Ultrafast Photo-Thermal Switching of Terahertz Spin Currents

Dissipationless and scattering-free spin-based terahertz electronics is the futuristic technology for energy-efficient information processing. Femtosecond light pulse provides an ideal pathway for exciting the ferromagnet (FM) out-of-equilibrium, causing ultrafast demagnetization and superdiffusive spin transport at sub-picosecond timescale, giving rise to transient terahertz radiation. Concomitantly, light pulses also deposit thermal energy at short timescales, suggesting the possibility of abrupt change in magnetic anisotropy of the FM that could cause ultrafast photo-thermal switching (PTS) of terahertz spin currents. Here, a single light pulse induced PTS of the terahertz spin current manifested through the phase reversal of the emitted terahertz photons is demonstrated. The switching of the transient spin current is due to the reversal of the magnetization state across the energy barrier of the FM layer. This demonstration opens a new paradigm for on-chip spintronic devices enabling ultralow-power hybrid electronics and photonics fueled by the interplay of charge, spin, thermal, and optical signals.     

Nonvolatile Reconfigurable Phase‐Change Metadevices for Beam Steering in the Near Infrared

The development of flat, compact beam‐steering devices with no bulky moving parts is opening up a new route to a variety of exciting applications, such as LIDAR scanning systems for autonomous vehicles, robotics and sensing, free‐space, and even surface wave optical signal coupling. In this paper, the design, fabrication and characterization of innovative, nonvolatile, and reconfigurable beam‐steering metadevices enabled by a combination of optical metasurfaces and chalcogenide phase‐change materials is reported. The metadevices reflect an incident optical beam in a mirror‐like fashion when the phase‐change layer is in the crystalline state, but reflect anomalously at predesigned angles when the phase‐change layer is switched into its amorphous state. Experimental angle‐resolved spectrometry measurements verify that fabricated devices perform as designed, with high efficiencies, up to 40%, when operating at 1550 nm. Laser‐induced crystallization and reamorphization experiments confirm reversible switching of the device. It is believed that reconfigurable phase‐change‐based beam‐steering and beam‐shaping metadevices, such as those reported here, can offer real applications advantages, such as high efficiency, compactness, fast switching times and, due to the nonvolatile nature of chalcogenide phase‐change materials, low power consumption.     

Ultrafast Relaxation of Charge Carriers Induced Switching in Terahertz Metamaterials

We demonstrate ultrafast switching of resonant mode in terahertz metamaterials through optical excitation of radiation-damaged silicon placed in the gap of a split-ring resonator. Upon optical excitation, we observe the dynamic transition of the fundamental resonance from ON-to-OFF state on a timescale of 4 picoseconds (ps) and then fast recovery of the resonance to the ON-state within the next 20 ps. Electric field distributions in the metamaterial unit cell derived through numerical simulations clearly support our experimental observations, showing that the high electric field at the resonator gaps, responsible for inductive-capacitive (LC) resonance, completely disappears and switches OFF the resonance after being optically excited. The ultrafast switching of the metamaterial resonance is attributed to the relaxation of the photo-carriers through the defect states of radiation-damaged silicon layer. Such ultrafast material–based active control of metamaterials can lead to the ultrafast terahertz metaphotonic devices.     

High Mobility 3D Dirac Semimetal (Cd3As2) for Ultrafast Photoactive Terahertz Photonics

The Dirac semimetal cadmium arsenide (Cd₃As₂), a 3D electronic analog of graphene, has sparked renewed research interests for its novel topological phases and excellent optoelectronic properties. The gapless nature of its 3D electronic band facilitates strong optical nonlinearity and supports Dirac plasmons that are of particular interest to realize high-performance electronic and photonic devices at terahertz (1 THz = 4.1 meV) frequencies, where the performance of most dynamic materials are limited by the tradeoff between power-efficiency and switching speed. Here, all-optical, low-power, ultrafast broadband modulation of terahertz waves using an ultrathin film (100 nm, λ/3000) of Cd₃As₂ are experimentally demonstrated through active tailoring of the photoconductivity. The measurements reveal the photosensitive metallic behavior of Cd₃As₂ with high terahertz electron mobility of 7200 cm² (Vs)⁻¹. In addition, optical fluence dependent ultrafast charge carrier relaxation (15.5 ps), terahertz mobility, and long momentum scattering time (157 fs) comparable to superconductors that invoke kinetic inductance at terahertz frequencies are demonstrated. These remarkable properties of 3D Dirac topological semimetal envision a new class of power-efficient, high speed, compact, tunable electronic, and photonic devices.     

Hybrid State Engineering of Phase-Change Metasurface for All-Optical Cryptography

Chalcogenide material Ge₂Sb₂Te₅ (GST) has bistable phases, the so-called amorphous and crystalline phases that exhibit large refractive index contrast. It can be reversibly switched within a nanosecond time scale through applying thermal bias, especially optical or electrical pulse signals. Recently, GST has been exploited as an ingredient of all-optical dynamic metasurfaces, thanks to its ultrafast and efficient switching functionality. However, most of these devices provide only two-level switching functionality and this limitation hinders their application to diverse all-optical systems. In this paper, the method to expand switching functionality of GST metasurfaces to three level through engineering thermo-optically creatable hybrid state that is co-existing state of amorphous and crystalline GST-based meta-atoms is proposed. Furthermore, the novel hologram technique is introduced for providing the visual information that is only recognizable in the hybrid state GST metasurface. Thanks to thermo-optical complexity to make the hybrid state, the metasurface allows the realization of highly secured visual cryptography architecture without the complex optical setup. The phase-change metasurface based on multi-physical design has significant potential for applications such as all-optical image encryption, security, and anti-counterfeiting.     

In Situ Reversible Ionic Control for Nonvolatile Magnetic Phases in a Donor/Acceptor Metal‐Organic Framework

Reversible magnetic control by electrical means, which is highly desired from the viewpoint of fundamentals and technological applications such as data storage devices, has been a challenging topic. In this study, the authors demonstrate in situ magnetic phase switching between the ferrimagnetic and paramagnetic states of an electron‐donor/‐acceptor metal‐organic framework (D/A‐MOF) using band‐filling control mediated by the Li⁺‐ion migration that accompanies redox reactions, i.e., “magneto‐ionic control”. By taking advantage of the rechargeability of lithium‐ion battery systems, in which Li⁺‐ions and electrons are simultaneously inserted into/extracted from a cathode material, the reversible control of nonvolatile magnetic phases in a D/A‐MOF has been achieved. This result demonstrates that the combination of a redox‐active MOF with porous flexibility and ion‐migration capability enables the creation of new pathways toward magneto‐electric coupling devices in the field of ionics.     

Tunable Volatility of Ge2Sb2Te5 in Integrated Photonics

The operation of a single class of optical materials in both a volatile and nonvolatile manner is becoming increasingly important in many applications. This is particularly true in the newly emerging field of photonic neuromorphic computing, where it is desirable to have both volatile (short‐term transient) and nonvolatile (long‐term static) memory operation, for instance, to mimic the behavior of biological neurons and synapses. The search for such materials thus far have focused on phase change materials where typically two different types are required for the two different operational regimes. In this paper, a tunable volatile/nonvolatile response is demonstrated in a photonic phase‐change memory cell based on the commonly employed nonvolatile material Ge₂Sb₂Te₅ (GST). A time‐dependent, multiphysics simulation framework is developed to corroborate the experimental results, allowing us to spatially resolve the recrystallization dynamics within the memory cell. It is then demonstrated that this unique approach to photonic memory enables both data storage with tunable volatility and detection of coincident events between two pulse trains on an integrated chip. Finally, improved efficiency and all‐optical routing with controlled volatility are demonstrated in a ring resonator. These crucial results show that volatility is intrinsically tunable in normally nonvolatile GST which can be used in both regimes interchangeably.     

Reliable Organic Nonvolatile Memory Device Using a Polyfluorene-Derivative Single-Layer Film

This letter describes the reversible switching performance of metal-organic-semiconductor (MOS) memory devices containing a polyfluorene-derivative single-layer film. The space-charge-limited current contributes to the switching behavior of WPF-oxy-F memory devices. The polyfluorene derivative reported here provides a significant advance to the field of organic semiconductors because it provides a type of organic memory material for nonvolatile memory devices. The following properties are responsible for its memory capabilities: its use of a single-layer film, a large on/off ratio (Ion/Ioff ~ 104), a long retention time (more than 10 000 s), acceptable thermal stability up to 120 degC, and an excellent device-to-device switching uniformity.     

Realization of the Switching Mechanism in Resistance Random Access Memory™ Devices: Structural and Electronic Properties Affecting Electron Conductivity in a Hafnium Oxide–Electrode System Through First-Principles Calculations

The resistance random access memory (RRAM?) device, with its electrically induced nanoscale resistive switching capacity, has attracted considerable attention as a future nonvolatile memory device. Here, we propose a mechanism of switching based on an oxygen vacancy migration-driven change in the electronic properties of the transition-metal oxide film stimulated by set pulse voltages. We used density functional theory-based calculations to account for the effect of oxygen vacancies and their migration on the electronic properties of HfO₂ and Ta/HfO₂ systems, thereby providing a complete explanation of the RRAM? switching mechanism. Furthermore, computational results on the activation energy barrier for oxygen vacancy migration were found to be consistent with the set and reset pulse voltage obtained from experiments. Understanding this mechanism will be beneficial to effectively realizing the materials design in these devices.     

Air‐Stable Cesium Lead Iodide Perovskite for Ultra‐Low Operating Voltage Resistive Switching

CsPbX₃ (X = halide, Cl, Br, or I) all‐inorganic halide perovskites (IHPs) are regarded as promising functional materials because of their tunable optoelectronic characteristics and superior stability to organic–inorganic hybrid halide perovskites. Herein, nonvolatile resistive switching (RS) memory devices based on all‐inorganic CsPbI₃ perovskite are reported. An air‐stable CsPbI₃ perovskite film with a thickness of only 200 nm is successfully synthesized on a platinum‐coated silicon substrate using low temperature all‐solution process. The RS memory devices of Ag/polymethylmethacrylate (PMMA)/CsPbI₃/Pt/Ti/SiO₂/Si structure exhibit reproducible and reliable bipolar switching characteristics with an ultralow operating voltage (<+0.2 V), high on/off ratio (>10⁶), reversible RS by pulse voltage operation (pulse duration < 1 ms), and multilevel data storage. The mechanical flexibility of the CsPbI₃ perovskite RS memory device on a flexible substrate is also successfully confirmed. With analyzing the influence of phase transition in CsPbI₃ on RS characteristics, a mechanism involving conducting filaments formed by metal cation migration is proposed to explain the RS behavior of the memory device. This study will contribute to the understanding of the intrinsic characteristics of IHPs for low‐voltage resistive switching and demonstrate the huge potential of them for use in low‐power consumption nonvolatile memory devices on next‐generation computing systems.     

二氧化钒复合薄膜太赫兹透射率模拟

二氧化钒(VO2)是一种具有可逆热致相变性质的材料,在太赫兹调制领域具有应用潜力。为了确认VO2复合薄膜对太赫兹调控的可行性,在进行相关实验之前,首先运用CST Studio Suite电磁仿真软件进行模拟研究,主要探究了VO2及其与金属光栅结合的复合薄膜的相关参数对太赫兹透射及调控的影响。仿真结果表明,VO2能很好地调控太赫兹的透射幅度,调控深度可由相变幅度控制。另外仿真发现,对VO2复合薄膜而言,通过控制光栅的参数变化可以改变太赫兹透射幅度,并且可以通过调控光栅周期实现对太赫兹调制深度控制。     

Fast Reversible Phase Change Silicon for Visible Active Photonics

Both amorphous and crystalline silicon are ubiquitous materials for electronics, photonics, and microelectromechanical systems. On‐demand control of Si crystallinity is crucial for device manufacturing and to overcome the limitations of current phase‐change materials (PCM) in active photonics. Fast reversible phase transformation in silicon, however, has never been accomplished due to the notorious challenge of amorphization. It is demonstrated that nanostructured Si can function as a PCM, since it can be reversibly crystallized and amorphized under nanosecond laser irradiation with different pulse energies. Reflection probing on a single nanodisk's phase transformations confirms the distinct mechanisms for crystallization and amorphization. The experimental results show that the relaxation time of undercooled silicon at 950 K is 10 ns. The phase change provides a 20% nonvolatile reflectivity modulation within 100 ns and can be repeated over 400 times. It is shown that such transformations are free of deformation upon solidification. Based on the switchable photonic properties in the visible spectrum, proof‐of‐concept experiments of dielectric color displays and dynamic wavefront control are shown. Therefore, nanostructured silicon is proposed as a chemically stable, deformation free, and complementary metal–oxide‐semiconductor compatible (CMOS) PCM for active photonics at visible wavelengths.     

Fragile‐to‐Strong Crossover in Supercooled Liquid Ag‐In‐Sb‐Te Studied by Ultrafast Calorimetry

Phase‐change random‐access memory relies on the reversible crystalline‐glassy phase change in chalcogenide thin films. In this application, the speed of crystallization is critical for device performance: there is a need to combine ultrafast crystallization for switching at high temperature with high resistance to crystallization for non‐volatile data retention near to room temperature. In phase‐change media such as nucleation‐dominated Ge₂Sb₂Te₅, these conflicting requirements are met through the highly “fragile” nature of the temperature dependence of the viscosity of the supercooled liquid. The present study explores, using ultrafast‐heating calorimetry, the equivalent temperature dependence for the growth‐dominated medium Ag‐In‐Sb‐Te. The crystallization shows (unexpectedly) Arrhenius temperature dependence over a wide intermediate temperature range. Here it is shown that this is evidence for a fragile‐to‐strong crossover on cooling the liquid. Such a crossover has many consequences for the interpretation and control of phase‐change kinetics in chalcogenide media, helping to understand the distinction between nucleation‐ and growth‐dominated crystallization, and offering a route to designing improved device performance.     

A New Family of Ultralow Loss Reversible Phase‐Change Materials for Photonic Integrated Circuits: Sb2S3 and Sb2Se3

Phase‐change materials (PCMs) are seeing tremendous interest for their use in reconfigurable photonic devices; however, the most common PCMs exhibit a large absorption loss in one or both states. Here, Sb₂S₃ and Sb₂Se₃ are demonstrated as a class of low loss, reversible alternatives to the standard commercially available chalcogenide PCMs. A contrast of refractive index of Δn = 0.60 for Sb₂S₃ and Δn = 0.77 for Sb₂Se₃ is reported, while maintaining very low losses (k < 10^?5) in the telecommunications C‐band at 1550 nm. With a stronger absorption in the visible spectrum, Sb₂Se₃ allows for reversible optical switching using conventional visible wavelength lasers. Here, a stable switching endurance of better than 4000 cycles is demonstrated. To deal with the essentially zero intrinsic absorption losses, a new figure of merit (FOM) is introduced taking into account the measured waveguide losses when integrating these materials onto a standard silicon photonics platform. The FOM of 29 rad phase shift per dB of loss for Sb₂Se₃ outperforms Ge₂Sb₂Te₅ by two orders of magnitude and paves the way for on‐chip programmable phase control. These truly low‐loss switchable materials open up new directions in programmable integrated photonic circuits, switchable metasurfaces, and nanophotonic devices.     

基于相位梯度光栅介电超表面的高效太赫兹波异常反射器

太赫兹(THz)波由于其诸多独特的性质,有着广泛的应用前景。然而由于相关材料和器件的发展相对滞后,太赫兹技术在实际中的应用尚有诸多限制。超材料和超表面概念的提出,能够对太赫兹波的相位、振幅、偏振进行有效操控,为太赫兹技术的发展提供了许多新的思路。其重要的功能之一是依靠相位不连续将入射波反射到非镜面方向,即通称的广义斯涅尔定律。然而,此前报道的大多数异常反射装置的效率都相对较低,在实际应用中存在局限性。针对这一问题,文中提出了一种太赫兹超表面异常反射器,将法向入射光反射到40°方向且不改变其偏振,并从理论上阐述了提高效率的思路,且通过数值模拟展示其有效性。通过使用全介质材料构建超表面从而消除材料损耗,并利用不同布洛赫波的耦合以提供非局部响应,令器件的工作效率超过99%。此外,这一设计理念可以推广到偏振无关器件中,并且对其他类似的器件也有一定参考意义。这一工作有潜力被应用于太赫兹波激光器、太赫兹波腔谐振器等太赫兹波实际器件中。     

Smart windows switch on the light

Optical switching devices for regulating incident solar energy that can replace traditional windows in buildings, vehicles, and aircraft are discussed. The chromogenic material in these smart windows exhibits a large change in optical properties with variation in applied electrical field, charge, light intensity, spectral composition, or temperature. The optical change transforms the material from a highly transmitting state to a partly reflecting (or absorbing) state over all or part of the visible and solar spectra. The optical switching can be activated electrically or nonelectrically. Electrically activated types, including electrochromic, liquid-crystal, and dispersed-particle (electrophoretic) devices, as well as devices based on reversible electrodeposition, and nonelectrically activated types, including devices based on photochromic and thermochromic materials, are described     

Forming-Free Reversible Bipolar Resistive Switching Behavior in Al-Doped HfO<Subscript>2</Subscript> Metal–Insulator–Metal Devices

Resistive switching (RS) characteristics are investigated in fabricated Al-doped HfO₂ metal–insulator–metal devices. It is proposed that oxygen vacancies in Al-doped HfO₂ devices play a key role as electron trap centers, leading to the forming-free reversible bipolar resistance switching behavior. The conduction mechanism can be explained by electron trapping and detrapping from such oxygen vacancy-related traps in the Al-doped HfO₂ films and is dominated by a trap-controlled space-charge-limited current (SCLC) mechanism. A large RS ratio (~10⁶) and excellent retention characteristics are also observed at room temperature as well as at 85°C. Such devices have potential for application in nonvolatile random-access memory.     

Phase change materials and their application to random access memory technology

Phase change materials can exist in two different phases, the amorphous and the crystalline phase, which exhibit distinctly different physical properties. It is possible to repeatedly switch the state of these materials, from the amorphous phase to the crystalline phase by heating the material above its crystallization temperature, and from the crystalline to the amorphous phase by melt-quenching. Phase change materials have been utilized very successfully in all modern optical re-writable storage media such as CDs, DVDs and Blu-ray disks. Recently, they have also been applied to solid-state memory devices where their large difference in electrical resistivity is used to store information. This paper reviews the unique properties of phase change materials in particular as they are important for their application to these devices.     

Nitrogen-doped Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> films for nonvolatile memory

Nitrogen-doped Ge₂Sb₂Te₅ (GST) films for nonvolatile memories were prepared by reactive sputtering with a GST alloy target. Doped nitrogen content was determined by using x-ray photoelectron spectroscopy (XPS). The crystallization behavior of the films was investigated by analyzing x-ray diffraction (XRD) and differential scanning calorimetry (DSC). Results show that nitrogen doping increases crystallization temperature, crystallization-activation energy, and phase transformation temperature from fcc to hexagonal (hex) structure. Doped nitrogen probably exists in the grain vacancies or grain boundaries and suppresses grain growth. The electrical properties of the films were studied by analyzing the optical band gap and the dependence of the resistivity on the annealing temperature. The optical band gap of the nitrogen-doped GST film is slightly larger than that of the pure GST film. Energy band theory is used to analyze the effect of doped nitrogen on electrical properties of GST films. Studies reveal that nitrogen doping increases resistivity and produces three relatively stable resistivity states in the plot of resistivity versus annealing temperature, which makes GST-based multilevel storage possible. Current-voltage (I-V) characteristics of the devices show that nitrogen doping increases the memory’s dynamic resistance, which reduces writing current from milliampere to microampere.     

Giant and Nonvolatile Control of Exchange Bias in Fe3GeTe2/Irradiated Fe3GeTe2/MgO Heterostructure Through Ultralow Voltage

The discovery of van der Waals magnets has provided a new platform for the electrical control of magnetism. Recent experiments have demonstrated that the magnetic properties of van der Waals magnets can be tuned by various gate modulations, although most of them are volatile and require gate voltages no lower than several volts. Here, the realization of nonvolatile control of exchange bias and coercive fields in Fe₃GeTe₂/MgO heterostructures, and the gate voltage is as low as tens of mV which is two orders of magnitude smaller than those in previous experiments is presented. The discovery of an ionic-irradiated phase formed in Fe₃GeTe₂ by MgO sputtering revealed that an exchange bias effect can be obtained in this heterostructure and tuned from ≈700 to 0 Oe through voltages ranging from 5 to 20 mV. Owing to the high stability of oxidized Fe₃GeTe₂, the voltage-driven oxygen incorporated into Fe₃GeTe₂ from the irradiated phase induces a nonvolatile magnetism modulation that can be retained after turning off the gate voltage. These findings demonstrate a methodology to modulate the magnetism of van der Waals magnets, opening new opportunities to fabricate all-solid, long-retention, and low-dissipation nano-electronic devices using van der Waals materials.