Tunable Bandwidth Research Articles

Broadband Acoustoelectric Conversion Based on Oriented Polyacrylonitrile Nanofibers and Slit Electrodes for Generating Power from Airborne Noise

Electrospun nanofiber acoustoelectric devices typically have a bandwidth in the range of 100-400 Hz, which limits their applications. This study demonstrates a novel device structure with tunable acoustoelectric bandwidth based on oriented electrospun polyacrylonitrile (PAN) nanofibers and slit electrodes. When the PAN nanofibers were arranged perpendicular to the slits, the devices had a much wider bandwidth than their parallel counterparts, while the latter had a bandwidth similar to that of randomly oriented nanofibers. In all devices, the electrical outputs follow a similar trend with the slit aspect ratio. However, the slit number only affected the electrical output without changing the bandwidth characteristic. We further showed that both the slit electrode and the oriented nanofiber membranes played a role in tuning the frequency response. Under sound, the vibration of the electrode caused the slit to be misaligned on both sides. The anisotropic tensile properties of the oriented nanofiber membranes allowed the fibers to stretch differently depending on their angle of alignment with the slits. Those perpendicular to the slits received more intense stretching, contributing to a wider bandwidth. The wider bandwidth increases the electrical output, especially when harvesting multifrequency sound. A 4 × 3 cm2 device made of five-slit electrodes (slit width × length, 2 mm × 30 mm) with PAN nanofibers perpendicular to the slits showed a bandwidth of 100-900 Hz and electrical outputs of 39.85 ± 1.34 V (current output 6.25 ± 0.18 μA) under 115 dB sound conditions, which is sufficient to power electromagnetic wireless transmitters. When one such slit device was used as a power supply and another as a sound sensor, they formed a completely self-powered wireless system that could detect sounds from various scenarios, such as high-speed trains, airports, highway traffic, and manufacturing industries. The energy can also be stored in lithium-ion batteries and capacitors. We hope that such novel devices will contribute to the development of highly efficient acoustoelectric technology for generating electrical energy from airborne noise.

A Tunable Gain and Bandwidth Low-Noise Amplifier with 1.44 NEF for EMG and EOG Biopotential Signal

This paper presents a low-noise inverter-based current-mode instrumentation amplifier with tunable gain and bandwidth for electromyogram (EMG) and electrooculogram (EOG) biopotential signals, targeting low input noise while maintaining low power consumption. The gain tuning method is based on pseudo-resistors, whereas the bandwidth is tunable due to a varactor system that is controlled by the same control voltage that tunes the gain. The circuit was designed and manufactured using the 110 nm UMC CMOS technology node, occupying an area of 0.624 mm2. The circuit presents a functioning mode for each biopotential signal with different characteristics, for the EMG a gain of 34.7 dB and a bandwidth of 1412 Hz was measured, with an input referred noise of 1.407 μV which matches a noise efficiency factor of 1.44. The EOG mode achieves a 39.5 dB gain and a 22.4 Hz bandwidth while presenting an input-referred noise of 0.829 μV corresponding to a noise efficiency factor of 6.37. For both modes, the supply voltage is 1.2 V and the circuit consumes 1 μA.

Photonic generation of tunable wideband dual-chirp microwave waveforms using a monolithic integrated three-section laser

A method for flexibly generating dual-chirp microwave waveform (DCMW) is proposed and experimentally demonstrated. The scheme uses a monolithic integrated three-section laser, which consists of a front section laser, a phase section and a rear section laser. Under the appropriate control current, the integrated laser works in the period-one (P1) state, and the two beams of different frequencies generated by the integrated laser can produce stable beat frequency. After photoelectric conversion, the generated DCMW has the flexible tunability of the central frequency. Through theoretical analysis and experimental demonstration, the DCMW with time-bandwidth-product (TBWP) of 6 × 105 and chirp rate of ± 0.03 GHz/μs is obtained at different centre frequencies. In addition, the auto-ambiguity function of the generated DCMW is also studied and it is proved that the proposed scheme can improve the range-velocity resolution, so it has application prospects in microwave photonic radar systems.

A Compact Bandwidth and Frequency-Range Reconfigurable CM Filter Using Varactor-Loaded Dual-Slot-Based Defected Ground Structure

A reconfigurable common-mode (CM) filter with a wide tunable bandwidth and frequency-range is proposed in this article. The filter adopts four compact dual-slot-based defected ground structure (DGS) cells to achieve a wide suppression of CM noise. To simplify the design, only one varactor and one dc-blocking capacitance are loaded on one DGS cell directly, which retains the original structure of the DGS cell. A wide continuously adjustable frequency range and bandwidth can be achieved when different voltages are applied to different varactors. A dynamic equivalent circuit model is built to explain the filter's working principle, which is helpful to obtain the filtering characteristics under different varactor parameters quickly and accurately. In order to verify the feasibility of the scheme, a DGS-based filter with an area of 19 mm × 9.2 mm is implemented under the microstrip line. The simulated results show the filter can achieve a continuously tunable CM suppression of frequency range from 1.9 to 7.3 GHz, and a tunable bandwidth using different voltage supplies. Good agreement between full-wave simulation and measured results is observed.

Oxygen vacancies in α-(Al x Ga1−x )2O3 alloys: a first-principles study

α-(Al x Ga1−x )2O3 alloys have attracted increasing interest as semiconductors with tunable wide band gaps. We report a systematic analysis of O vacancies in α-Al2O3, α-Ga2O3, and α-(Al x Ga1−x )2O3 alloys using first-principles calculations. The formation energies and electronic levels of the O vacancies are sensitive to not only the nearest-neighbor Al/Ga ratio but also the atomic relaxation around the vacancies. Consequently, the vacancy formation energies vary by up to ∼2 eV, reflecting diverse local atomic environments in the alloys. These results provide insight into further understanding and controlling the properties of α-(Al x Ga1−x )2O3 alloys.

Sub-kHz-Linewidth External-Cavity Laser (ECL) With Si3N4 Resonator Used as a Tunable Pump for a Kerr Frequency Comb

Combining optical gain in direct-bandgap III-V materials with tunable optical feedback offered by advanced photonic integrated circuits is key to chip-scale external-cavity lasers (ECL), offering wideband tunability along with low optical linewidths. External feedback circuits can be efficiently implemented using low-loss silicon nitride (Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) waveguides, which do not suffer from two-photon absorption and can thus handle much higher power levels than conventional silicon photonics. However, co-integrating III-V-based gain elements with tunable external feedback circuits in chip-scale modules still represents a challenge, requiring either technologically demanding heterogeneous integration techniques or costly high-precision multi-chip assembly, often based on active alignment. In this work, we demonstrate Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> -based hybrid integrated ECL that exploit 3D-printed structures such as intra-cavity photonic wire bonds and facet-attached microlenses for low-loss optical coupling with relaxed alignment tolerances, thereby overcoming the need for active alignment while maintaining the full flexibility of multi-chip integration techniques. In a proof-of-concept experiment, we demonstrate an ECL offering a 90 nm tuning range (1480 nm–1570 nm) with on-chip output powers above 12 dBm and side-mode suppression ratios of up to 59 dB in the center of the tuning range. We achieve an intrinsic linewidth of 979 Hz, which is among the lowest values reported for comparable feedback architectures. The optical loss of the intra-cavity photonic wire bond between the III-V gain element and the Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> -based tunable feedback circuit amounts to approximately (1.6 ± 0.2) dB. We use the ECL as a tunable pump laser to generate a dissipative Kerr soliton frequency comb. To the best of our knowledge, our experiments represent the first demonstration of a single-soliton Kerr comb generated with a pump that is derived from a hybrid ECL.

Nonlinear kernel mode‐based regression for dependent data

Under stationary ‐mixing dependent samples, we in this article develop a novel nonlinear regression based on mode value for time series sequences to achieve robustness without sacrificing estimation efficiency. The estimation process is built on a kernel‐based objective function with a constant bandwidth (tuning parameter) that is independent of sample size and can be adjusted to maximize efficiency. The asymptotic distribution of the resultant estimator is established under suitable conditions, and the convergence rate is demonstrated to be the same as that in nonlinear mean regression. To numerically estimate the kernel mode‐based regression, we develop a modified modal‐expectation‐maximization algorithm in conjunction with Taylor expansion. A robust Wald‐type test statistic derived from the resulting estimator is also provided, along with its asymptotic distribution for the null and alternative hypotheses. The local robustness of the proposed estimation procedure is studied using influence function analysis, and the good finite sample performance of the newly suggested model is verified through Monte Carlo simulations. We finally combine the recommended kernel mode‐based regression with neural networks to develop a kernel mode‐based neural networks model, the performance of which is evidenced by an empirical examination of exchange rate prediction.

Tuning the Reflection Bandwidth of Polarization Volume Gratings by Guest Material Penetration

Polarization volume gratings (PVGs) based on polymerized cholesteric liquid crystal (CLC) films can potentially be applied as optical combiners in augmented reality (AR) smart glasses. The reflection bandwidth of PVGs is determined by the helical pitch and birefringence of the CLC, which must be tuned to match the desired color for reflection, but tuning the material properties is challenging as the materials must also be polymerizable. Here, we report a postprocessing method based on guest material penetration for the tuning of the reflection bandwidth of CLC polymer films. Solution coating of a high-birefringence guest liquid crystal on a CLC film modifies the birefringence as well as elongates the helical pitch. Penetration of a high-birefringence 5CT liquid crystal was found to cause a birefringence increase of the CLC film by approximately 18%.

Design and analysis of Minkowski fractal antenna for wideband THz frequency tuning/multiband operation in a MIMO antenna system

The proposed Minkowski fractal antenna design achieves wideband and continuous frequency tuning in a multiple-input and multiple-output (MIMO) antenna system. By manipulating the fractal geometry of the unit antenna element, the resonance frequency of the antenna can be adjusted simply by changing its electrical length. The Minkowski fractal operator generates an increasing current path, resulting in a leftward frequency shift as the antenna side length increases with each iteration. In the first iteration, the proposed fractal antenna demonstrated a 97.9% continuous frequency shift from 0.204 to 0.404 THz with maximum return loss values of −31.23 and −21.6 dB, respectively. In the second iteration, a 38.6% continuous resonance frequency shift from 0.413 to 0.578 THz was achieved with return loss values of −18.22 and −40.47 dB, respectively. The maximum tunable bandwidths of the first and second iterations were approximately 0.2 and 0.16 THz, respectively. The proposed correlation between the dimensions of a single antenna and its resonance frequency provides the foundation for designing and implementing MIMO antenna systems in high-speed wireless devices, cognitive radio, and other multiband MIMO applications. A 2 × 2 MIMO antenna system has been designed from the results of the proposed single antenna to achieve multiband operation or frequency tuning through selective switching of the antenna feed.

Generation of Large-Bandwidth High-Power X-Ray Free-Electron-Laser Pulses Using a Hollow-Channel Plasma

Large-bandwidth x-ray free-electron-laser (XFEL) facilities are desirable scientific tools in various fields, such as molecular structural dynamics and spectroscopy diagnosis. Various methods are proposed to broaden the FEL spectra. Here, a method is proposed to generate high-power XFEL radiation of tunable spectral bandwidth using plasma wakefield acceleration. An ultrabroad bandwidth is achieved by chirping the electron beam in a hollow-channel plasma without noticeable slice-beam-quality degradation. A dedicated beamline can match the beams with a large energy chirp in the undulators almost without beam loss. Numerical simulations demonstrate that a relative spectral bandwidth (full width) of up to 24% can be obtained with optimized beam and plasma parameters.

Energy‐Saving High‐Bandwidth Perovskite Sub‐Micro‐Encoder

AbstractOptical coding is widely applied in communication and data processing fields due to its practical and powerful characteristics, such as high transmission rate and operational convenience. However, improvements in the loading information capacity of optical encoders always result in increased energy consumption. It is a challenge to realize an optical encoder that combines energy savings, high bandwidth, and miniaturization. Here, both the concept and the demonstration of a perovskite sub‐micro‐encoder with an 84% energy‐saving efficiency and a tunable bandwidth of up to 200 GHz are reported. The way to code the radiative dynamics of the light source allows for the reduction of unnecessary energy consumption from the initial stage through the control of the radiation core. Moreover, the miniature encoder generates code sequences of good coherence, stimulating the development of perovskite‐based optical microchips for ultrafast information processing.

Tunable topological phase transition in the telecommunication wavelength

Recent progress in the Valley Hall insulator has demonstrated a nontrivial topology property due to the distinct valley index in 2D semiconductor systems. In this work, we propose a highly tunable topological phase transition based on valley photonic crystals. The topological phase transition is realized by the inversion symmetry broken due to the refractive index change of structures consisting of optical phase change material (OPCM) with thermal excitation of different sites in a honeycomb lattice structure. Besides, simulations of light propagation at sharp corners and pseudo-spin photon coupling are conducted to quantitatively examine the topological protection. Compared with other electro-optical materials based on reconfigurable topological photonics, a wider bandwidth and greater tunability of both central bandgap frequency and topological phase transition can happen in the proposed scheme. Our platform has great potential in practical applications in lasing, light sensing, and high-contrast tunable optical filters.

Hybrid-integrated wideband tunable optoelectronic oscillator.

As a photonic-based microwave signal generation method, the optoelectronic oscillator (OEO) has the potential of meeting the increasing demand of practical applications for high frequency, broadband tunability and ultra-low phase noise. However, conventional OEO systems implemented with discrete optoelectronic devices have a bulky size and low reliability, which extremely limits their practical applications. In this paper, a hybrid-integrated wideband tunable OEO with low phase noise is proposed and experimentally demonstrated. The proposed hybrid integrated OEO achieves a high integration level by first integrating a laser chip with a silicon photonic chip, and then connecting the silicon photonic chip with electronic chips through wire-bonding to microstrip lines. A compact fiber ring and an yttrium iron garnet filter are also adopted for high-Q factor and frequency tuning, respectively. The integrated OEO exhibits a low phase noise of -128.04 dBc/Hz @ 10 kHz for an oscillation frequency of 10 GHz. A wideband tuning range from 3 GHz to 18 GHz is also obtained, covering the entire C, X, and Ku bands. Our work demonstrates an effective way to achieve compact high-performance OEO based on hybrid integration, and has great potential in a wide range of applications such as modern radar, wireless communication, and electronic warfare systems.

Extended topological interface modes with tunable frequency in the piezoelectric phononic crystal

Acoustic metamaterials have provided a versatile platform to explore more degrees of freedom for tunable topological wave manipulation. Recently, extended topological interface modes (ETIMs) with heterostructure have been proposed to extend the spatial degree of freedom. However, the absence of frequency tunability still restricts the application of the wave transports of ETIMs. Here, we propose a one-dimensional piezoelectric topological phononic crystal (PTPC) with electrically tunable working frequency by introducing external capacitor circuit. With the bandgap frequency actively controlled by appropriately tuning the capacitances, we construct the heterostructured PTPCs possessing high-energy-capacity ETIMs with electrically tunable working frequency range and bandwidth. This work paves the way to wide engineering applications on acoustic sensing enhancement, nondestructive testing, energy harvesting, information processing, and reconfigurable topological wave transports.

A Simple Frequency- and Polarization-Reconfigurable Bent Monopole Antenna Based on Liquid Metal

In this letter, a frequency- and polarization-reconfigurable bent monopole antenna using liquid metal is proposed. The proposed antenna is designed to operate at continuously tunable frequencies and switch between left-handed circular polarization (LHCP) and right-handed circular polarization (RHCP) states. The frequency reconfigurability is achieved by controlling the length of the liquid metal to change the electrical length of the antenna. The circularly polarization can be achieved by injecting from the inlet, and excited by orthogonal horizontal radiation as well as vertical radiation. 3 dB axial ratio bandwidth of 37.5%(3.12–4.63 GHz) in LHCP state and 48.8% (3.07–5.05 GHz) in RHCP state can be achieved. The whole antenna is characterized by continuous tunability, reconfigurable frequency and polarization, and wide tunable bandwidth. The measured results of constructed prototypes are in close agreement with the simulation results, and its feasibility is fully proved.

A Theoretical Study on Mid-Infrared Difference Frequency Generation Based on Periodically Poled Thin-Film LiNbO3

A mid-infrared difference frequency generator (DFG) based on a periodically poled thin-film lithium niobate rib waveguide on a sapphire substrate is theoretically studied. A mode analysis is carried out at the mid-infrared region, and the analysis focuses on the effects of waveguide geometry on effective refractive indices of a few lower-order modes. A complete theory suitable for modeling a DFG based on a waveguide structure is described. Its validity is confirmed by comparing the theoretical results with previously reported experimental data. Explicit expressions are presented for nonlinear conversion efficiency, thermal tunability and quasi-phase matching (QPM) bandwidth. The effects of waveguide geometry and mode hybridization on the effective mode field area and mode overlap factor, which are either inversely or linearly proportional to nonlinear conversion efficiency, are studied in detail. In this article, an optimized mid-infrared DFG with improved geometry that exhibits excellent performance, including a higher nonlinear conversion efficiency of 230–273% W−1cm−2 in the temperature range of 20–120 °C; a larger temperature tunability of 2.2 nm/°C; a larger QPM bandwidth of ~130 nm; and a higher idler wave output power, as much as −2 dBm when Pp = 20 dBm and Ps = 11.5 dBm, is suggested.

Doubly-resonant enhanced second-harmonic generation and its wide-band tunability

The second-harmonic generation (SHG) from the doubly-resonant metasurfaces composed of the U-shaped and the rectangular apertures on an optically opaque gold film is investigated theoretically. By engineering the fundamental plasmonic resonances of the composite metasurfaces to match the excitation and the second-harmonic wavelengths, respectively, the doubly-resonant enhanced SHG intensity of almost an order of magnitude is obtained, compared with the singly-resonant U-apertures. In addition, the satisfaction of doubly-resonant condition can be extended to a wide wavelength range by changing the refractive index of the surrounding medium, which benefits from the two fundamental plasmonic resonances oscillating at frequencies of a two-fold relationship. Our work provides a new way to enhance the efficiency of nonlinear harmonic generation.

Design of a broadband frequency-tunable planar filtering power divider

ABSTRACT In order to meet the development requirements of multi-frequency and functional integration of radio frequency (RF) modules, a broadband tunable filtering power divider with center frequency is designed. A new structure is composed of open branch line, step impedance resonator, and double branch resonator. The filter response is realized by branch line and double branch resonator. The resonant frequency of the device can be adjusted flexibly by controlling the varactor diode loaded on the two resonators with external voltage. The double branch resonator can realize the broadband frequency adjustability, the step impedance resonator can improve part of the return loss performance of the device, so that the central frequency can achieve high performance in a wide tuning range. The research results show that the center frequency of the designed filtering power divider is adjustable in the range of 1.6 GHz to 2.7 GHz, the tuning bandwidth is 51.2%, the input return loss of each center point is better than 20 dB, and the isolation between output ports is better than 19 dB. The device is expected to be widely used in RF communication systems such as array antennas and transceiver components.

Active tunable terahertz bandwidth absorber based on single layer graphene

In this paper, an active tunable terahertz bandwidth absorber based on single-layer graphene is proposed, which consists of a graphene layer, a photo crystal plate, and a gold substrate. When the Fermi energy (E f ) of graphene is 1.5 eV, the absorber shows high absorption in the range of 3.7 THz–8 THz, and the total absorption rate is 96.8%. By exploring the absorption mechanism of the absorber, the absorber shows excellent physical regulation. The absorber also shows good adjustability by changing the E f of graphene. This means that the absorber exhibits excellent tunability by adjusting the physical parameters and E f of the absorber. Meanwhile, the absorber is polarization independent and insensitive to the incident angle. The fine characteristics of the absorber mean that the absorber has superior application value in many fields such as biotechnology and space exploration.

Experimental Study on the Frequency Tunability of a Magnetically Insulated Transmission Line Oscillator

In the present study, the frequency tunability of a magnetically insulated transmission line oscillator (MILO) is investigated by simulation in conjunction with an experiment. In the simulation, the tunable MILO reaches a maximum output power of approximately 5.0 GW with an applied voltage of 590 kV and an input power of 29 GW. The corresponding power efficiency is 17.2%, and the frequency is 1.575 GHz. In addition, by adjusting the outer radius of the vane from 77 to 150 mm, the 3-dB tunable frequency of the tunable MILO ranges from 0.85 to 2.12 GHz, and the tuning bandwidth is 85.5%. The experimental results show that when the voltage and current are 405 kV and 42 kA, respectively, the peak microwave power of the tunable MILO is approximately 2.3 GW with a microwave frequency of approximately 1.900 GHz. The difference in the two frequencies at the peak microwave power is significant because of the tuning mechanism. When the outer radius of the vane varies in the range of 76.6–93.0 mm, the 3-dB tunable frequency range is between 1.552 and 2.168 GHz, corresponding to a 3-dB tuning bandwidth of 33.1%. The experimental measurements verify the simulation results.