Subash Vegesna

Tunable dual-band terahertz metamaterial bandpass filters

We report metamaterial terahertz (THz) bandpass filters with tunable dual-band selectivity. The shift in the center frequency of the device is achieved by actively modifying the effective length of the resonators. This was realized by introducing vanadium dioxide (VO2) bridges interconnecting specific regions of each resonator. Raising the temperature across the phase transition shifted the resonance frequency by ~32% due to changes in the electrical conductivity of the VO2. Measured THz transmission response of the proposed dual-band filter was in good correspondence with simulations.

Terahertz bandpass filters using double-stacked metamaterial layers

Bandpass filters are reported based on double-stacked metamaterial layers separated by an air gap for operation at terahertz frequencies. Several stacking configurations were investigated designed for a ~0.5 THz center frequency. The filters exhibited improved spectral transmission properties when compared with conventional ones based on single metamaterial layers. 3 dB bandwidth of ~78 GHz and sidelobe suppression ratio >16 dB were determined when symmetric or asymmetric double layers were stacked. We demonstrate that superior frequency selectivity can be achieved when metamaterial layers with different unit cells are used. Good agreement was found between measured and simulated transmission response.

Terahertz Two-Layer Frequency Selective Surfaces With Improved Transmission Characteristics

In this paper, a cascaded configuration for two-layer frequency selective surfaces (FSSs) at terahertz (THz) frequencies with improved filtering characteristics is realized for electrically thick substrates. At THz frequencies, the thicknesses of commercially available substrates are comparable to the free-space wavelength. As a result, the substrate plays a critical role in determining the transmission characteristics of THz multilayer FSS structures. Proper coupling method between FSS structures should be chosen to avoid unwanted substrate resonances or Fabry-Pérot resonances, which otherwise degrade the transmission characteristics of the cascaded FSS structure. In this paper, a cascaded structure to avoid multiple reflections within the substrate is presented and the same is used to realize two double-layered FSS structures to improve the transmission response. The transmission response is improved by introducing an extra transmission zero at a frequency location lower than the resonant frequency, thereby achieving high roll-off rate for the lower side of the stop band, and to suppress unwanted resonances, thereby increasing the rejection bandwidth of the filter. The proposed cascaded FSS structures were fabricated and tested using THz time-domain spectroscopy. Good agreement between simulations and experiments were obtained.

NOVEL COMPACT DUAL-BAND BANDPASS MICROSTRIP FILTER

In this paper, a novel microstrip structure is developed to realize a dual-band bandpass fllter. The proposed bandpass structure uses a microstrip resonator with two independently controlled resonance frequencies producing two frequency bands of interest controlled by adjusting the dimensions of the resonator. Parametric analysis is performed on the structure to determine the optimum dimensions to obtain the desired frequency response and is explained in the paper. The dual-band bandpass fllter developed in this paper exhibits dual operating frequencies at 1390MHz and 2520MHz with 9.85% and 9.92% fractional bandwidths respectively. We achieved a compact second-order dual-band bandpass fllter with controllable resonance frequencies and low insertion losses in the passband with high selectivity. The measured results are in good agreement with simulated results. Additionally, it can be easily fabricated and can be used in applications where miniaturization and compatibility with microstrip technology are of primary concern.

Compact Two-Layer Microstrip Bandpass Filters Using Broadside-Coupled Resonators

This paper presents a design methodology for realizing broadside-coupled microstrip bandpass fllters on multilayer substrates to reduce the size of the fllter. The new fllter conflguration consists of broadside coupled split-ring resonators on two layers backed by a ground plane. With the proposed new method, miniaturization to a greater extent can be achieved compared to the conventional method of realizing microstrip multilayer fllters. In addition, coupling apertures in the ground plane used to achieve coupling among the resonators in conventional multilayer structures are eliminated. The proposed design is more ∞exible compared to traditional multilayer fllters. Layers can be easily added to increase the fllter order. To demonstrate the method, a miniaturized two-layered bandpass fllter centered at 728MHz with low insertion loss is implemented and investigated. Miniaturization of more than 25% is achieved compared to the conventional broadside coupled structure and more than 40% miniaturization compared to the edge coupled structure. The new microstrip fllter discussed in this paper can be realized using simple fabrication techniques.

Reconfigurable terahertz frequency selective structures using vanadium dioxide

Terahertz frequency selective surfaces with tunable characteristics are presented. Vanadium dioxide patches and metallic resonators were used. Vanadium dioxide behaves as an insulator at low temperatures and as a metal at high temperatures with transition temperature of ~ 68°C. Single to dual polarized bandpass FSS structure at 0.5 THz and tunable bandpass FSS structure were realized. We measured extinction ratio of 25 dB for the polarizer and 31% shift in the resonance frequency for the FSS.

Terahertz frequency selective surface with reconfigurable polarization characteristics using vanadium dioxide

In this paper, we present a design technique to realize reconfigurable terahertz (THz) frequency selective surface (FSS) polarizer. Our approach relies on combining vanadium dioxide (VO2) patches with metallic resonators. Vanadium dioxide behaves as an insulator at room temperatures and as a metal at high temperatures with a characteristic insulator–metal transition temperature of ~68 °C. We used this attribute to realize a reconfigurable single- to dual-polarized bandpass FSS structure at 0.5 THz. Along with the simulation results, FSS structures fabricated on sapphire substrates were measured using THz time-domain spectroscopy. Measured extinction ratio of ~25 dB was achieved for the THz polarizer with just a single FSS layer. Good agreement between simulation and experiments were obtained.

Non-destructive technique for broadband characterization of carbon nanotubes at microwave frequencies

This paper presents a broadband microwave non-destructive technique for characterization of carbon nanotubes (CNTs). Typically, commercially available carbon nanotubes are powder-like samples and, therefore, in this paper, a broadband characterization technique to extract electrical conductivity of powder materials is developed. The technique uses a microstrip line configuration in conjunction with a cavity resonator technique. The electrical conductivity of CNTs is extracted from the measured attenuation in the signal response (|S 21|dB) for the microstrip configuration with a signal trace made of copper and a ground plane filled with CNT samples. A resonant cylindrical cavity is also used in the measurement process to help determine a correction factor for the surface roughness of the CNT microstrip ground plane. A novel method to take attenuation due to surface roughness into account to determine conductivity of CNTs is introduced. Experimental and numerical verification of the proposed method is provided. The method developed in this work provides a cost-effective solution where significant amount of time and cost are reduced in the sample preparation process. Measurement results for the electrical conductivity of single-walled CNTs and multi-walled CNTs are presented.

Miniaturized microstrip bandpass filter with interdigitated fingers

In this paper, a compact microstrip bandpass filter with improved selectivity and spurious harmonic suppression is presented. The proposed filter design makes use of a resonator with interdigitated fingers to achieve compactness as well as high-selectivity by introducing two transmission zeros, with just two resonators. Miniaturization of about 48% and 20% is achieved in comparison to filters realized using open-loop resonators and stepped-impedance resonators, respectively. In addition, the filter design is modified to suppress the spurious resonances without significant increase in the filter size. Spurious resonances are suppressed to frequencies more than 5 times the fundamental resonance frequency. Good agreement between simulated and measured results is obtained.

Suppression of spurious frequencies in microwave dual-band bandstop filters

This paper presents a design methodology for suppressing spurious frequencies in dual-band bandstop filters. Traditional methods used for suppression of spurious frequency for bandpass filter are not effective for bandstop filters. Two techniques are discussed in this paper for achieving spurious response suppression in bandstop filters. With the first technique, the spurious response is pushed to a higher frequency expanding the useable band, while with the second proposed technique spurious response is suppressed for dual-band bandstop filters. Initially, a bandstop filter with dual operating frequencies at 1.5 GHz and 2.6 GHz is designed and both techniques discussed in the paper are used to suppress the spurious response.