Guangjun Wen

Configurable metamaterial absorber with pseudo wideband spectrum.

Metamaterials attain their behavior due to resonant interactions among their subwavelength components and thus show specific designer features only in a very narrow frequency band. There is no simple way to dynamically increase the operating bandwidth of a narrowband metamaterial, but it may be possible to change its central frequency, shifting the spectral response to a new frequency range. In this paper, we propose and experimentally demonstrate a metamaterial absorber that can shift its central operating frequency by using mechanical means. The shift is achieved by varying the gap between the metamaterial and an auxiliary dielectric slab parallel to its surface. We also show that it is possible to create multiple absorption peaks by adjusting the size and/or shape of the dielectric slab, and to shift them by moving the slab relative to the metamaterial. Specifically, using numerical simulations we design a microwave metamaterial absorber and experimentally demonstrate that its central frequency can be set anywhere in a 1.6 GHz frequency range. The proposed configuration is simple and easy to make, and may be readily extended to THz frequencies.

Analysis of metamaterial absorber in normal and oblique incidence by using interference theory

Metamaterial absorber (MMA), which is a kind of thin electromagnetic absorber consisting of sub-wavelength metamaterial resonators and can exhibit near-perfect absorption characteristics, has been widely investigated in recently years. The impedance matching theory was proposed to analyze the configuration of MMA in most literatures. Such theory, however, may not suitable to analyze the interactions of metamaterial resonators and the ground plane. The interference theory, on another hand, can play effective approach for this kind of problem presented in recent studies, whereas little attention has been paid on the oblique incidence conditions. In this paper, we firstly extend the interference theory model to make it applicable for oblique incident waves and analyze MMA using the extended interference theory model. Secondly, we further explore the sufficient condition for the maximum absorptivity at both normal and oblique incidence cases. Thirdly, with the sufficient condition, we can obtain the absorbing frequency directly if the thickness of MMA is given. These theoretical results have significant effects on the design and analyze of MMA. And lastly, we point out that absorptivity is not absolutely insensitive to the incidence angle in TM mode as what previous study claims, but insensitive when the dielectric slab is high loss, which can also be explained by interference theory.

WIDEBAND CIRCULARLY POLARIZED UHF RFID READER ANTENNA WITH HIGH GAIN AND WIDE AXIAL RATIO BEAMWIDTHS

A broadband circularly polarized patch antenna with high gain and wide axial ratio beamwidths is proposed for ultra-high- frequency (UHF) RF identiflcation (RFID) applications in this paper. The antenna is composed of a square patch, a feed network printed on the bottom side of the substrates and an antenna radome. The CP radiation of the proposed antenna is excited by four cylinder probes which transmit four signals that have equal amplitude with quadrature phase difierence (0 - , 90 - , 180 - , and 270 - ) generated from the feed network. To obtain an optimum peak gain and a broad CP bandwidth, 100› isolation resistor is omitted in the feed network for obtaining high radiation e-ciency, and the efiects of varying the feed positions and dimensions of the various parameters on the antenna performances are respectively investigated. Simulation results are compared with the measurements, and a good agreement is obtained. The measured results show that the proposed antenna can provide broad impendence bandwidth of 19.7% (815{993MHz) (re∞ection coe-cient less than i15dB), a maximum gain of 9.65dBi, and a 3-dB axial ratio (AR) bandwidth of about 11% (860{960MHz). The results indicate that the proposed antenna is an excellent candidate for UHF RFID reader system. At last, read performance of the proposed antenna array in RFID systems is presented, which verify the superior features of the proposed antenna in practical RFID system applications.

Experimental demonstration of a magnetically tunable ferrite based metamaterial absorber

We synthesize and systematically characterize a novel type of magnetically tunable metamaterial absorber (MA) by integrating ferrite as a substrate or superstrate into a conventional passive MA. The nearly perfect absorption and tunability of this device is studied both numerically and experimentally within X-band (8-12 GHz) in a rectangular waveguide setup. Our measurements clearly show that the resonant frequency of the MA can be shifted across a wide frequency band by continuous adjustment of a magnetic field acting on the ferrite. Moreover, the effects of substrate/superstrates thickness on the MAs tunability are discussed. The insight gained from the generic analysis enabled us to design an optimized tunable MA with relative frequency tuning range as larger as 11.5% while keeping the absorptivity higher than 98.5%. Our results pave a path towards applications with tunable devices, such as selective thermal emitters, sensors, and bolometers.

Wide-angle and polarization-independent metamaterial absorber based on snowflake-shaped configuration

A metamaterial absorber (MA) based on snowflake-shaped resonators is designed and experimentally demonstrated in this paper. Such an MA possesses a nearly perfect absorption with wide angle of incidence and polarization independence. The measurement in a rectangular waveguide system shows the absorptivity as high as 99.5% at 11.28 GHz. Furthermore, the operating frequency can be flexibly controlled by adjusting the lengths of the snowflakes branches, while keep the peak absorptivities nearly uniform (>97.5%). The proposed MA is sure to find wide applications in, for instance, stealth technology, thermal detector and imaging.

Dual-Band Negative Permittivity Metamaterial Based on Cross Circular Loop Resonator With Shorting Stubs

In this letter, a novel design of dual-band negative permittivity metamaterial is presented, which is based on a cross circular loop resonator (CCLR) with shorting stubs. The electromagnetic characteristics of this metamaterial are analyzed straightforwardly by an equivalent circuit. Numerical simulations and microwave experiments are further employed to illustrate the theoretical expectations. Moreover, the dependence between both the two frequencies and the positions of shorting stubs are discussed. Our results open a way to design the dual-notched ultrawideband (UWB) antennas and filters and dual-band metamaterial absorbers.

Wideband giant optical activity and negligible circular dichroism of near-infrared chiral metamaterial based on a complementary twisted configuration

We theoretically analyze the near-infrared properties of a chiral metamaterial constituting an array of twisted crosses and complementary crosses made of silver. Through rigorous full-wave numerical simulations, we demonstrate that this type of metamaterial exhibits wideband giant optical activity, with a polarization azimuth rotation angle reaching values as large as 1900∘ per wavelength. Owing to the negligible loss at optical frequencies in the dielectric (magnesium fluoride) making up the metamaterial, we observe negligible circular dichroism and low dispersion of the polarization azimuth rotation angle over a wide frequency band. We envision that this type of chiral metamaterial will find extensive applications in optical communication systems and biological sensing.

An integrated low phase noise radiation-pressure-driven optomechanical oscillator chipset.

High-quality frequency references are the cornerstones in position, navigation and timing applications of both scientific and commercial domains. Optomechanical oscillators, with direct coupling to continuous-wave light and non-material-limited f × Q product, are long regarded as a potential platform for frequency reference in radio-frequency-photonic architectures. However, one major challenge is the compatibility with standard CMOS fabrication processes while maintaining optomechanical high quality performance. Here we demonstrate the monolithic integration of photonic crystal optomechanical oscillators and on-chip high speed Ge detectors based on the silicon CMOS platform. With the generation of both high harmonics (up to 59th order) and subharmonics (down to 1/4), our chipset provides multiple frequency tones for applications in both frequency multipliers and dividers. The phase noise is measured down to −125 dBc/Hz at 10 kHz offset at ~400 μW dropped-in powers, one of the lowest noise optomechanical oscillators to date and in room-temperature and atmospheric non-vacuum operating conditions. These characteristics enable optomechanical oscillators as a frequency reference platform for radio-frequency-photonic information processing.

Design and Characterization of Tunable Terahertz Metamaterials With Broad Bandwidth and Low Loss

Achievable tunable left-handed metamaterials are physically designed and numerically characterized at terahertz (THz) frequency in this letter. The Lu2.1Bi0.9Fe5O12 (LuBiIG) garnet films prepared by liquid phase epitaxy (LPE) method on a gadolinium gallium garnet (GGG) substrate are used to achieve negative permeability, while the silver films are used to achieve negative permittivity. Both the LuBiIG garnet films and silver films are made physically available using the present techniques. The transmission and tunability characteristics of such metamaterials at THz frequency are numerically investigated, and the effective refractive index is retrieved in terms of the simulated transmission parameters. The numerical results obtained demonstrate that such metamaterials have a negative passband centered at 0.1415 THz. The passband can also be shifted by changing the applied dc magnetic field. These results depict a new way of designing low-loss THz transmission media and the resulted waveguides.

Polarization conversion of metasurface for the application of wide band low-profile circular polarization slot antenna

In this letter, based on a corner-cut square metasurface, a planar polarization conversion structure is presented and the application for wide band low-profile circular polarization (CP) slot antenna is proposed. The mechanisms for achieving the CP state from a linearly polarized incident wave and for broadening the working bandwidth of conventional slot antenna are analyzed theoretically. The wide band low-profile CP slot antenna is achieved with numerical optimizations and parameter studies. Both simulations and measurements are performed to demonstrate the proposed antenna, and good agreements are obtained. Such results will open the path for polarization conversion metasurfaces used in the CP antenna area.