Li-Ming Si

CPW-Fed Multi-Band Omni-Directional Planar Microstrip Antenna Using Composite Metamaterial Resonators for Wireless Communications

A novel approach for the design of a compact multiband planar microstrip antenna is presented. This type of antenna is composed of composite metamaterial resonators (including conditional microstrip resonators and metamaterial resonators), and fed by signal feed. A sample antenna with composite closed-ring resonator and split-ring resonator (SRR) fed by 50 Ω coplanar waveguide (CPW) developed on FR4 epoxy substrate for multi-band wireless communication applications is presented. Appropriate design of the composite structure resulted in three discontinuous resonant bands. The fundamental magnetic resonant and electric resonant frequency of SRR and the first electric resonant frequency of the closed-ring resonator were combined to form low, middle, and high resonant band. The properties of this antenna are investigated by theoretical analysis and finite element method (FEM) simulations. The numerical results show that the proposed antenna has good impedance bandwidth and radiation characteristics in the three operating bands which cover the required band widths of the 2.4/5.2/5.8GHz wireless local-area networks (WLAN) and 3.5/5.5 GHz worldwide interoperability for microwave access (WiMax) with return loss of better than 10 dB. The antenna also has stably omni-directional H-plane radiation patterns within the three operating bands.

A Compact, Planar, and CPW-Fed Metamaterial-Inspired Dual-Band Antenna

A novel compact, planar, and coplanar waveguide (CPW)-fed dual-band antenna is designed and proposed using composite metamaterial. Such composite metamaterial consists of an inner split-ring resonator (SRR) and an outer closed-ring resonator (CRR). The composite metamaterial can provide dual-band operation at 2.595-2.654 and 3.185-4.245 GHz with reflection coefficient better than -10 dB by the two resonant modes of SRR and CRR, respectively. A CPW-fed line with trapeziform ground plane and tapered impedance transformer line is employed to improve the impedance matching of the antenna. The uniqueness of this design is that the inner SRR with size much smaller than the resonant wavelength is used for obtaining the lower narrow frequency band, which makes the dual-band antenna very compact. Antenna parameters, including reflection coefficient, radiation pattern, radiation efficiency, and gain, are analyzed with numerical simulation and experimental measurement. Good agreement between the simulation and measurement is observed.

Graphene-enabled tunability of optical fishnet metamaterial

We present an effective method for actively controlling intrinsic resonances of optical metamaterials using graphene. Exploiting the Fermi level shift and associated variations in optical transitions of graphene due to voltage biasing, we attain the ability to significantly modulate the intrinsic resonance of the fishnet structure. Despite being atomically thin and having a weak optical response, graphene can be strongly coupled with the left-handed resonance of the fishnet metamaterial. We unambiguously demonstrate that the resonant transmission, absorption, and effective constitutive parameters of the graphene-coupled fishnet metamaterial can be precisely controlled by varying the bias voltage.

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.

Some Recent Developments of Microstrip Antenna

Although the microstrip antenna has been extensively studied in the past few decades as one of the standard planar antennas, it still has a huge potential for further developments. The paper suggests three areas for further research based on our previous works on microstrip antenna elements and arrays. One is exploring the variety of microstrip antenna topologies to meet the desired requirement such as ultrawide band (UWB), high gain, miniaturization, circular polarization, multipolarized, and so on. Another is to apply microstrip antenna to form composite antenna which is more potent than the individual antenna. The last is growing towards highly integration of antenna/array and feeding network or operating at relatively high frequencies, like sub-millimeter wave or terahertz (THz) wave regime, by using the advanced machining techniques. To support our points of view, some examples of antennas developed in our group are presented and discussed.

Characterization and application of planar terahertz narrow bandpass filter with metamaterial resonators

A planar terahertz (THz) narrow bandpass filter for the sensing of small amounts of overlaid dielectrics is presented. A two-dimensional (2D) periodic array of four sub-wavelength rectangular split-ring resonators (SRRs) is developed on a benzocyclobutene (BCB) substrate to realize the novel compact, low insertion-loss, and narrow bandpass microstrip filter with the center frequency of 1 THz and 3-dB bandwidth of 6.2 GHz. Due to center frequency shifts occurs with the change of equivalent permittivities at the surface of the filter when it covered by dielectrics of different permittivity and thickness, this narrow bandpass filter is especially suitable for detecting the properties of overlaid dielectrics.

A Uniplanar Triple-Band Dipole Antenna Using Complementary Capacitively Loaded Loop

A uniplanar compact metamaterial-inspired triple-band dipole antenna is designed and proposed for wireless local area network (WLAN) and Worldwide Interoperability for Microwave Access (WiMAX) applications. By introducing two pairs of complementary capacitively loaded loop (CCLL) slots into a uniplanar bow-tie antenna, two notched bands can be generated to form a triple-band operation. The antenna is directly fed by a 50- Ω microstrip line and a microstrip-to-coplanar-stripline (CPS) transition as a wideband balun. Antenna parameters, including voltage standing-wave ratio (VSWR), radiation patterns, and gain, are obtained by numerical simulations and experimental measurements. The results show that the proposed antenna can provide triple-band operation at 2.21-2.77, 3.33-3.95, and 5.04-6 GHz with the VSWR less than 2 to achieve 2.4/5.2/5.8-GHz WLAN and 2.5/3.5/5.5-GHz WiMAX applications.

Low-index-metamaterial for gain enhancement of planar terahertz antenna

We theoretically present a high gain planar antenna at terahertz (THz) frequencies by combing a conventional log-periodic antenna (LPA) with a low-index-metamaterial (LIM, |n| < 1). The LIM is realized by properly designing a fishnet metamaterial using full-wave finite-element simulation. Owing to the impedance matching, the LIM can be placed seamlessly on the substrate of the LPA without noticeable reflection. The effectiveness of using LIM for antenna gain enhancement is confirmed by comparing the antenna performance with and without LIM, where significantly improved half-power beam-width (3-dB beam-width) and more than 4 dB gain enhancement are seen within a certain frequency range. The presented LIM-enhanced planar THz antenna is compact, flat, low profile, and high gain, which has extensive applications in THz systems, including communications, radar, and spectroscopy.

Experimental Realization of High Transmittance THz 90

We present a high efficiency terahertz (THz) 90°-bend electromagnetic crystal (EMXT) waveguide using Si-based microelectromechanical systems technology. The 2-D EMXT consists of a periodical array of square-shaped gold rods (13 × 13 cells) deposited on a silicon substrate, and a 200-nm gold layer is further sputter-coated on the gold rod array. The dimension of each rod is 48 × 48 × 241 μm3, and the total size of the EMXT is 1756 × 1756 ×787 μm3. The 90°-bend waveguide is fabricated by removing three rows of rods from the EMXT. Through rigorous numerical simulation, it is found that transmittance higher than 90% are found in the whole range from 0.365 to 0.578 THz. The bending performance of the waveguide is further experimentally measured, where transmittance as high as 91.2% is observed at 0.5 THz. The proposed 90°-bend EMXT waveguide can be widely used for applications in THz circuit integrations and THz communications.

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A novel terahertz (THz) waves band-pass filter with the center frequency of 880 GHz and low insertion loss using microstrip line hairpin resonators is introduced. The properties of this filter are investigated by finite element method (FEM) simulations. The numerical result shows that the filter has a 3 dB bandwidth from 865 GHz to 892 GHz and has 80 GHz bandwidth better than -10 dB. Resonance frequency shifts of the filter covered by dielectric materials of different permittivity and thickness are calculated, which shows that this type of filter is suitable for detecting the properties of overlaid dielectric materials.