Relative Tuning Range Research Articles

Research Progress in Tunable Metamaterial Absorbers

Since the initial discovery of metamaterial absorbers (MAs), they generate significant interest across the electromagnetic device industry due to their capacity to attain subwavelength magnitudes, adaptable design, and almost complete absorption. In recent years, there has been a growing focus on incorporating active materials into MAs, making it a current focal point of investigation. In the past few years, MAs have seen the integration of various active materials and configurations thanks to continuous exploration and innovation. Herein, comprehensive and systematic research on different tuning methods of MAs is reviewed, highlighting innovative materials and unique designs to accurately control the electromagnetic absorption characteristics of MAs. Key parameters, such as the operating frequency and relative tuning range, are summarized and compared to provide guidance for optimization design. Finally, the challenges of tunable MAs and their future prospects are also summarized.

Broadband Achromatic Metalens for Tunable Focused Vortex Beam Generation in the Near-Infrared Range.

Vortex beams accompanied with orbital angular momentum have attracted significant attention in research fields due to their formidable capabilities in various crucial applications. However, conventional devices for generating vortex beams still suffer from bulky sizes, high cost, and confined performances. Metalens, as an advanced platform to arbitrarily control the optical waves, has promising prospects to address the predicament for conventional devices. Although great progress has been demonstrated in the applications of vortex beams, they are still confronted with fixed functionality after fabrication that severely hinders their application range. In this work, the phase-change material of Ge2Sb2Te5 (GST) is employed to design the meta-atoms to realize tunable optical responses. Moreover, the focused vortex beam can be accomplished by superimposing a helical phase and hyperbolic phase, and the chromatic aberrations in near-infrared (NIR) range can be corrected by introducing an additional phase compensation. And the design strategy is validated by two different metalenses (BAMTF-1 and BAMTF-2). The numerical results indicate that the chromatic aberrations for two metalens can be corrected in 1.33-1.60 μm covering the telecom range. Moreover, the average focusing efficiency of BAMTF-1 is 51.4%, and that of BAMTF-2 is 39.9%, indicating the favorable performances of designed BAMTF. More importantly, their average focal lengths have a relative tuning range of 38.82% and 33.17% by altering the crystallization ratio of GST, respectively. This work may provide a significant scheme for on-chip and tunable devices for NIR imaging and communication systems.

Dual-Band Reconfigurable Flexible Antenna With Independent Frequency Tunability

A dual-band frequency-reconfigurable flexible textile and foam antenna with independent tunability is presented in this letter. The proposed antenna operates simultaneously in a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{TM}_{0(0.5)0}^{z}$</tex-math></inline-formula> mode and a higher order mode with individual frequency tunability centered at the 2.45 and 5.8 GHz industrial, scientific, and medical bands. The relative frequency tuning range at the lower band is 41.1%, while that at the higher band is 29.9%. The antenna is fabricated and measured to validate the concept. The antenna prototype operates as expected and maintains robust performance in various bending configurations through its individual dual-band frequency reconfigurability, which suggests its practicality for wearable applications.

An ultra-broadband frequency-agile terahertz perfect absorber with perturbed MoS2 plasmon modes.

Multidomain dynamic manipulations for terahertz (THz) absorbers usually necessitate the orchestrated actions of several active elements, inevitably complicating the structural design and elongating the modulation time. Herein, we utilize the coupling between the total reflection prism and electrically-driven MoS2 to activate a tight field confinement in a deep-subwavelength interlayer, ultimately achieving frequency-agile absorption adjustments only with a gate voltage. Theoretical and simulation analysis results indicate that the redistributed electric field and susceptible dielectric response are attributed to the limited spatial near-field perturbation of surface plasmon resonances. We also demonstrate that perturbed MoS2 plasmon modes promote the formation of dual-phase singularities, significantly suppressing the attenuation of the absorption amplitude as large-scale frequency shifts, thereby extending the relative tuning range (WRTR) to 175.4%. These findings offer an efficient approach for expanding the horizon of THz absorption applications that require ultra-broadband and swift-response capabilities.

Metamaterial Absorbers: From Tunable Surface to Structural Transformation.

Since the first demonstration, remarkable progress has been made in the theoretical analysis, structural design, numerical simulation, and potential applications of metamaterial absorbers (MAs). With the continuous advancement of novel materials and creative designs, the absorption of MAs is significantly improved over a wide frequency spectrum from microwaves to the optical regime. Further, the integration of active elements into the MA design allows the dynamical manipulation of electromagnetic waves, opening a new platform to push breakthroughs in metadevices. In the last several years, numerous efforts have been devoted to exploring innovative approaches for incorporating tunability to MAs, which is highly desirable because of the progressively increasing demand on designing versatile metadevices. Here, a comprehensive and systematical overview of active MAs with adaptive and on-demand manner is presented, highlighting innovative materials and unique strategies to precisely control the electromagnetic response. In addition to the mainstream method by manipulating periodic patterns, two additional approaches, including tailoring dielectric spacer and transforming overall structure are called back. Following this, key parameters, such as operating frequency, relative tuning range, and switching speed are summarized and compared to guide for optimum design. Finally, potential opportunities in the development of active MAs are discussed.

A Vibration Energy Harvester With Targeted Frequency-Tuning Capability

One of the major challenges for vibration energy harvesters is to match their resonant frequency with ambient vibration frequency. The main limitation of the existing frequency-tuning techniques is the uncontrollable tuning process, i.e., the incapability of tuning the frequency to a desired value. In this article, we report a tunable electromagnetic vibration energy harvester capable of being tuned to a specific target frequency. The device adjusts the resonant frequency by changing the effective length of two polyimide springs through a set of smart tuning structures with scales. A maximum frequency tuning range of 58 Hz (46 Hz-104 Hz, 126% relative tuning range) is realized with the proposed device. At a sinusoidal acceleration of 1g, an output peak-to-peak voltage of 263.4 mV and an output power of 143.6 μW are obtained at the frequency of 56 Hz.

A tunable broadband bandpass filter based on quarter wavelength resonators

In this letter, a tunable broadband bandpass filter (BPF) based on quarter wavelength resonators was proposed. The resonators and coupling structures were mainly consisted of quarter wavelength transmission lines and tunable devices loaded at the two ends of the transmission lines. The resonant frequency of the BPF can be adjusted through changing the total capacitance at the both ends of the resonators. Especially, the coupling strength between resonators can be adjusted by adjusting the value ratio of the capacitor between the two ends. The tunable BPF in this scheme had the advantages of simply design. The bandwidth and center frequency of the tunable BPF can be tuned independently at the same time. As a demonstrator, an eight-orders filter was designed and fabricated on Rogers3006 substrate. The center frequency can be tuned in the range of 0.81 to 1.28 GHz, the relative tuning range was 45%. The relative bandwidth of the tunable filter can be tuned in the range of 14% to 38%. The simulated results were agreement well with the measured ones.

A Compact, Low-Power and Constant Output Power 330 GHz Voltage-Controlled Oscillator in 130-nm SiGe BiCMOS

Nowadays, terahertz and sub-terahertz frequencies are experiencing a significant interest from both academic and industrial world, since they offer a wide and unused spectrum available for different applications, as high-throughput communications, efficient imaging systems and medical uses. In this work an innovative sub-THz voltage-controlled oscillator (VCO) realized with a 130-nm SiGe process is proposed and described in detail. The technology is the SG13G2 bipolar-complementary-metal-oxide-semiconductor (BiCMOS) provided by IHP foundry. The signal source is conceived for high-resolution THz imaging camera; at design level it has been defined with a push-push architecture and adopts an oscillator core based on a Colpitts topology. State-of-the-art performance has been obtained thanks to the proposed design choices that are uncommon at these frequencies, as for instance, the VCO output taken from the common base node, the use of varactors in anti-series connection and the possibility to define a double output at different harmonic frequencies. These, as well as other innovative expedients, allow to simplify the integration at system level and to achieve compact sizes, very low power consumption and almost constant output power level over the full tuning bandwidth. A relative tuning range of about 5.5% has been obtained around a central frequency of 330 GHz, while the output power is about − 8 dBm with a maximum variation of 0.4 dB. Also the phase noise of − 110 dBc/Hz at 10 MHz offset is very promising. Measurements over different chip samples demonstrate the robustness of the proposed solution and the reliability of the design.

An ultra-wideband 6–14 GHz frequency modulated continuous wave primary radar with 3 cm range resolution

This paper describes the design and verification of an ultra-wideband 6–14 GHz frequency modulated continuous wave (FMCW) primary radar system with very high range resolution. The design and measurement results of the utilized signal generator and receiver are presented. The signal generator features a 86% relative continuous tuning range and average phase noise of −106 dBc/Hz at 1 MHz offset to the carrier. The radar system utilizes a hybrid structure where the voltage controlled oscillator was fabricated using 130 nm SiGe BiCMOS technology and the lower frequency components are off-the-shelf. Free field measurements of the radar showcase an unprecedented combination of 3 cm range resolution, low phase noise and low operating frequency of 6–14 GHz for inherently higher range.

A Self-adaptive and Self-Sufficient Energy Harvesting System.

Self-adaptive vibration energy harvesters convert the kinetic energy from vibration sources into electrical energy and continuously adapt their resonance frequency to the vibration frequency. Only when the two frequencies match can the system harvest energy efficiently. The harvesting of vibration sources with a time-variant frequency therefore requires self-adaptive vibration harvesting systems without human intervention. This work presents a self-adaptive energy harvesting system that works completely self-sufficiently. Using magnetic forces, the axial load on a bending beam is changed and thus the resonance frequency is set. The system achieves a relative tuning range of 23% at a center frequency of 36.4 Hz. Within this range, the resonance frequency of the harvester can be set continuously and precisely. With a novel optimized method for frequency measurement and with customized electronics, the system only needs 22 µW to monitor the external vibration frequency and is therefore also suitable for environments with low vibration amplitudes. The system was verified on a vibrational test bench and can easily be tailored to a specific vibration source.

Design of a CMOS 65‐nm inductor‐less VCO for ISM applications in the VHF band

SummaryThis paper presents a design of a CMOS cross‐coupled voltage‐controlled oscillator (VCO) using active inductors (AIs) for wide‐band applications and can also be applied to various wireless technologies standards. The compatibility of this design to different wireless standards highlights its potential to be implemented at the core of the communication front end in the Internet of Things (IoT). The proposed AI design employs a gyrator‐C topology as the basic structure to generate an inductance. The VCO uses a cross‐coupled oscillator structure with a pair of varactors to sweep the frequency. Two extra capacitors, between the AIs and the outputs of the VCO core tank, are employed to enhance the performance of the phase noise and make the VCO work similarly to a linear transconductance (LiT) oscillator. Both the AIs and the VCO are designed in the TSMC 65‐nm CMOS technology, and the performance is analyzed using postsimulation results, as well as through measurements. The fundamental frequency spans from 140 to 463 MHz. Thus, the relative tuning range of this design is approximately 107%. The optimal phase noise of the design is around −97 dBc/Hz at 1‐MHz offset. Furthermore, it achieves an excellent figure of merit (FOM) around −163 dBc/Hz with a direct current (DC) power consumption less than 3 mW. The proposed design shows an advantage in phase noise and power consumption in comparison with previous active inductor VCO and ring VCO designs, respectively. The final layout occupies only 0.4 × 0.62 mm2 including the pads. The proposed AI‐VCO shows a compact size, linear tuning, low power consumption, and good phase noise performance.

Tunable optofluidic liquid metal core microbubble resonator.

This study introduces design and coupling techniques, which bridge an opaque liquid metal, optical WGM mode, and mechanical mode into an opto-mechano-fluidic microbubble resonator (MBR) consisting of a dielectric silica shell and liquid metal core. Benefiting from the conductivity of the liquid metal, Ohmic heating was carried out for the MBR by applying current to the liquid metal to change the temperature of the MBR by more than 300 °C. The optical mode was thermally tuned (>3 nm) over a full free spectral range because the Ohmic heating changed the refractive index of the silica and dimeter of the MBR. The mechanical mode was thermally tuned with a relative tuning range of 9% because the Ohmic heating changed the velocity and density of the liquid metal.

An RF-MEMS-Based Digitally Tunable SIW Filter in X-Band for Communication Satellite Applications

This paper presents a digitally tunable SIW (substrate integrated waveguide) filter designed for X-band, based on RF-MEMS (radio frequency micro-electrical-mechanical systems) technology. Four commercial off-the-shelf RF-MEMS single-pole single-throw (SPST) switches were directly mounted on the upper surface of the filter, with metallic tuning posts specifically located within each cavity to define the potential achievable frequency range. Fabricated on standard alumina substrate, the design of the filter and the biasing network enabled fine digital frequency control of up to four functional states by the inclusion of wire bondings between each switch and the substrate. A relative tuning range of 2.3% was achieved between the lower and upper discrete states of 2.76% and 2.89% in the 3 dB fractional bandwidths.

Enhancing performance of a InGaP/GaAs VCO by means of a switching architecture

The introduction of a compact switching architecture in the design of a monolithic microwave integrated voltage-controlled oscillator (VCO) is described, that is able to provide enhanced characteristics in term of both linearity and tunability. The proposed solution is applied to the design of a C-band GaAs MMIC (monolithic microwave integrated circuit) VCO. The chip has been realised with the heterojunction bipolar transistor (HBT) HB20M technology process provided by UMS Foundry. Measurements show a relative tuning range of about 30%, an average sensitivity of about 83 MHz/V, a minimum output power of 11.4 dBm and a phase noise at 100 kHz of frequency offset from the carrier lower of −107 dBc/Hz.

A Frequency- and Pattern-Reconfigurable Two-Element Array Antenna

A frequency- and pattern-reconfigurable two-element array antenna based on a stub-loaded configuration is presented. The frequency-tuning mechanism is implemented using varactor diodes loaded with open stubs. Two independent bias voltages allow to independently add pattern reconfigurability to the array. This is achieved by slightly detuning the resonances of the two patches, thus introducing a beam-steering relative phase difference between them. The design is optimized for a relative frequency tuning range of $\text{10}\%$ extending from 2.15 to 2.38 GHz, within which it presents a continuously beam-steerable radiation pattern covering scanning angles from $-\text{23}^{\circ }$ to $+ \text{23}^{\circ }$ across broadside. An antenna prototype is experimentally characterized, which validates the proposed concept.

High selectivity tunable filtering power divider based on liquid crystal technology for microwave applications

A novel electrically tunable filtering power divider, based on liquid crystal (LC) technology, is designed and experimentally demonstrated in this paper. Using low-cost T-shaped inverted-microstrip structures integrated with two dual-mode open-loop resonators, the tunable power divider possesses bandpass-filtering frequency response. Moreover, transitions between inverted-microstrip and microstrip structures are specifically designed at the I/O ports for the self-packaged. The experimental results indicate that the central frequency can be tuned from 5.75 to 6.25 GHz, i.e. a relative tuning range of 8.3%, the 3-dB bandwidth is 6.8 ± 0.2% over the whole tuning range, and the maximum amplitude and phase imbalance are less than 0.8 dB and 2.5°, respectively. In addition, the proposed filtering power divider has three transmission zeros to improve the frequency selectivity.

Staked Graphene for Tunable Terahertz Absorber with Customized Bandwidth

Terahertz (THz) absorber with dynamically tunable bandwidth possesses huge application value in the fields of switches, sensors, and THz detection. However, the perfect absorbers based on photonic crystals and metamaterials are not intelligent enough to capture the electromagnetic wave in a tunable way. In this paper, we utilized only patterned graphene to tune the absorption positions and the bandwidth in the terahertz regime. More distinguished than some dynamic absorbers proposed before, the performances with peak frequency relative tuning range of 68 % and nearly unity absorbance are obtained by a single cross-shaped graphene layer. Additionally, the working bandwidth can be broadened with stacked structured graphene. The almost perfect absorption shifted from 2.36∼3.2 to 3.26∼3.99 THz continuously via changing the chemical potential of graphene.

Design of 5 GHz‐compact reconfigurable DGS‐bandpass filter using varactor‐diode device and coupling matrix technique

ABSTRACTIn this work, a novel electrical reconfigurable second order microstrip bandpass filter is presented. The investigated filter structure consists of two neighbor coupled octagonal‐DGS resonators. The capacitor and varactor are placed on the DGS‐shape. According to the tunable characteristic of discrete lumped capacitors and varactor diode, the resonant frequencies can be tunable. The proposed filter occupies an area of 2.5 × 1.5 cm2. The coupling method has been applied and leads to an extended coupling matrix that assumes multiple couplings between both filter resonators. The measured relative tuning range of the bandpass filter is 23.5% with a fractional bandwidth of 12%. Varactors loaded on the octagonal DGS resonator allow us to control the center frequency with quasi constant bandwidth as well as to have a large tuning bandwidth. Based on the detailed analysis, the varactor‐tuned DGS bandpass filter is designed, in which the tuning bandwidth reaches 12% of the center frequency at 5 GHz. The proposed reconfigurable bandpass filter was fabricated and tested. The measured results show a variation of the center frequency from 4.3 GHz (0V) to 5.4 GHz (15V) tunable range, while the insertion loss varies from 5 to 3 dB. The measured performance of the tunable bandpass filter agrees well with the simulation results. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:304–309, 2016

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/superstrate's thickness on the MA's 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.

Improving the Pass-Band Return Loss in Liquid Crystal Dual-Mode Bandpass Filters by Microstrip Patch Reshaping.

In this paper, the design and experimental characterization of a tunable microstrip bandpass filter based on liquid crystal technology are presented. A reshaped microstrip dual-mode filter structure has been used in order to improve the device performance. Specifically, the aim is to increase the pass-band return loss of the filter by narrowing the filter bandwidth. Simulations confirm the improvement of using this new structure, achieving a pass-band return loss increase of 1.5 dB at least. Because of the anisotropic properties of LC molecules, a filter central frequency shift from 4.688 GHz to 5.045 GHz, which means a relative tuning range of 7.3%, is measured when an external AC voltage from 0 Vrms to 15 Vrms is applied to the device.