Ultrafast Speed Research Articles

Ultrasensitive and high-speed AlGaN/AlN solar-blind ultraviolet photodetector: a full-channel-self-depleted phototransistor by a virtual photogate

High sensitivity, high solar rejection ratio, and fast response are essential characteristics for most practical applications of solar-blind ultraviolet (UV) detectors. These features, however, usually require a complex device structure, complicated process, and high operating voltage. Herein, a simply structured n-AlGaN/AlN phototransistor with a self-depleted full channel is reported. The self-depletion of the highly conductive n-AlGaN channel is achieved by exploiting the strong polarization-induced electric field therein to act as a virtual photogate. The resulting two-terminal detectors with interdigital Ohmic electrodes exhibit an ultrahigh gain of 1.3 × 10 5 , an ultrafast response speed with rise/decay times of 537.5 ps/3.1 μs, and an ultrahigh Johnson and shot noise (flicker noise) limited specific detectivity of 1.5 × 10 18 ( 4.7 × 10 16 ) Jones at 20-V bias. Also, a very low dark current of the order of ∼ pA and a photo-to-dark current ratio of above 10 8 are obtained, due to the complete depletion of the n - Al 0.5 Ga 0.5 N channel layer and the high optical gain. The proposed planar phototransistor combines fabrication simplicity and performance advantages, and thus is promising in a variety of UV detection applications.

Femtosecond Thermal and Nonthermal Hot Electron Tunneling Inside a Photoexcited Tunnel Junction.

Efficient operation of electronic nanodevices at ultrafast speeds requires understanding and control of the currents generated by femtosecond bursts of light. Ultrafast laser-induced currents in metallic nanojunctions can originate from photoassisted hot electron tunneling or lightwave-induced tunneling. Both processes can drive localized photocurrents inside a scanning tunneling microscope (STM) on femto- to attosecond time scales, enabling ultrafast STM with atomic spatial resolution. Femtosecond laser excitation of a metallic nanojunction, however, also leads to the formation of a transient thermalized electron distribution, but the tunneling of thermalized hot electrons on time scales faster than electron–lattice equilibration is not well understood. Here, we investigate ultrafast electronic heating and transient thermionic tunneling inside a metallic photoexcited tunnel junction and its role in the generation of ultrafast photocurrents in STM. Phase-resolved sampling of broadband terahertz (THz) pulses via the THz-field-induced modulation of ultrafast photocurrents allows us to probe the electronic temperature evolution inside the STM tip and to observe the competition between instantaneous and delayed tunneling due to nonthermal and thermal hot electron distributions in real time. Our results reveal the pronounced nonthermal character of photoinduced hot electron tunneling and provide a detailed microscopic understanding of hot electron dynamics inside a laser-excited tunnel junction.

MXene‐Germanium Schottky Heterostructures for Ultrafast Broadband Self‐Driven Photodetectors

AbstractA novel 2D layered material Ti3C2Tx (MXene) is favored by researchers in the application field of optoelectronics due to its tunable work function, great light transmittance, and excellent electrical conductivity. In this work, Ti3C2Tx/n‐germanium (MXene/n‐Ge) Schottky heterostructures are fabricated and investigated. Schottky contacts of MXene with n‐Ge are identified by ultraviolet photoelectron spectroscopy (UPS). Based on the MXene/n‐Ge Schottky junctions, an ultrafast, broadband, and self‐powered photodetector is demonstrated and studied. The MXene/n‐Ge device exhibits excellent photoresponse from ultraviolet to near‐infrared light illumination. In particular, it shows an excellent on/off ratio (≈104), a high responsivity (3.14 A/W), a larger specific detectivity (2.14 × 1011 Jones), and an ultrafast response speed (trise of 1.4 µs and tdecay of 4.1 µs). Moreover, the MXene/n‐Ge Schottky heterostructure photodetector also shows excellent low‐temperature work characteristics of 73 K. It is believed that this work will attract more researchers’ attention to MXene in the field of optoelectronic devices.

All-polymeric multilayer para/ferroelectric dielectric films utilizing a gradient structure toward concurrent high discharge efficiency and energy density

Polymeric dielectrics exhibit huge prospects for power electronic devices due to their ultrafast charge-discharge speed. However, a persisting challenge is that enhanced energy density always brings about deteriorated efficiency. Herein, a novel design of all-organic dielectric films, consisting of gradient transition layers sandwiched between a paraelectric layer and a ferroelectric layer, are prepared. It is interesting to find that the breakdown behaviors of the multilayer films are strongly dependent on the configuration of gradient transition layers. Consequently, via adjusting the configuration of gradient transition layers, a high breakdown strength of 623 kV/mm, an excellent efficiency of 89.5%, along with a high-energy density of 7.34 J/cm3 are achieved. This work offers a unique route to develop polymeric dielectrics with concurrent high breakdown strength, high-energy density, and high efficiency.

Ultrasensitive and broadband polarization-sensitive topological insulator photodetector induced by element substitution

Topological insulators are considered as one of the preferred materials for high-performance optoelectronic devices due to their small bulk bandgaps and ultra-high carrier mobility. However, the existence of their unique Dirac like surface states makes the corresponding optoelectronic devices to have high dark current, and the logic circuit cannot be turned off effectively. Opening the surface state gap by element doping is an effective means to achieve high performance of devices. Here, we design and prepare single crystal Bi2Se2.15S0.85 nanowires by a facile iodine-assisted chemical vapor transport method and a fabricated individual Bi2Se2.15S0.85 nanowire based photodetector. The devices exhibit remarkable photoresponse over the broadband wavelength ranging from ultraviolet C (275 nm) to near-infrared (1310 nm) with the low dark current of 10−12 A. They show superior optoelectrical properties with an ultrafast response speed of 170 ns, detectivity of 9.35 × 1011 Jones, a competitive responsivity of 1.31 A/W, and superb stability to keep great photoresponse for at least one year, which are superior to the reported photodetectors. Additionally, benefiting from the anisotropic crystal structure of Bi2Se2.15S0.85, the devices also display good polarization detection performance in a wide spectral range from 266 nm to 1064 nm with a dichroic ratio of 1.81 at 360 nm.

Achieving ultrafast discharge speed and excellent energy storage efficiency in environmentally friendly niobate-based glass ceramics

Environmentally friendly Na2O–BaO–Nb2O5–SiO2 glass ceramics (GCs) with different vanadium pentoxide (V2O5) contents were successfully synthesized using the conventional melt-quenching method and heat treatment. The microstructure and dielectric energy storage properties of the GCs could be related to the addition of V2O5. When 1 mol% of V2O5 was added, the activation energy of crystallization decreased from 282 to 221 kJ/mol, and the GCs had the highest energy efficiency of 95.96% and breakdown strength of 1326 kV/cm. Moreover, the GCs could also achieve an ultrafast discharge time of 14 ns and an actual discharge energy density of 0.724 J/cm3. These results indicate that these environmentally friendly GCs can be used in energy storage devices.

Performance of metal-semiconductor-metal structured diamond deep-ultraviolet photodetector with a large active area

In this work, the uniformity is significantly improved of the photoresist film spinning-coated on the small-size diamond wafer by inlaying the diamond wafer into a 1-inch polytetrafluoroethylene substrate; consequently, the utilizable surface area of the diamond wafer is remarkably increased. As a result, the interdigital electrodes of 2.5 mm × 2.5 mm in dimension are prepared on the single crystal diamond (5 mm × 5 mm × 0.5 mm) and a metal–semiconductor–metal structured diamond deep-ultraviolet photodetector with a large active area of 3.093 mm2 has been fabricated. Compared to the maximum values of the interdigital-typed intrinsic diamond deep-ultraviolet photodetectors, the active area is increased by more than six times, and the photocurrent reaches the order of milliampere, which is about two orders of magnitude larger. Meanwhile, the responsivity and external quantum efficiency are 56.3 A W−1 and 328, respectively, at 50 V bias under 3.125 μW mm−2 213 nm illumination, and the corresponding mobility-lifetime product of the diamond wafer is 1.11 × 10−5 cm2 V−1. As the voltage continued to increase, which still maintained an upward trend and did not appear saturated; the corresponding responsivity is up to 275.9 A W−1 at 120 V. In addition, the ultraviolet-visible light discrimination ratio is 1.4 × 104 at 10 V, and the carrier transit time between interdigital electrodes is measured to be only about 1 ns (excited by a 213 nm pulse laser), which shows that the photodetector has an ultrafast response speed.

Ternary compound MgTiO3 combined with graphene for solar-blind deep ultraviolet photodetection

Ternary materials have gained great popularity in the field of solar-blind ultraviolet (UV) photodetectors due to their diverse band gaps. Whereas, quality problems brought by lattice damage and phase separation appear frequently during preparation, which greatly limit the application of ternary solar-blind photoresponsive materials. MgTiO3 (MTO), a ceramic material, has extensive application in microwave capacitors and dielectric resonators but takes no place in the field of photodetectors. Faced with this situation, this study investigates the effect of annealing on the structure of MTO thin film, and fabricates a solar-blind UV detector through combining MTO with graphene, which expands the application of ternary compound MTO to the field of photodetectors. Under solar-blind UV illumination, this MTO-based detector exhibits excellent performance with an open circuit voltage as high as 1.7 V. At 0 V bias, the detector exhibits an ultrafast response speed with the rise and decay time as short as 30 ms and 25 ms, respectively. Besides, a responsivity up to 46.5 mA/W, an external quantum efficiency (EQE) up to 20.4%, and a specific detectivity (D*) up to 6.14 × 1011 Jones also prove the great performance this detector owns. All the results achieved in this work not only suggest the great potential of MTO in being applied to fabricating solar-blind UV detectors, but also provide some guidance for the development of photovoltaic detectors based on other ternary semiconductor materials.

Thermally stable gold nanorod dispersed silicone composite with plasmonic resonance in the optical communication window

Gold nanorods (AuNRs) possess a high optical nonlinear coefficient, ultrafast optical response speed and widely tunable localized surface plasmon resonance (LSPR) wavelength covering the visible and near infrared region. Therefore, they are extensively investigated for many optical applications. However, the poor thermal stability of the AuNRs seriously restricts their practical performance. In addition, for many applications, such as optical communication or laser modulation, AuNRs have to be combined with transparent solids, for example polymers, glass or crystals to make devices. Here, we report on the preparation of 0.23 mg AuNR dispersed methyl silicone resin (MSR) with longitudinal LSPR (L-LSPR) wavelength (1450 nm) in the optical communication window. We found that AuNR-silicone composites possess high thermal stability. After calcination in ambient environment at a temperature of 250 °C for 10 h, the L-LSPR peak of the sample can remain longer than 1380 nm, implying that the NR shape of the Au particles was well maintained. Using the open-aperture Z-scan technique, the nonlinear absorption coefficient of the composites was measured as −11.71 cm GW−1, higher than many nonlinear materials. Thus, the thermally stable AuNR@SiO2-MSR composite with high nonlinearity is promising for practical applications in the optical communication window.

Radiation Hardening Design of Nonvolatile Hybrid Flip-Flop Based on Spin Orbit Torque MTJ and SRAM

Spin orbit torque magnetic tunnel junction (SOT-MTJ) has been widely used for designing nonvolatile flip-flop (NVFF), thanks to its ultrafast switching speed, zero static power consumption, nearly unlimited endurance, etc. However, in the space environment, the write pulse width of the SOT-MTJ is comparable to radiation-induced current duration. Thus, the SOT-MTJ-based NVFF may suffer from soft error induced by the nonvolatile single event upset (NVSEU) in the backup mode. In this paper, we analyze the sensitive nodes in different work modes and propose the radiation hardening circuits for master–slave NVFF (MS-NVFF) to solve the NVSEUs. Based on a physics-based SOT-MTJ compact model and a 40[Formula: see text]nm CMOS design kit, the reliability of MS-NVFF is verified by simulation with 2[Formula: see text]pC charge, which could be reliably integrated into aerospace and avionics electronics in hostile environments.

Deep learning-enabled compact optical trigonometric operator with metasurface

In this paper, a novel strategy based on a metasurface composed of simple and compact unit cells to achieve ultra-high-speed trigonometric operations under specific input values is theoretically and experimentally demonstrated. An electromagnetic wave (EM)-based optical diffractive neural network with only one hidden layer is physically built to perform four trigonometric operations (sine, cosine, tangent, and cotangent functions). Under the unique composite input mode strategy, the designed optical trigonometric operator responds to incident light source modes that represent different trigonometric operations and input values (within one period), and generates correct and clear calculated results in the output layer. Such a wave-based operation is implemented with specific input values, and the proposed concept work may offer breakthrough inspiration to achieve integrable optical computing devices and photonic signal processors with ultra-fast running speeds.

Fast Operations of Nonvolatile Ferroelectric Domain Wall Memory with Inhibited Space Charge Injection

The microampere-level domain wall currents in LiNbO3 single crystals have promising applications in nonvolatile ferroelectric domain wall random access memory and logic with high-density integration, ultrafast operation speeds, and almost unlimited switching cycles. For the memory commercialization, the improvements of the reliability and operation speed of the devices are challenging due to the high-field charge injection. The injected charge could compensate the domain-wall boundary charge that screens the domain switching field and reduces the domain wall current. In this work, two kinds of memory nanocells were fabricated on the surfaces of X-cut LiNbO3 single crystals to study the geometry-dependent charge injection. The striped memory cell due to the appearance of the size-driven reconstruction has a smaller coercive field than that of a clamped memory cell without relaxation of the lattice matching stress, which reduces low-frequency charge injection and increases the domain switching speed. At an operating voltage of 5 V, we observed a retention time of more than 1 week and an on/off current ratio of 2 × 104 for a striped-like cell, paving the route to integrate energy-efficient high-density domain wall memory in high reliability.

Coherent modulation of chiral nonlinear optics with crystal symmetry

Light modulation is of paramount importance for photonics and optoelectronics. Here we report all-optical coherent modulation of third-harmonic generation (THG) with chiral light via the symmetry enabled polarization selectivity. The concept is experimentally validated in monolayer materials (MoS2) with modulation depth approaching ~100%, ultra-fast modulation speed (<~130 fs), and wavelength-independence features. Moreover, the power and polarization of the incident optical beams can be used to tune the output chirality and modulation performance. Major performance of our demonstration reaches the fundamental limits of optical modulation: near-unity modulation depth, instantaneous speed (ultra-fast coherent interaction), compact footprint (atomic thickness), and unlimited operation bandwidth, which hold an ideal optical modulation solution for emerging and future nonlinear optical applications (e.g., interconnection, imaging, computing, and quantum technologies).

A Surgeless Diode-Clamped Multilevel Solid-State Circuit Breaker for Medium-Voltage DC Distribution Systems

In conventional medium-voltage solid-state circuit breakers (SSCBs), series-connected semiconductor switches are used with metal oxide varistors (MOVs) in parallel with each of them to extend the voltage ratings of the breakers without incurring the dynamic voltage unbalance. Besides, for isolating the faults at faster speeds, the clamped surge voltages of the MOVs are always much higher than the system voltages. For those reasons, the voltage ratings of all the switches in the SSCBs have to be overdesigned according to the surge voltage values, which causes high material costs along with nonideal conduction losses in the SSCBs employed for medium-voltage dc distribution systems. To solve this dilemma, a surgeless diode-clamped multilevel SSCB is proposed in this article. It offers an ultrafast isolation speed with no significant surge voltages or dynamic unbalanced voltage across the semiconductor switches. With these attributes, the proposed SSCB can potentially use the semiconductor switches rated at lower voltages compared to conventional SSCBs and have higher efficiency along with faster isolation speed. The simulation and experimental results are presented to validate the technical feasibility and practical values of the proposed SSCB.

5G: Next Generation Mobile Wireless Technology for A Fast Pacing World

The quest for faster and seamless mobile communication services has given rise to the Fifth Generation (5G) mobile communication technology. 5G presents enormous opportunities for end-users, businesses, and the global economy at large. The major edge 5G has over its predecessor technologies; the 3rd and 4th Generation (3G and 4G), is ultra-low latency, which translates to ultra-fast data speed, which has the potential to peak to about 1Gbps at the frequency range of 6GHz to 100GHz. Its application of Multiple Input, Multiple Output (MIMO) antenna technology enables it to beam dedicated radio signals to 5G enabled User Equipment (UEs) using the advanced beam steering algorithm. This results in maximum data throughput with ultra-high bandwidth transmission to end-user applications resulting in new and never-seen-before business opportunities across different industries, investments, and business services. By expanding the scope of wireless technologies and making devices more autonomous, 5G has the potential to reduce carbon emission footprint and encourage greener communities. The application of 5G cuts across all spheres of our lives; industry, transportation, education, communications, health, and businesses. 5G is more inclusive, progressive, proven, and powerful than any of its predecessor generations of communications technology. With more power and improved efficiency, 5G is on its way to creating a truly connected world.

Ultralow Power Wearable Organic Ferroelectric Device for Optoelectronic Neuromorphic Computing

In order to imitate brain-inspired biological information processing systems, various neuromorphic computing devices have been proposed, most of which were prepared on rigid substrates and have energy consumption levels several orders of magnitude higher than those of biological synapses (∼10 fJ per spike). Herein, a new type of wearable organic ferroelectric artificial synapse is proposed, which has two modulation modes (optical and electrical modulation). Because of the high photosensitivity of organic semiconductors and the ultrafast polarization switching of ferroelectric materials, the synaptic device has an ultrafast operation speed of 30 ns and an ultralow power consumption of 0.0675 aJ per synaptic event. Under combined photoelectric modulation, the artificial synapse realizes associative learning. The proposed artificial synapse with ultralow power consumption demonstrates good synaptic plasticity under different bending strains. This provides new avenues for the construction of ultralow power artificial intelligence system and the development of future wearable devices.

Influences of crystallization temperature on the structure, dielectric, and energy storage characteristics of KBaSrNb5O15‐based glass–ceramics

AbstractGlass–ceramics capacitors have great application potential in pulsed power systems, due to ultrafast discharge speed and high dielectric breakdown strength (BDS). Here, lead‐free niobate glass–ceramic dielectric materials were synthesized, and the effects of heat treatment temperature on the dielectric, ferroelectric, and energy storage properties of glass–ceramics were investigated comprehensively. The results exhibit that the dielectric permittivity first increases and then decreases as the crystallinity increases; however, the dielectric BDS diminishes. At the optimum crystallization temperature of 740°C, the maximum value of discharge energy density is 2.2 J/cm3 at 600 kV/cm, which is about 7.6 times that of mother glass. Furthermore, an ultrahigh power density of about 380.9 MW/cm3 and ultrafast discharge speed of about 11.2 ns were achieved simultaneously. Meanwhile, great thermal stability of charge–discharge property was verified in this glass–ceramics. According to P–E loops and dielectric test result, a high dielectric constant (∼207) and low dielectric loss (&lt;0.005) as well as high energy storage efficiency of about 94.9% were achieved for G740 sample. The previous results make the obtained glass–ceramic as potential candidates in dielectric capacitors.

Colloidal MoS2 quantum dots for high-performance low power resistive memory devices with excellent temperature stability

Conventional memory technologies are facing enormous problems with downscaling, and are hence unable to fulfill the requirement of big data storage generated by a huge explosion of digital information. A resistive random access memory device (RRAM) is one of the most emerging technologies for next-generation computing data storage owing to its high-density stacking, ultrafast switching speed, high non-volatility, multilevel data storage, low power consumption, and simple device structure. In this work, colloidal MoS2 quantum dots (QDs) embedded in an insulating matrix of poly-(4vinylpyridine) (PVP) were used as an active layer to fabricate a RRAM device. The MoS2 QDs-PVP based RRAM device reveals an excellent nonvolatile resistive switching (RS) behavior with a maximum current on-off ratio (ION/IOFF) of 105. High endurance, long retention time, and successive “write-read-erase-read” cycles indicate high-performance RRAM characteristics. The ultimate power consumption by this RRAM device is considerably low for energy saving. In addition, the MoS2 QDs-PVP based device shows RS behavior even at 130 °C. High ION/IOFF, low operating power, high endurance, long retention time, and excellent stability with temperatures reveal that the MoS2 QDs-PVP based device can be a promising candidate for high-performance low power RRAM devices that can be operated at relatively higher temperatures.

High-performance self-driven ultraviolet photodetector based on SnO2 p-n homojunction

Taking advantage of the high-quality p-type epitaxial SnO2:Mg film achieved in our previous study, a p-SnO2:Mg/n-SnO2 homojunction based ultraviolet photodetector was fabricated on a c-sapphire substrate by pulsed laser deposition in this work. The p-SnO2:Mg/n-SnO2 homojunction exhibits a typical diode behavior, with a rectifying ratio approaching 201 at 2 V and −2 V. Compared with the traditional metal-semiconductor-metal SnO2-based ultraviolet photodetector, the present p-n homojunction based photodetector is self-powered under 0 V bias. The built-in electric field in the interface depletion region of the p-SnO2:Mg/n-SnO2 homojunction separates photogenerated electron-hole pairs rapidly, which renders the photodetector an ultra-fast response speed (τr = 0.57 ms, τd = 0.64 ms). In addition, the detector exhibits a maximum photocurrent Ip of 1.5 nA under illumination with 290 nm light at 1.177 mW/cm2. At the same time, it shows a higher responsivity of 1.55 mA/W, and a higher detectivity of 9.8 × 1010 Jones, than traditional detectors. These results imply great application potential of SnO2 homojunction for UV photodetectors as well as other power devices.

Light in AI: Toward Efficient Neurocomputing With Optical Neural Networks—A Tutorial

In the post Moore&#x2019;s era, conventional electronic digital computing platforms have encountered escalating challenges to support massively parallel and energy-hungry artificial intelligence (AI) workloads. Intelligent applications in data centers, edge devices, and autonomous vehicles have restricted requirements in throughput, power, and latency, which raises a high demand for a revolutionary neurocomputing solution. Optical neural network (ONN) is a promising hardware platform that could represent a paradigm shift in efficient neurocomputing with its ultra-fast speed, high parallelism, and low energy consumption. In recent years, efforts have been made to facilitate the ONN design stack and push forward the practical application of optical neural accelerators. In this tutorial, we give an overview of state-of-the-art cross-layer co-design methodologies for scalable, robust, and self-learnable ONN designs across the circuit, architecture, and algorithm levels. Besides, we analyze challenges and highlight emerging directions targeting next-generation optics for AI.