidth and the attenuation coefficient is approximately 1.5dB µm-1. [ABSTRACT FROM AUTHOR]

A low-profile, directive, high-power, simple radio frequency (RF) source can play a crucial role in long-distance communication, sensing, and anti-interference research. Conventional approaches to achieving high-power radiation require a microwave source, an amplifier, and a separate radiating structure. In this article, we transform a static voltage to electromagnetic radiation, which can be electronically steered, using only a thin sheet of metasurface. The proposed approach can be scaled in both power and frequency. The concept makes use of the k = 0 mode in a periodic resonant surface to excite a series of phase-locked individual sources on the surface. The phase-locked sources create an overall coherent mode in the far-field toward the desired direction. The proposed idea is inspired by spark-gap transmitters, pulsed ring-down sources, and antenna arrays to provide a novel and highly scalable electromagnetic source. We envision that the proposed active metasurface for converting a static field to electromagnetic radiation can be potentially used in long-range communication, sensing devices, and radars. [ABSTRACT FROM AUTHOR]

Receptor coated resonant nanoparticles and quantum dots are proposed to provide a cellular-level resolution image of neural activities inside the brain. The functionalized nanoparticles and quantum dots in this approach will selectively bind to different neurotransmitters in the extra-synaptic regions of neurons. This allows us to detect neural activities in real time by monitoring the nanoparticles and quantum dots optically. Gold nanoparticles (GNPs) with two different geometries (sphere and rod) and quantum dots (QDs) with different sizes were studied along with three different neurotransmitters: dopamine, gamma-Aminobutyric acid (GABA), and glycine. The absorption/emission spectra of GNPs and QDs before and after binding of neurotransmitters and their corresponding receptors are reported. The results using QDs and nanorods with diameter 25nm and aspect rations larger than three were promising for the development of the proposed functional brain mapping approach. [ABSTRACT FROM AUTHOR]

The total-internal-reflection (TIR) principle and the concept of an epsilon-near-zero (ENZ) material are combined to form an antenna exhibiting sum and difference patterns with good impedance properties. The ENZ material has been realized using a uniaxial wire medium (WM) metamaterial, and the departure from the behavior of an ideal ENZ material is discussed. The radiation pattern of the fabricated antennas has been measured and compared with simulation results. [ABSTRACT FROM PUBLISHER]

Using a relaxation oscillator circuit, breakdown (VBD) and quench (VQ) voltages of a DC discharge microplasma between two needle probes are measured. High resolution modified Paschen curves are obtained for argon microplasmas including a quench voltage curve representing the voltage at which the plasma turns off. It is shown that for a point to point microgap (e.g., the microgap between two needle probes) which describes many realistic microdevices, neither Paschen's law applies nor field emission is noticeable. Although normally VBD > VQ, it is observed that depending on environmental parameters of argon, such as pressure and the driving circuitry, plasma can exist in a different state with equal VBD and VQ. Using emission line spectroscopy, it is shown that VBD and VQ are equal if the atomic excitation by the electric field dipole moment dominantly leads to one of the argon's metastable states (4P5 in our study). [ABSTRACT FROM AUTHOR]

Using a realistic quantum master equation, we show that the resonance fluorescence spectra of a two-level artificial atom (quantum dot) can be tuned by adjusting its photonic local density of states via biasing one or more graphene monolayers. The structured photon reservoir is included using a photon Green function theory which fully accounts for the loss and dispersion. The field-driven Moliow triplet spectrum can be actively controlled by the graphene bias in the THz frequency regime. We also consider the effect of a dielectric support environment and multiple graphene layers on the emitted fluorescence. Finally, thermal bath effects are considered and are shown to be important for low THz frequencies. [ABSTRACT FROM AUTHOR]

The canalization of terahertz surface plasmon polaritons using a modulated graphene monolayer is investigated for subwavelength imaging. An anisotropic surface conductivity formed by a set of parallel nanoribbons with alternating positive and negative imaginary conductivities is used to realize the canalization regime required for hyperlensing. The ribbons are narrow compared to the wavelength and are created electronically by gating a graphene layer over a corrugated ground plane. Good quality canalization of surface plasmon polaritons is shown in the terahertz even in the presence of realistic loss in graphene, with relevant implications for subwavelength imaging applications. [ABSTRACT FROM AUTHOR]

The epsilon-near-zero (ENZ) condition in natural and artificial plasmas is considered for spatially dispersive materials. In the presence of spatial dispersion the ENZ condition must be judged by vanishing of the electric displacement field in real-space. Unlike the simple case of local materials where ENZ occurs at the plasma frequency, in spatially dispersive materials the matter is more complicated. To consider the spatially dispersive case, we obtain the momentum-dependent permittivity in real-space, and define a characteristic length parameter, in addition to the Debye length, which governs polarization screening. Using this formulation, conditions are investigated under which the electric displacement field (equivalently, the real-space permittivity) can vanish or be strongly diminished, even in the presence of spatial dispersion, implementing an ENZ material. [ABSTRACT FROM AUTHOR]

An infinite sheet of graphene lying above a perturbed ground plane is studied. The perturbation is a two-dimensional ridge, and a bias voltage is applied between the graphene and the ground plane, resulting in a graphene nanoribbon-like structure with a soft boundary (SB). The spatial distribution of the graphene conductivity forming the SB is studied as a function of the ridge parameters and the bias voltage. The current distribution of the fundamental transverse magnetic surface plasmon polariton (SPP) is considered. The effect of the ridge parameters and shape of the SB on the current distributions are investigated, and the conditions are studied under which the mode remains confined to the vicinity of the ridge region. [ABSTRACT FROM AUTHOR]

A transport-based formulation is used to model three-dimensional objects made of a uniaxial wire medium. The resulting drift-diffusion equation is used to calculate the extinction/scattering cross section of various uniaxial wire medium objects. A comparison is made with measurement and full wave simulation results. This method is faster than full wave simulation and provides physical insight into electrodynamic processes within the homogenized medium. The effect of wire period and thickness on the homogenized model is investigated, and a Debye length parameter is used to access the effect of wire length. [ABSTRACT FROM AUTHOR]

Scattering problems involving wire media are computationally intensive due to the spatially dispersive nature of homogenized wire media. In this work, an integro-differential equation based on a transport formulation is proposed instead of the convolution-form integral equation that directly arises from spatial dispersion. The integro-differential equation is much faster to solve than the convolution equation form, and its effectiveness is confirmed by solving several examples in one-, two-, and three-dimensions. As experimental confirmation of both the integro-differential equation formulation and the homogenized wire medium parameters, several isotropic connected wire medium spheres have been fabricated on a rapid-prototyping machine, and their measured extinction cross sections compared with simulation results. Wire parameters (period and diameter) are varied to the point where homogenization theory breaks down, which is reflected in the measurements. [ABSTRACT FROM PUBLISHER]

The excitation of transverse magnetic (TM) surface plasmons by a point dipole in the vicinity of a multilayered graphene/dielectric system is examined. It was previously shown that the surface plasmon (SP) excited by a vertical dipole on an isolated graphene sheet exhibits a strong excitation peak in the THz region; here we show that, in the presence of a finite-thickness dielectric support layer such as SiO2, considerable spectral content is transferred to a second (perturbed dielectric slab) mode, greatly decreasing and redshifting the excitation peak. The presence of a Si half-space also diminishes the excitation strength, but for graphene on top of SiO2-Si the presence of the SiO2 layer creates a spacer restoring the excitation peak. A two-level quantum emitter is also considered, where it is shown that the addition of a thin dielectric support slab and SiO2-Si geometries affects the spontaneous decay rate in a manner similar to the classical dipole SP excitation peak. [ABSTRACT FROM AUTHOR]

Electromagnetic absorption and subsequent heating of nanoparticle solutions and simple NaCl ionic solutions is examined for biomedical applications in the radiofrequency range at 13.56 MHz. It is shown via both theory and experiment that for in vitro measurements the shape of the solution container plays a major role in absorption and heating. [ABSTRACT FROM PUBLISHER]

Natural and artificial wire materials exhibiting spatial dispersion are considered using a transport (drift-diffusion) model. The connection between drift-diffusion and electron transport in natural materials is highlighted, and then applied to various forms of wire media, leading to the definition of effective conductivity and diffusion parameters that characterize the material. It is shown that the effective material parameters lead to a Debye length that provides a quantitative measure of the strength of spatial dispersion for wire mediums. Further, it is shown that Pekar's additional boundary condition applies in many instances to natural materials as well as artificial wire media, and can be derived from elementary electromagnetics. [ABSTRACT FROM PUBLISHER]

Pocklington's integro-differential equation for thin wires with the exact kernel is reformulated using a second derivative formula for improper integrals. This allows for analytical evaluation of the second derivatives, resulting in a pure integral equation, in a similar manner to what is done when using the approximate kernel. However, as opposed to using the approximate kernel, the resulting integral equation developed here is numerically stable even with a simple pulse function/point matching solution. Good convergence for the current is obtained using pulse functions, and the severe unphysical oscillations of the current that are encountered when using pulse functions with the approximate kernel are avoided. [ABSTRACT FROM AUTHOR]

In this work, a novel circuit topology is introduced for the implementation of multiplexers and demultiplexers, called 'active distributed (de)multiplexers.' The desired characteristics of active distributed (de)multiplexers as being wideband, planar and active are realized by the proposed microwave circuit. A numerical methodology is presented for its design. An active distributed detriplexer is designed, simulated, fabricated, and measured by the proposed numerical procedure. Computer simulation and measurement results show the effectiveness and suitability of the proposed topology as a detriplexer. © 2010 Wiley Periodicals, Inc. Int J RF and Microwave CAE , 2010. [ABSTRACT FROM AUTHOR]

In this article, a novel approach is introduced for the design of wideband antennas in a specified frequency bandwidth (BW). In the proposed approach, the frequency BW is divided into some sub-bands. Then, the narrowband antennas are designed for the consecutive frequency sub-bands, and the antennas are connected together by a proposed active circuit, so that their frequency BWs combined. The proposed active circuit may be used to connect as many as antenna needed for the design. An active microstrip antenna is designed for the frequency BW of 4–10 GHz with a gain better than 5 dB by the proposed method. Fabrication and measurement results show the effectiveness of the proposed methodology. © 2010 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2010. [ABSTRACT FROM AUTHOR]

A correction to the article "Soft-boundary graphene nanoribbon formed by a graphene sheet above a perturbed ground plane: conductivity profile and SPP modal current distribution," published in August 1, 2014 issue is presented.

A graphene sheet gated with a ridged ground plane, creating a soft-boundary graphene nanoribbon, is considered. By adjusting the ridge parameters and bias voltage a channel can be created on the graphene which can guide transverse magnetic surface plasmon polaritons. Two types of modes are found; fundamental and higher-order modes with no apparent cutoff frequency and with energy distributed over the created channel, and edge modes with energy concentrated at the soft-boundary edge. Dispersion curves, electric near-field patterns, and current distributions of these modes are determined. Since the location where energy is concentrated in the edge modes can be easily controlled electronically by the bias voltage and frequency, the edge-mode phenomena is used to propose a voltage controlled plasmonic switch and a plasmonic frequency demultiplexer. [ABSTRACT FROM AUTHOR]

We have investigated a magnet-free, nonreciprocal surface wave absorber based on high impedance surfaces (HISs) using a spatial-temporal modulation approach. By controlling embedded switches with a travelling wave, the HIS metasurface is modulated to break the time and spatial symmetry, which enables surface waves to propagate in one direction but be absorbed when propagating in the reverse direction. The nonreciprocity has been demonstrated by an EM-circuit co-simulation. We envision that this could be possibly applied in future communication systems that preferably transmit unidirectionally but absorb interference from the reverse direction caused by reflections or other devices.

A high power (~ 1W) continuous wave (CW) laser was focused on argon microplasma generated in the microgap between two electrodes with submillimeter diameters. Dependence of breakdown (V(BD)) and quench (V(Q)) voltages of microplasma to the laser power, wavelength, and spot location were studied as the gap size and pressure varied. It was observed that the laser-plasma interaction can only occur thermally through the electrodes. Also, the thermal effect of the laser was noticeable at relatively higher pressures (> 10Torr), and in most cases led to a decrease in V(BD), proportional to the pressure.