Ultrafast Response Time Research Articles

Cu3 (HHTP)2 c-MOF/ZnO Ultrafast Ultraviolet Photodetector for Wearable Optoelectronics.

Two-dimensional conductive metal-organic frameworks (2D c-MOFs) are a family of highly tunable and electrically conducting materials that can be utilized in optoelectronics. A major issue of 2D c-MOFs for photodetection is their poor charge separation and recombination dynamics upon illumination. This study demonstrates a Cu3 (HHTP)2 /ZnO type-II heterojunction ultraviolet (UV) photodetector fabricated by layer-by-layer (LbL) deposition, in which the charge separation of photogenerated carriers is enhanced. At optimized MOF layer cycles, the device achieves a responsivity of 78.2 A/W and detectivity of 3.8×109 Jones at 1 V. Particularly, the device can be operated in the self-powered mode with an ultrafast response time of 70 μs, which is the record value for MOF-based photodetectors. In addition, even after 1000-time bending of 180°, the flexible device maintains stable performance. This flexible MOF-based UV photodetector with anti-fatigue and anti-bending properties provides strong implication to wearable optoelectronics.

Near-IR Light-Tunable Omnidirectional Broadband Terahertz Wave Antireflection Based on a PEDOT:PSS/Graphene Hybrid Coating.

Omnidirectional broadband terahertz (THz) antireflection (AR) with an actively configurable coating promises the achievement of next-generation efficient and versatile THz components with high performance. We demonstrate a near-infrared (NIR) light-tunable and omnidirectional broadband THz AR coating based on an impedance matching method and composed of a poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/graphene composite film. The omnidirectional broadband properties of the active AR coating can be efficiently achieved by tunable NIR optical excitation of less than 0.27 W·cm-2, which exhibits omnidirectional suppression of THz-wave reflection for incidence angles from 0 to 70°, concerning the broadband frequency range of 0.1-3.0 THz, with an ultrafast response time of ∼5 ps. Furthermore, we demonstrate that the active AR coating can improve the performance of a reflectance-tunable THz-wave polarization reflector by the elimination of Fabry-Pérot interference. The NIR irradiance-dependent active AR mechanism of the hybrid system is investigated, which demonstrates the essential role of the PEDOT:PSS/graphene layers in promoting the charge separation at the interface and therefore changing the photoconductivity of the composite film to achieve impedance matching under optical excitation. Several crucial advantages of the proposed and proven concept, including the wide-angle range, broad spectral range, flexible tunability, and easier fabrication, may revolutionize the AR strategy at THz frequencies for a wide range of THz applications.

Kirigami‐Inspired Reconfigurable Thermal Mimetic Device

AbstractMimicry, a biological phenomenon whereby organisms can change their morphological characteristics to resemble another object, has stimulated the unprecedented development of devices that allow the manipulation of thermal emission. Herein, inspired by biological mimicry and the pattern evolution of Chinese Kirigami, this study demonstrates a universal and effective method to prepare devices that exhibit a reconfigurable and multilevel evolution of infrared (IR) thermal images based on vanadium dioxide (VO2). The significant discrepancy between the phase transition temperatures of VO2 with different tungsten doping fractions affords the reconfigurable response in thermal images to varying environmental temperatures. Particularly, the large degree of freedom afforded by evolution steps and local IR emissivity further consummates the thermal mimicry ability for diverse demands. Furthermore, the ultrafast response time, notable observation angle independence, and the active control system are also demonstrated. This study offers a new perspective for reconfigurable thermal mimetic devices and provides a new path for distinct applications such as IR communication, information encryption, and virtual/augmented reality.

All-optical nano logical gates AND, NOR, OR, and NOT based on plasmonic waveguides with Kerr nonlinear cavity

In this paper, all-optical logical gates were designed and simulated based on Kerr nonlinear circular resonators in order to increase the computational power of an optical central processing unit (CPU) using the two-dimensional finite-difference time-domain (FDTD) method. All the logical gates were designed with a simple and single plasmonic composition in which the switch between the gates is done only by changing the intensity and location of the pumps, signal, and monitor. And this is the novelty of this study, which has been optimized by changing the offset and pulse length of the springs to achieve the maximum transmittance between the OFF and ON states. In addition to the above, a manufacturing tolerance simulation was performed for each gate, which takes into account the probability of error in the experimental production process. Overall, the results showed that the difference in transmission rate for AND, NOR, OR, and NOT was 42%, 95%, 55%, and 92% respectively with an ultrafast time response of up to about 50 fs. The benefits of the present study’s design include the number of gates, high transmission rate, manufacturing tolerance, and a smaller device footprint dimension, all of which make it a good base for the future of photonic computational circuits.

Coordination of Ultrafast Solid-State Circuit Breakers in Radial DC Microgrids

This article proposes a comprehensive method for optimal coordination of ultrafast solid-state circuit breakers (SSCBs) in radial dc microgrids. The proposed method consists of two consecutive steps. In the first step, SSCBs are optimally coordinated for overload, short circuit, and instantaneous protections. Inverse time-current characteristics (ITCC) curves are formulated to systematically coordinate SSCBs. The trip curve of each SSCB includes overload, short-circuit, and instantaneous profiles. In determining overload protection, the thermal capability of SSCBs is considered; while the short-circuit and instantaneous tripping profiles are realized based on short circuit thermal capability and ultrafast response time of SSCBs. Coordination time interval (CTI) is included between upstream and downstream SSCBs to achieve selectivity. To avoid miscoordination in practice, CTI is chosen by incorporating the SSCBs’ metal-oxide varistor (MOV) conduction time interval during turn-off process and dc current measurement error. In the second step, the operating speed of the protective scheme is optimized by applying distance-based secondary protection (DBSP). The DBSP is used to minimize the tripping time when short circuit faults occur close to upstream SSCBs. This significantly reduces the consumed energy by faults, prevents damage, reduces absorbed energy by SSCBs, and enhances the safety of operators. The proposed coordination removes the need for communication links between SSCBs and presents high reliability. A 400-V dc microgrid benchmark with photovoltaic (PV) and energy storage systems (ESSs) is examined as a case study in simulation. Besides, experiments are conducted in a 400-V dc system to evaluate the effectiveness and accuracy of the DBSP.

3D Ionic Olefin-Linked Conjugated Microporous Polymers for Selective Detection and Removal of TcO4-/ReO4- from Wastewater.

Technetium (99Tc) is a highly toxic radioactive nuclear wastewater contaminant. Real-time detection of 99Tc is very difficult due to its difficult-to-complex nature. Herein, a novel three-dimensional ionic olefin-linked conjugated microporous polymer (TFPM-EP-Br) is constructed using tetrakis(4-aldehyde phenyl)methane (TFPM) as the central monomer. The unique cationic cavity and highly hydrophobic framework enable TFPM-EP-Br to act as a fluorescent sensor for TcO4-. The fluorophores of TFPM-EP-Br can be quenched due to electron transfer from TFPM-EP-Br to TcO4- and the formation of strongly nonfluorescent complexes. Meanwhile, the regular pore channels are beneficial for the fast mass transfer of TcO4-, resulting in an ultrafast response time (less than 2 s) with an ultralow detection limit (33.3 nM). In addition, the ultrahigh specific surface area enables TFPM-EP-Br to combine the ability to synergistically detect and remove radioactive 99Tc. From this perspective, the novel conjugated microporous polymer has made a breakthrough in the detection and extraction of radioactive contaminants.

Compact breath monitoring based on helical intermediate-period fiber grating

Tiny and soft breath sensors offer unique capabilities in a continuous and long-term recording of vital physiological parameters for human health monitoring. Here, we propose an all fiber-optic breath sensor based on helical intermediate-period fiber grating (E-HIPFG) written in an elliptic core polarization-maintaining fiber. This E-HIPFG-based breath sensor shows ultrafast response time and recovery time, which are 34.46 ms and 41.34 ms, respectively. The response time and recovery time are three orders faster than the corresponding values of conventional optical fiber-based breath sensors with humidity-sensitive material. In addition, this sensor does not combine with any additional coating materials or fiber architectures, increasing the stability in a continuous and long-term detection for breath monitoring. The E-HIPFG sensor is insensitive to external influences, such as temperature, torsion and strain, endowing the possibility of stable detection in the moving body. Particularly, the temperature sensitivity of the sensor is 2.75 pm/°C, which is two orders lower than the corresponding value of previous reports. Due to its considerable stability, quick response, and insensitivity to external influences, the E-HIPFG-based breath sensor offers significant potential for long-term and compact breathing monitoring devices.

Ultra-sensitive and stable N-doped carbon dots for selective detection of uranium through electron transfer induced UO2+(V) sensing mechanism

Unintended leakage of toxic and radioactive uranyl ion (UO22+) ions poses high harmful to human health and environment, hence its monitoring and detection is of utmost significance. Here, we developed a mild synthetic route to prepare stable N-doped carbon nanodots (CDs) with special hydrophilicity functional groups and graphitic N by polymerization and carbonization of novel norfloxacin precursor with nitrogen heterocyclic structure. The morphology, chemical structure, fluorescent properties, sensing competition and selectivity behaviors of CDs-UO22+ were systematically analyzed. Controllable experiments including low and high concentration CDs-UO22+ and another type of carbon dots (TEA-CDs) with low graphitic N amount using triethanolamine (TEA) precursor without nitrogen heterocyclic structure were designed, which further confirmed the effective coordination and interaction mechanism between CDs and UO22+ that graphitic N in CDs induced the formation UO2+(V) by electron transfer. It is very suitable for on-site and real-time monitoring of UO22+ with an ultra-fast response time (∼20 s) and low detection limit of 20.38 nM (4.7 μg L−1), which is lower than the permissible limits (30 μg L−1) defined by the United States Environmental Protection Agency (EPA). More importantly, different form the liquid sensor, solid sensors (printed test strip and hydrogel) were also successfully employed to monitor UO22+ targets quickly, broadening the potential application for the advanced image encryptions.

Ultrafast intramolecular charge transfer dynamics and nonlinear optical properties of phenothiazine-based push–pull zinc porphyrin

Comprehensive investigations of the ultrafast photophysical properties and charge injection dynamics (adsorbed on mesoporous TiO2) of phenothiazine-functionalized push–pull Zinc porphyrin (LG6) utilizing a benzothiadiazole (BTD) as an acceptor unit is presented herewith. This includes density functional theory (DFT) calculations, fluorescence measurements, time-dependent fluorescence measurements, and femtosecond transient absorption (fs-TA) spectroscopy measurements. The charge injection dynamics of LG6 were compared with another porphyrin, LG5, with a similar molecular structure but thiophene as the acceptor unit instead of BTD. The multicomponent electron injection analysis from different excited electronic energy levels reveals ultrafast electron injection times of 434 fs and 336 fs for LG6 and LG5, respectively, revealing the impact of acceptor units on charge injection times. Further, femtosecond nonlinear optical (NLO) properties were characterized using the femtosecond Z-scan measurements with 70 fs, 800 nm pulses. The analysis of Z-scan open aperture data has been performed by solving three-level rate equations incorporating excited-state lifetimes calculated from the target/global analysis of fs-TA data set. The estimated NLO parameters and ultrafast NLO response times of LG6 indicated the strong potential for optical limiting and optical signal processing applications. The second hyperpolarizability was estimated to be γ=2.5×10-31esu, which was found to be in close agreement with the theoretically calculated value of 0.4×10-31e.s.u.

Printed GaAs Microstructures‐Based Flexible High‐Performance Broadband Photodetectors

AbstractNano/microstructures of compound semiconductors such as gallium arsenide (GaAs) demonstrate enormous potential for advanced photonic technologies as they provide realistic means for miniaturization of optoelectronic devices that feature better performance and low power consumption. However, intimately integrating them onto flexible substrates is challenging and restricts their use in the next generation of applications such as wearables and soft robotics. Herein, printed arrays of well‐defined and laterally aligned semi‐insulating (undoped) and doped GaAs microstructures are presented to develop high‐performance flexible broadband photodetectors. The direct roll transfer printed GaAs microstructures‐based photodetectors exhibit excellent performance under ultraviolet and near‐infrared illumination, including ultrafast response (2.5 ms) and recovery (8 ms) times, high responsivity (>104 AW–1), detectivity (>1014 Jones), external quantum efficiency (>106), and photoconductive gain (>104) at low operating voltage of 1 V. The achieved performance is among the best reported for broadband photodetectors but with an added benefit of the developed devices having a flexible form factor. Further, the photodetectors show stable performance under mechanical bending (500 cycles) and twisting loading. The developed materials and manufacturing route can enable high‐speed communications and computation via high‐performance flexible electronics and optoelectronics and transform numerous emerging applications such as wearable systems and internet of things.

Hierarchical zinc oxide nanobrushes ultraviolet photodetector

In order to synthesise hierarchical zinc oxide (ZnO) nanomaterials with a high surface-to-volume ratio, a well-controlled multistage hydrothermal method was developed. Hierarchical ZnO nanobrushes (ZNBs) built from initial mono-morphological nanomaterials, ZnO nanowires (ZNWs), and ZnO nanoplates (ZNPs) were developed using sequential nucleation and growth following a hydrothermal process. Hierarchical nanomaterials which are comprised of one-dimensional (1D) nanowire building blocks were obtained via a zinc ion source (zinc nitrate) during the second growth phase. In comparison to their first mono-morphological equivalents, the hierarchical nanomaterials which were grown showed improved ultraviolet (UV) detection, as well as improved sensitivity (∼104), an ultra-fast response time (235 ms), and a good recovery time (310 ms). Improvements to the response and recovery times were a result of distinct nanojunction barrier-dominated resistance. Moreover, the enhanced sensitivity of the ZNB photodetector (PD) was because of the reduced dimensionality and the ultrahigh surface-to-volume ratio. This work lays the groundwork for lower-cost, larger-scale, and lower-temperature production of high-performance nanostructured ZnO-based PDs on transparent and flexible substrates.

Revealing a Highly Sensitive Sub-ppb-Level NO2 Gas-Sensing Capability of Novel Architecture 2D/0D MoS2/SnS Heterostructures with DFT Interpretation.

In this work, we use a chemical method to design novel 2D-material/0D-quantum dot (MoS2/SnS) heterostructures. Furthermore, the unique 2D/0D heterostructure enhanced the NO2 gas-sensing capability 3 times and increased the sensing recoverability by more than 90%. Advanced characterization tools such as SEM, TEM, XRD, and AFM confirm the formation of MoS2/SnS heterojunction nanomaterials. Using AFM data, the average thickness of the MoS2 layer was found to be 5 nm. The highest sensor response of 0.33 with good repeatability was observed at 250 ppb of NO2. Sensing characterization reveals the ultra-fast response time, that is, 74 s, at 50 ppb of NO2. The limit of detection for detecting NO2 was also found to be very low, that is, 0.54 ppb, by using MoS2/SnS heterostructures. The theoretical calculations based on density functional theory well corroborated and quantified the intermolecular interaction and gas adsorption on the surface of MoS2/SnS.

High responsivity and flexible deep-UV phototransistor based on Ta-doped β-Ga2O3

Deep-ultraviolet (DUV) phototransistors have shown great potential applications in UV imaging, artificial intelligence, and wearable optoelectronics. Among a large number of wide bandgap semiconductors, the quasi-two-dimensional β-Ga2O3 is considered as an ideal candidate for DUV photodetector applications. Herein, we report a high responsivity (R) and fully flexible Ta-doped β-Ga2O3 DUV phototransistor which exhibits outstanding optoelectrical properties with a high R of 1.32 × 106 A/W, a large detectivity of 5.68 × 1014 Jones, a great photo-to-dark current ratio of 1.10 × 1010%, a high external quantum efficiency of 6.60 × 108%, and an ultra-fast response time of ~3.50 ms. Besides, the flexible Ta-doped β-Ga2O3 device also displays high reliability and mechanical flexibility that can sustain well after over 1 × 104 bending cycles. Moreover, high-contrast imaging of UV light was obtained on the flexible DUV detector arrays, which can be efficiently trained and recognized by an artificial neural network. Our findings offer a perspective to develop wearable optoelectronics and UV imaging based on high-performance flexible β-Ga2O3 DUV phototransistors, providing an inspiration for the future work in artificial intelligence and bionic robot fields.

Ultra‐fast dynamic nonlinear mechanism of Fabry–Perot cavity with femtosecond pumping

AbstractWe numerically investigate the ultrafast dynamic nonlinear mechanism of an Fabry‐Perot (FP) cavity with femtosecond (fs) pumping by the transmission change of an incident signal pulse. The nonlinear effect simulated by a two‐level system model can decrease the dielectric constant. Therefore, the temporally transmitted pulse is compressed owing to the faster phase velocity, and a sloped change in the transmission spectra is observed, which can be explained by the mechanism of pulse deformation. In addition, we stretched a certain part of the temporally transmitted pulse to further demonstrate the deformation of the pulse mechanism. This dynamic nonlinear mechanism based on the FP cavity can not only be used to design new optical devices with ultrafast response times, such as pulse modulators and passive amplifiers, but can also be easily constructed and integrated, which has great practical applications.

Smartphone-assisted paper-based multifunctional assay for on-site tandem detection of inorganic phosphate, neutral red and heparin

Today, easy availability and efficient detection assays in low-resource settings are mandatory. Therefore, a smartphone-assisted paper-based detection strategy using eosin Y (EY)-embed ZIF-8 nanocomposite for phosphate (Pi), neutral red (NR) and heparin (HP) on site testing was fabricated. After adding the targets in a specific order, the prepared strips produced brilliant color transitions (astonishing colors, ranging from pistachio to bright yellow for Pi, from bright yellow to jacinth for NR and from jacinth to light orange for HP). These variation on tonality can be evaluated by the smartphone-based readout device (Color Assist App) through the ratio change of B/(R + G + B), which exhibits superior performance of on-site detection in wastewater, including high sensitivity (LOD is 0.015, 0.031 and 0.014 mM for Pi, NR and HP, respectively), ultra-fast response time and excellent selectivity. This strips and smartphone dual-mode monitoring strategy may offer a convenient idea to identify Pi, NR and HP, with great prospects for environmental wastewater applications. Additionally, the probe was enabled for bioimaging of Pi, NR and HP in living cells. Finally, the related anti-counterfeiting fluorescent ink of probe was proposed on schedule.

Microcavity Exciton‐Polariton Quantum Spin Fluids

AbstractMicrocavity exciton‐polaritons are attractive quantum quasi‐particles resulting from strong light–matter coupling in a quantum‐well‐cavity structure. They have become one of the most stimulating solid‐state material platforms to explore beautiful collective quantum phenomena originating from macroscopic coherence in condensation and superfluidity, Berezinskii–Kosterlitz–Thouless transition, and various topological excitations in the form of solitons, vortices, and skyrmions. They can also provide opportunities for the development of pioneering photonic devices by exploiting bistability and parametric scatterings due to strong nonlinearity that possess remarkable performance advantages of power‐efficient operation, ultrafast response time, and scalable planar geometries. This story becomes profound and fascinating when the spins of excitons are taken into account, that can be directly accessed through light polarization states. The purpose of this review is to give central principles of microcavity exciton‐polariton spins and their anisotropic interactions, which can couple with the effective magnetic fields from mode‐splitting of microcavity photons and spin‐dependent relaxation processes of quantum‐well excitons. Furthermore, notable theoretical and experimental research activities are summarized to reveal extraordinary quantum phenomena of spin‐resolved topological states and exotic spin textures and to devise novel spin‐based photonic devices based on microcavity exciton‐polaritons.

Dirac Point Modulated Self-Powered Ultrasensitive Photoresponse and Color-Tunable Electroluminescence from Flexible Graphene/Metal–Organic Frameworks/Graphene Vertical Phototransistor

Integrated wearable optoelectronic devices synchronously exhibit contradictory phenomena of exciton generation and recombination with their control by an external electric field, and the mechanical strain associated with this has widespread applications. However, a demonstration of such single-miniaturized multifunctional wearable optoelectronics remains a daunting challenge. A versatile metal–organic framework (MOF) possesses tailorable outstanding optical properties through an inherent multiple charge-transfer mechanism between metal and ligand. Yet, its commercialization remains unsuccessful because of large porosity, poor conductivity, and deficient crystallinity. Graphene is honorable for its outstanding optoelectronic and mechanical behaviors. Combining excellent features of a versatile MOF with monolayer graphene, unprecedently, we report a self-powered ultrasensitive (external quantum efficiency ∼3 × 1010%) and ultrafast (response time ≈ 220 μs) wearable vertical phototransistor by utilizing a graphene/MOF/graphene/poly(vinylidene fluoride-co-trifluoroethylene) heterojunction on a flexible polydimethylsiloxane substrate through the formation of asymmetric Schottky junctions at MOF/graphene interfaces by a judicious incorporation of a piezo-sensitive poly(vinylidene fluoride) layer. Importantly, by the Dirac point modulation of graphene through a mechanical strain, we achieve optical strain sensors that can modify the amplitude and polarity of the photocurrent. Additionally, controlling the Dirac point of graphene and charge-transfer mechanisms of the MOF by a suitable electrical bias and strain, we demonstrate color-tunable broadband light emission from a flexible vertical phototransistor. Our unique discovery will enrich wearable integrated optoelectronics.

A Nanoscale Hot-Wire Flow Sensor Based on CMOS-MEMS Technology

In this paper, we proposed an ultrafast and nanoscale hot-wire flow (NHF) sensor implemented in a 0.18 µm complementary metal-oxide-semiconductor microelectromechanical system (CMOS-MEMS) technology. The nanoscale wire was released and reduced in thickness and width by an in-house developed post-CMOS fabrication process, hence the heat conduction loss is greatly suppressed, while the response time of the NHF sensor is significantly improved. Benefiting from the nano size of the hot-wire (a width of 622 nm), the NHF sensor exhibits an ultrafast response time of 30 µs (@ airflow velocity of 0 m/s), a wide flow range of 0–30 m/s, and a cut-off frequency of 21 kHz under the constant temperature (CT) mode. In addition, an equivalent circuit model (ECM) was established in PSPICE to predict the NHF sensor performance, and the theoretical simulation results were in good agreement with the experiment results.

3D single-pixel imaging at the near-infrared wave band.

Focal plane detector array technology in the infrared wave band is expensive or underdeveloped, and the detection efficiency is low, while single-pixel imaging (SPI) offers better performance, such as ultrafast time response and high quantum efficiency in wide wave bands. Therefore, SPI technology can be used for infrared imaging. In this work, a near-infrared raster scan SPI system is proposed. By means of a grating to modulate height information of objects, we can further achieve three-dimensional imaging in the framework of Fourier transform profilometry. The proposed approach is demonstrated with experiments at the wavelength of 1064 nm.

A Highly Conducting Polymer for Self-Healable, Printable, and Stretchable Organic Electrochemical Transistor Arrays and Near Hysteresis-Free Soft Tactile Sensors.

A stretchable and self-healable conductive material with high conductivity is critical to high-performance wearable electronics and integrated devices for applications where large mechanical deformation is involved. While there has been great progress in developing stretchable and self-healable conducting materials, it remains challenging to concurrently maintain and recover such functionalities before and after healing. Here, a highly stretchable and autonomic self-healable conducting film consisting of a conducting polymer (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), PEDOT:PSS) and a soft-polymer (poly(2-acrylamido-2-methyl-1-propanesulfonic acid), PAAMPSA) is reported. The optimal film exhibits outstanding stretchability as high as 630% and high electrical conductivity of 320 S cm-1 , while possessing the ability to repair both mechanical and electrical breakdowns when undergoing severe damage at ambient conditions. This polymer composite film is further utilized in a tactile sensor, which exhibits good pressure sensitivity of 164.5 kPa-1 , near hysteresis-free, an ultrafast response time of 19ms, and excellent endurance over 1500 consecutive presses. Additionally, an integrated 5 × 4 stretchable and self-healable organic electrochemical transistor (OECT) array with great device performance is successfully demonstrated. The developed stretchable and autonomic self-healable conducting film significantly increases the practicality and shelf life of wearable electronics, which in turn, reduces maintenance costs and build-up of electronic waste.