Reflectionless potentials for slow whispering gallery modes in surface nanoscale axial photonic fiber resonators

We consider an optical fiber with a nanoscale variation of the effective fiber radius that supports whispering gallery modes slowly propagating along the fiber, and reveal that the radius variation can be designed to support the reflectionless propagation of these modes. We show that reflectionless modulations can realize control of the transmission amplitude and temporal delay, while enabling close packing due to the absence of cross talk, in contrast to the conventional potentials.

Assembly of a high-dielectric constant thin TiOX layer directly on H-terminated semiconductor diamond

A high-dielectric constant (high-k) TiOx thin layer was fabricated on hydrogen-terminated diamond (H-diamond) surface by low temperature oxidation of a thin titanium layer in ambient air. The metallic titanium layer was deposited by sputter deposition. The dielectric constant of the resultant TiOx was calculated to be around 12. The capacitance density of the metal-oxide-semiconductor (MOS) based on the TiOx/H-diamond was as high as 0.75 µF/cm2 contributed from the high-k value and the very thin thickness of the TiOx layer. The leakage current was lower than 10-13 A at reverse biases and 10-7A at the forward bias of -2 V. The MOS field-effect transistor based on the high-k TiOx/H-diamond was demonstrated. The utilization of the high-k TiOx with a very thin thickness brought forward the features of an ideally low subthreshold swing slope of 65 mV per decade and improved drain current at low gate voltages. The advantages of the utilization high-k dielectric for diamond MOSFETs are anticipated.

Permanent matching of coupled optical bottle resonators with better than 0.16 GHz precision

The fabrication precision is one of the most critical challenges to the creation of practical photonic circuits composed of coupled high Q-factor microresonators. While very accurate transient tuning of microresonators based on local heating has been reported, the record precision of permanent resonance positioning achieved by post-processing is still within 1 and 5 GHz. Here we demonstrate two coupled bottle microresonators fabricated at the fiber surface with resonances that are matched with a better than 0.16 GHz precision. This corresponds to a better than 0.17 Å precision in the effective fiber radius variation. The achieved fabrication precision is only limited by the resolution of our optical spectrum analyzer and can be potentially improved by an order of magnitude.

Sub-wavelength resonators as seismic Rayleigh waves shield

We explore the thesis that tall structures can be protected by means of seismic metamaterials. Seismic metamaterials can be built as some elements are created over soil layer with different shapes, dimensions, patterns and from different materials. Resonances in these elements are acting as locally resonant metamaterials for Rayleigh surface waves in the geophysics context. Analytically we proved that if we put infinite chain of SDOF resonator over the soil layer as an elastic, homogeneous and isotropic material, vertical component of Rayleigh wave, longitudinal resonance of oscillators will couple with each other, they would create a Rayleigh bandgap frequency, and wave will experience attenuation before it reaches the structure. As it is impossible to use infinite chain of resonators over soil layer, we considered finite number of resonators throughout our simulations. Analytical work is interpreted using finite element simulations that demonstrates the observed attenuation is due to bandgaps when oscillators are arranged at sub-wavelength scale with respect to the incident Rayleigh wave. For wavelength less than 5 meters, the resulting bandgaps are remarkably large and strongly attenuating when impedance of oscillators matches impedance of soil. Since longitudinal resonance of SDOF resonator are proportional to its length inversely, a formed array of resonators that attenuates Rayleigh waves at frequency ≤10 Hz could be designed starting from vertical pillars coupled to the ground. Optimum number of vertical pillars and their interval spacing called effective area of resonators are investigated. For 10 pillars with effective area of 1 meter and resonance frequency of 4.9 Hz, bandgap frequency causes attenuation and a sinusoidal impulsive force illustrate wave steering down phenomena. Simulation results proved analytical findings of this work.

Thermal stability study of AlGaN/GaN MOS-HEMTs using Gd2O3 as gate dielectric

Thermal stability of AlGaN/GaN MOS-HEMTs and -diodes using Gd_(2)O_(3) are investigated by means of different thermal cycles and storage tests up to 500ºC for one week. IV DC and pulsed characteristics of the devices before and after the processes are evaluated and compared with conventional HEMTs. Results show that the devices with Gd_(2)O_(3) dielectric layer have lower leakage current and a more stable behavior during thermal treatment processes compared with conventional devices. In fact, an excellent on/off ratio of about 108 and a stable V_(t) is observed after storage at high temperature. The beneficial effects of Gd_(2)O_(3) on trapping effects of MOS-HEMTs are also dis-cussed.

Ferroelectric and dielectric properties of Nd2−xCexTi2O7 ceramics

The solid solution system Nd2−xCexTi2O7 has been investigated. The solubility limit of Ce in Nd2−xCexTi2O7 was found to be 0·5–0·75 according to X-ray diffraction and X-ray photoelectron spectroscopy results. Ce substitution increases the b and c axes and the volume of the unit cell due to its larger ionic radius. Nd2−xCexTi2O7 (x  = 0·05, 0·25, 0·5, 0·75) textured ceramics were fabricated using spark plasma sintering. The ferroelectric and dielectric properties of the ceramics were studied. Ce substitution decreases the Curie point Tc of Nd2−xCexTi2O7 compounds. The results suggest that the Tc of Ce2Ti2O7 is <1445°C.

Graphene-based waveguide resonators for submillimeter-wave applications

Utilization of graphene covered waveguide inserts to form tunable waveguide resonators is theoretically explained and rigorously investigated by means of full-wave numerical electromagnetic simulations. Instead of using graphene-based switching elements, the concept we propose incorporates graphene sheets as parts of a resonator. Electrostatic tuning of the graphene surface conductivity leads to changes in the electromagnetic field boundary conditions at the resonator edges and surfaces, thus producing an effect similar to varying the electrical length of a resonator. The presented outline of the theoretical background serves to give phenomenological insight into the resonator behavior, but it can also be used to develop customized software tools for design and optimization of graphene-based resonators and filters. Due to the linear dependence of the imaginary part of the graphene surface impedance on frequency, the proposed concept was expected to become effective for frequencies above 100 GHz, which is confirmed by the numerical simulations. A frequency range from 100 GHz up to 1100 GHz, where the rectangular waveguides are used, is considered. Simple, all-graphene-based resonators are analyzed first, to assess the achievable tunability and to check the performance throughout the considered frequency range. Graphene–metal combined waveguide resonators are proposed in order to preserve the excellent quality factors typical for the type of waveguide discontinuities used. Dependence of resonator properties on key design parameters is studied in detail. Dependence of resonator properties throughout the frequency range of interest is studied using eight different waveguide sections appropriate for different frequency intervals. Proposed resonators are aimed at applications in the submillimeter-wave spectral region, serving as the compact tunable components for the design of bandpass filters and other devices.