dB Cutoff Research Articles

Vertically‐Aligned GaS Nanosheets‐Based Photoelectrochemical Photodetector for Fast‐Speed, High‐Sensitivity, and Self‐Powered UV–vis Underwater Optical Communication

AbstractUnderwater optical communication (UOC) based on the novel photoelectrochemical (PEC) type photodetector has garnered substantial interest in the field of marine resource exploration and ocean investigation due to its strong operability, self‐powered capability, and large sensitive area in harsh underwater environments. To achieve high sensitivity, fast transmission speed, and broadband response characteristics, vertical‐GaS nanosheets‐based PEC photodetectors are prepared by chemical vapor deposition for UOC application. The GaS based PEC photodetector exhibits excellent self‐powered photodetection and a broad UV–vis photoresponse range as well as high stability performance in both Na2SO4 electrolyte and seawater. Notably, it also demonstrates an exceptional photoresponsivity of 11 mA W−1 coupled with a fast response and recovery time of 12.9 and 13 µs which are superior to those of 2D layered materials and conventional wide‐bandgap semiconductors. These improvements are mainly due to the large surface area, short diffusion length, as well as high carrier mobility of vertically‐oriented GaS nanosheets. Strikingly, the constructed GaS photodetector showcases a broad bandwidth exceeding the −3 dB cutoff frequency of 55.3 kHz during UOC testing, successfully enabling transmission and decoding of character information. This work establishes the groundwork for developing high‐performance, self‐powered UV–vis photodetectors adaptable to UOC using PEC technology.

High‐Speed Flexible Near‐Infrared Organic Photodetectors for Self‐Powered Optical Integrated Sensing and Communications

AbstractIntegrated sensing and communications (ISAC) based on radio frequency (RF), merging the sensing and communication functionalities into a single platform, can effectively address the increasing demand for spectrums due to the proliferation of wireless devices. However, the ISAC commonly suffers from spectrum scarcity, high power consumption, and limited sensing capabilities. Here flexible self‐powered optical ISAC (O‐ISAC) is proposed and fabricated by preparing a high‐speed near‐infrared organic photodetector (NIR OPD) that exhibits a broad photodetection ranging from 300 to 1100 nm, specific detectivity over 1013 Jones, and −3 dB cutoff frequency over 1 MHz (λ = 1050 nm) in self‐powered mode, ranking as the fastest among self‐powered NIR OPDs in the region above 1000 nm. The device is fabricated by incorporating a narrow‐bandgap non‐fullerene acceptor BTPSV‐4Cl into the PCE‐10:BTPSV‐4F system to finely tune the film morphology and reduce trap states and energetic disorder. The semitransparent and regular NIR OPDs are integrated on one substrate to fabricate flexible O‐ISAC, realizing simultaneous accurate vital signs monitoring and high‐speed data transmission under ambient, sunlight, and NIR lighting conditions. It is envisioned that the proposed O‐ISAC will inspire a new generation of wireless technology to address RF limitations for emerging applications, such as intelligent healthcare, autonomous vehicles, and security.

Compact microstrip lowpass filter with wide stopband and sharp transition band using radial stub resonator

This study introduces a compact microstrip lowpass filter that offers a wide stopband and a low transition band. The couple radial stubs are designed for having sharp roll-off. The filter has a −3 dB cut off frequency of 1.6 GHz, with a transition band ranging from 1.6 to 1.7 GHz with attenuation levels of −3 dB and −20 dB, respectively. To ensure a wide stopband, the filter utilizes bended transmission lines with stepped impedance suppressing cells, resulting in a stopband attenuation of 20 dB from 1.7 to 28 GHz. The insertion loss in the passband is less than 0.1 dB, and the overall size of the filter is 0.127 × 0.087 λg2.

Ultrafast (≈10 GHz) mid-IR modulator based on ultrafast electrical switching of the light–matter coupling

We demonstrate a free-space amplitude modulator for mid-infrared radiation (λ ≈ 9.6 μm) that operates at room temperature up to at least 20 GHz (above the −3 dB cutoff frequency measured at 8.2 GHz). The device relies on the ultrafast transition between weak- and strong-coupling regimes induced by the variation of the applied bias voltage. Such transition induces a modulation of the device reflectivity. It is made of a semiconductor heterostructure enclosed in a judiciously designed array of metal–metal optical resonators, that—all-together—behave as an electrically tunable surface. At negative bias, it operates in the weak light–matter coupling regime. Upon application of an appropriate positive bias, the quantum wells populate with electrons, and the device transitions to the strong-coupling regime. The modulator transmission remains linear with input radio frequency power in the 0–9 dBm range. The increase in optical powers up to 25 mW exhibit a weak beginning of saturation a little bit below.

Spin-vertical-cavity surface-emitting lasers with by-design-defined birefringence for high-speed modulation.

We propose a new, to the best of our knowledge, type of spin-vertical-cavity surface-emitting laser (VCSEL) with controlled by design birefringence. To this aim, we utilize the so-called columnar thin films (CTFs) in the VCSEL dielectric distributed Bragg mirror and/or in a second dielectric cavity. We design such CTF-VCSELs with pre-defined birefringence and calculate their polarization-resolved resonant longitudinal modes and the corresponding quantum-well confinement factors and threshold gains. Using the spin-flip VCSEL model, we show that such spin CTF-VCSELs can achieve small-signal modulation response with a 3 dB cutoff frequency of several hundreds of GHz.

A fast and effective approach for microstrip filter design using GA and TL-model

Microwaves and RF technology and their components like filters, antennas, etc. are commonly used in wireless networking and communication systems, wireless security systems, radar systems, and environmental remote sensing. In this paper, a fast and effective procedure has been proposed for microstrip filter design using a genetic algorithm (GA) with a transmission line model (TL). GA is modified to be highly efficient and accurate by encoding the topology. For the fixed filter topology, the electrical parameters of the filter are encoded in a single chromosome. To make the proposed procedure fast and effective, a transmission line model has been employed to compute the fitness value in GA. To demonstrate the effectiveness of the proposed procedure, a wideband second-order bandpass filter with a center frequency of 2.3 GHz is examined with a pair of short-circuited stubs and a pair of open-circuited stubs. The optimized design is validated using full-wave methods (MoM and EM simulator CST). The results show a low insertion loss of 0.1 dB and return loss better than 30 dB and a wide bandwidth of 46.95% with − 3 dB cutoff frequencies at 1.76 GHz and 2.84 GHz.

Designing optimal prototype filters for maximally decimated Cosine Modulated filter banks with rapid convergence

An analytic design of a prototype filter for M-channel maximally decimated cosine-modulated Near Perfect Reconstruction (NPR) filter banks is proposed in this work. The prototype filter is created using the least-square (CLS) method with weighted constraints, which is one-dimensional and requires single-parameter optimization. Compared to existing approaches, this suggested method achieves rapid convergence by analytically determining the optimal step size, ensuring the 3 dB cutoff frequency at π/2 M. The simulation results for design examples outperform the techniques in the available literature in terms of amplitude and aliasing distortion, reaching distortion around 2.4489 × 10−4 and 3.4907 × 10−9, respectively. This optimization algorithm's usefulness is further demonstrated with the sub-band coding of ECG signals. Implementing optimal prototype filters has tangible real-world effects, especially in critical sectors like healthcare and communications, improving diagnostics accuracy, data transmission efficiency, and overall performance.

A fast 2D MoS2 photodetector with ultralow contact resistance.

Two-dimensional (2D) transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS2), hold great promise for next-generation nanoelectronic and nanophotonic devices. While high photoresponsivity and broad spectral coverage (UV-IR) have been reported, the slow response time of MoS2 photodetectors caused by their unfavorable RC characteristics is still a major limit in current devices. Once the RC limit issue is resolved, the intrinsic saturation drift velocity of electrons in TMDs (∼106 cm s-1) may enable GHz opto-electronic operations. Recent breakthroughs in device fabrication technology have enabled significant progress in exploring the possibilities of high-speed TMD photodetectors. In this work, using semi-metallic bismuth contacts to suppress metal-induced gap states (MIGS), an MoS2 photodetector with ultra-low contact resistance (<400 Ω μm) was fabricated. The device exhibited a broad bandwidth and high photoresponsivity (>1 A W-1). In particular, using an acousto-optic modulator (AOM)-modulated 532 nm laser, a -3 dB cutoff frequency of ∼70 kHz was obtained, which was corroborated by directly observed rise/fall times (on a scale of 10 μs). An extrinsic effect, where defective states of BN induce a negative shift in the photocurrent baseline was further identified and attributed to charge-induced screening, elucidating where a device can exhibit different dynamic and static response behaviors simultaneously. Our results may shed light for future GHz optoelectronic applications employing TMDs as a platform.

Ultralow dark current and high on–off ratio of perovskite photodetectors enabled by 3D/2D/3D heterostructures unipolar barrier

Lead halide perovskite photodetectors exhibit outstanding photoelectric performance in multiple parameters and have become a widely studied hotspot. The perovskite with single crystal structures significantly reduces the density of grain boundaries and defect states, thereby improving the carrier lifetime and mobility. However, the suppression of dark current is one of the most important concerns for the research of perovskite photodetectors. Here, we constructed a pBp unipolar barrier photodetector by inserting two-dimensional perovskite into three-dimensional perovskite through a pressure bonding process. The dark current of the photodetector decreased by more than two orders of magnitude, while the photocurrent remained unchanged. The perovskite unipolar barrier photodetectors exhibit excellent photoelectric performance, with an on–off ratio of 106, a specific detectivity of 4.11 × 1012 Jones, a −3 dB cutoff frequency of 344 kHz, and a linear dynamic range of 160 dB, providing a prospective path for weak light detection.

Development of n-Type Small-Molecule Acceptors for Low Dark Current Density and Fast Response Organic Photodetectors.

Suppressing the dark current density (Jd) while maintaining sufficient charge transport is important for improving the specific detectivity (D*) and dynamic characteristics of organic photodetectors (OPDs). In this study, we synthesized three novel small-molecule acceptors (SMAs) densely surrounded by insulating alkyl side chains to minimize the Jd in OPDs. Introducing trialkylated N-annulated perylene diimide as a terminal moiety to the alkylated π-conjugated core structure was highly efficient in suppressing Jd in the devices, resulting in an extremely low Jd of 4.60 × 10-11 A cm-2 and 10-100 times improved D* values in the devices. In addition, SMAs with a geometrically aligned backbone structure exhibited better intermolecular ordering in the blended films, resulting in 3-10 times as high responsivity (R) values in the OPDs. Outstanding OPD performances with a D* of 8.09 × 1012 Jones, -3 dB cutoff frequency of 205.2 kHz, and rising response time of 16 μs were achieved under a 530 nm illumination in photoconductive mode. Geometrically aligned core-terminal SMAs densely surrounded by insulating alkyl side chains are promising for improving the static and dynamic properties of OPDs.

Design and analysis of highly selective ultrawide stopband lowpass filter using lumped and distributed equivalent circuit models

AbstractAn ultrawide stopband lowpass filter is reported using three stepped impedance resonators with high selectivity. The filter extends the stopband frequency range and attenuation, and two quarter‐wave open stubs and three circular ground slots are introduced. The lumped and distributed equivalent models are derived and analyzed. The corresponding results are validated experimentally in a fabricated prototype. The prototype lowpass filter has a 3 dB cutoff frequency (fc) of 2.9 GHz, and the stopband is extended up to 35 GHz (12.07fc), with an attenuation level better than 20 dB throughout. The passband‐to‐stopband transition (3 dB–20 dB) bandwidth is 0.18 GHz, and the roll‐off factor is 135 dB/GHz at 30 dB. The insertion loss is 0.3 dB at 1.6 GHz. The normalized circuit size of the proposed filter with respect to the guided wavelength is 0.04.

Low-noise and high-speed trans-impedance amplifier for nanopore sensor.

The small current detection circuit is the core component of the accurate detection of the nanopore sensor. In this paper, a compact, low-noise, and high-speed trans-impedance amplifier is built for the nanopore detection system. The amplifier consists of two amplification stages. The first stage performs low-noise trans-impedance amplification by using ADA4530-1, which is a high-performance FET operational amplifier, and a high-ohm feedback resistor of 1 GΩ. The high pass shelf filter in the second stage recovers the higher frequency above the 3 dB cutoff in the first stage to extend the maximum bandwidth up to 50 kHz. The amplifier shows a low noise below sub-2 pA rms when tuned to have a bandwidth of around 5 kHz. It also guarantees a stable frequency response in the nanopore sensor.

Breaking the Responsivity‐Bandwidth Trade‐Off Limit in GaN Photoelectrodes for High‐Response and Fast‐Speed Optical Communication Application

AbstractUnderwater optical communication (UOC) has attracted considerable interest in the continuous expansion of human activities in marine/ocean environments. The water‐durable and self‐powered photoelectrodes that act as a battery‐free light receiver in UOC are particularly crucial, as they may directly face complex underwater conditions. Emerging photoelectrochemical (PEC)‐type photodetectors are appealing owing to their intrinsic aqueous operation characteristics with versatile tunability of photoresponses. Herein, a self‐powered PEC photodetector employing n‐type gallium nitride (GaN) nanowires as a photoelectrode, which is decorated with an iridium oxide (IrOx) layer to optimize charge transfer dynamics at the GaN/electrolyte interface, is reported. Strikingly, the constructed n‐GaN/IrOx photoelectrode breaks the responsivity‐bandwidth trade‐off limit by simultaneously improving the response speed and responsivity, delivering an ultrafast response speed with response/recovery times of only 2 µs/4 µs while achieving a high responsivity of 110.1 mA W−1. Importantly, the device exhibits a large bandwidth with 3 dB cutoff frequency exceeding 100 kHz in UOC tests, which is one of the highest values among self‐powered photodetectors employed in optical communication system.

A Miniaturized wide Stopband Low-pass Filter using T and Modified L Shapes Resonators

A new structure of microstrip-based low-pass filter with wide stopband and sharp roll-off is introduced, in this paper. In the proposed topology, resonators with T and modified L Shapes have been used. To improve the suppression factor and relative stopband bandwidth, the second resonator has been added to the first resonator. The designed filter has been fabricated on a 20 mm thickness RO4003 substrate, which has a loss tangent of 0.0021 and a relative dielectric constant equal to 3.38. All parameters including roll of rate, stopband, bandwidth, return loss, insertion loss, and figure of merit have significant coefficients. Simulation has been ran using advanced design system software. The 3dB cutoff frequency is appropriate. The value of the insertion loss parameter is &lt;0.1 dB and the S11 parameter is −22 dB at this point. The stopband is extended from 2.42 up to 24 GHz, which shows an ultra-stopband. The results of the simulation and experiment are almost similar, which indicates a proper performance of the designed structure.

Self-Powered p-GaN/i-ZnGa2O4/n-ITO Heterojunction Broadband Ultraviolet Photodetector With High Working Temperature

A self-driven p-GaN/i-ZnGa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> /n-ITO heterojunction broadband ultraviolet (BUV) photodetector was firstly demonstrated in this work with a high working temperature. In the 25-300 °C temperature range, the device exhibits excellent and stable BUV photodetection performance. Even at 300 °C, a large peak responsivity of ~132 mA/W, a broad UV response band ranging from 250 to 400 nm, a high UV-to-visible rejection ratio of nearly 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> , and a high -3 dB cutoff frequency of 20 kHz can be still observed at 0 V, which is obviously superior to the other reported high-temperature BUV heterojunction photodetectors. The remarkable performance of our device at high temperature can be attributed to the excellent insulation and high crystalline quality of i-ZnGa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> layer, as well as the good electrical properties of p-GaN and n-ITO. Moreover, their wide and complementary band gaps make the device have a very broad UV detection band.

THz optical beat-note detection with a fast superconducting hot electron bolometer operating up to 31 GHz.

We study the performance of a hot-electron bolometer (HEB) operating at THz frequencies based on superconducting niobium nitride films. We report on the voltage response of the detector over a large electrical detection bandwidth carried out with different THz sources. We show that the impulse response of the fully packaged HEB at 7.5 K has a 3 dB cutoff around 2 GHz. Remarkably, detection capability is still observed above 30 GHz in an heterodyne beating experiment using a THz quantum cascade laser frequency comb. Additionally, the HEB sensitivity has been evaluated and an optical noise equivalent power NEP of 0.8 pW/√H z has been measured at 1 MHz.

Ultrafast and Ultrabroadband UV–vis-NIR Photosensitivity under Reverse and Self-Bias Conditions by n+-ZnO/n-Si Isotype Heterojunction with &gt;1 kHz Bandwidth

A high-performance broadband photodetector has attracted significant attention due to its wide range of applications. We report an n+-ZnO/n-Si isotype heterojunction by depositing ZnO nanorods followed by a ZnO thin film on an n-type undoped Si wafer for detecting a broad wavelength from 300 to 940 nm with a high speed under reverse as well as zero bias conditions. Under 1.5 V reverse and zero bias, the device provides responsivity values up to ∼200 and ∼13 mA/W respectively. Under self-bias, the n+-ZnO/n-Si heterojunction exhibits up to 1.2 × 103 photosensitivity. The n+-ZnO/n-Si heterojunction can detect power as low as 10–12 μW/cm2. A photoresponse up to 2 kHz modulated illuminations has been measured, and the ultrafast n+-ZnO/n-Si heterojunction provides a stable and rapid photoresponse with a response time in the 60–120 μs range. The n+-ZnO/n-Si heterojunction exhibits a −3 dB cutoff frequency of >2000 and 1100 Hz under reverse and zero bias, respectively. Therefore, we unprecedentedly demonstrate an n+-ZnO/n-Si isotype heterojunction as a potential candidate for detecting light in a broad ultraviolet to infrared region with a very high speed and >1 kHz frequency bandwidth as well.

Liquid-Crystal Spin-VCSEL with Electro-Optically Controllable Birefringence

We suggest a new construction of spin-VCSEL with an embedded nematic liquid crystal (LC) in a second cavity. We design such a coupled-cavity LC-VCSEL and develop a procedure for calculating its LC-voltage dependent polarization resolved resonant longitudinal modes and their quantum-well confinement factors. Using these characteristics, we are able to slightly modify the spin-flip VCSEL model to include the voltage dependent birefringence and anisotropy. Then, we show that such an LC-VCSEL can reach small signal modulation response with a 3dB cut off frequency of several hundreds of GHz.

A Precise Rotary Piezoelectric Actuator Based on the Spatial Screw Compliant Mechanism

Rotary piezoelectric actuators (RPAs) are widely used in various precise applications. But given the rotary axis offset caused by the single off-center driving force or the discrepancy of multiple piezoelectric elements, most previous works present rotary roundness loss. Besides, there is a contradiction between the motion range of RPA and its response speed. For solving these problems, an RPA based on the spatial screw compliant mechanism (SSCM) is hereby proposed; a theoretical model and an finite element (FE) model are built to verify the actuation principle and guide the structural design; and a prototype is fabricated and tested. The experimental results indicate that the static rotary angle changes from 0 to 4.935 mrad as the voltage changes from 0 to 100 V with a high resolution of 45.53 nrad. The first resonant and –3 dB cutoff frequencies are measured as 1361.5 and 2040.1 Hz, respectively. With the application of the SSCM, the RPA is proven capable of avoiding the rotary roundness loss in theory, and it can balance the milliradian-level rotary range with millisecond-level response speed driving by one lead zirconium titanate (PZT) stack under a small size of Φ32 × 68 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Effect of CNT Parameter Variations on CNTFET Amplifier Performance

In this paper we propose a procedure to study the dependence of a CNTFET amplifier performance on CNT parameters that fully identify the geometrical properties of a regular CNT. In particular these parameters are the length and structural indices (n,m) of CNT. We analyse the effects on a linear common source amplifier using a N type CNTFET with a resistive gate polarization. We polarize at a fixed drain voltage and current, gradually changing the CNT parameters. As regards the indices, we present the analysis for a zig-zag CNT. However the proposed procedure can be applied to other types of CNT. We show the effect on gain, on the 3 dB cut off frequency, F3dB, on the input and output resistances, obtaining that F3dB has a clear inverse dependence on the length and that also decreases at higher structural indices for zig-zag CNT.