Chalcogenide Phase Change Material for Active Terahertz Photonics

The strikingly contrasting optical properties of various phases of chalcogenide phase change materials (PCM) has recently led to the development of novel photonic devices such as all‐optical non‐von Neumann memory, nanopixel displays, color rendering, and reconfigurable nanoplasmonics. However, the exploration of chalcogenide photonics is currently limited to optical and infrared frequencies. Here, a phase change material integrated terahertz metamaterial for multilevel nonvolatile resonance switching with spatial and temporal selectivity is demonstrated. By controlling the crystalline proportion of the PCM film, multilevel, non‐volatile, terahertz resonance switching states with long retention time at zero hold power are realized. Spatially selective reconfiguration at sub‐metamaterial scale is shown by delivering electrical stimulus locally through designer interconnect architecture. The PCM metamaterial also features ultrafast optical modulation of terahertz resonances with tunable switching speed based on the crystalline order of the PCM film. The multilevel nonvolatile, spatially selective, and temporally tunable PCM metamaterial will provide a pathway toward development of novel and disruptive terahertz technologies including spatio‐temporal terahertz modulators for high speed wireless communication, neuromorphic photonics, and machine‐learning metamaterials.     

Metamaterials for Enhanced Optical Responses and their Application to Active Control of Terahertz Waves

Metamaterials, artificially constructed structures that mimic lattices in natural materials, have made numerous contributions to the development of unconventional optical devices. With an increasing demand for more diverse functionalities, terahertz (THz) metamaterials are also expanding their domain, from the realm of mere passive devices to the broader area where functionalized active THz devices are particularly required. A brief review on THz metamaterials is given with a focus on research conducted in the authors' group. The first part is centered on enhanced THz optical responses from tightly coupled meta-atom structures, such as high refractive index, enhanced optical activity, anomalous wavelength scaling, large phase retardation, and nondispersive polarization rotation. Next, electrically gated graphene metamaterials are reviewed with an emphasis on the functionalization of enhanced THz optical responses. Finally, the linear frequency conversion of THz waves in a rapidly time-variant THz metamaterial is briefly discussed in the more general context of spatiotemporal control of light.     

Phase Engineering of Transition Metal Dichalcogenides with Unprecedentedly High Phase Purity,Stability, and Scalability via Molten-Metal-Assisted Intercalation

The crystalline phase of layered transition metal dichalcogenides (TMDs) directly determines their material property. The most thermodynamically stable phase structures in TMDs are the semiconducting 2H and metastable metallic 1T phases. To overcome the low phase purity and instability of 1T-TMDs, which limits the utilization of their intrinsic properties, various synthesis strategies for 1T-TMDs have been proposed in phase-engineering studies. Herein, a facile and scalable synthesis of 1T-phase molybdenum disulfide (MoS₂) via the molten-metal-assisted intercalation (MMI) approach is introduced, which exploits the capillary action of molten potassium and the difference between the electron affinity of MoS₂ and the ionization potential of potassium. Highly reactive molten potassium metal can readily intercalate into the MoS₂ interlayers, inducing an efficient phase transition from the 2H to 1T crystal structure. The ionic bonding between the intercalated potassium and sulfur lowers the energy barrier of the 1T-phase transition, enhancing the phase stability of the 1T crystals. Owing to the high purity and stability of the 1T phase, the electrocatalytic performance for the hydrogen evolution reaction is significantly higher in 1T-MoS₂ (MMI) than in 2H-MoS₂ and even in 1T-MoS₂ synthesized using n-butyllithium.     

A New Ba0.6Sr0.4TiO3–Silicon Hybrid Metamaterial Device in Terahertz Regime

Metamaterials, offering unprecedented functionalities to manipulate electromagnetic waves, have become a research hotspot in recent years. Through the incorporation of active media, the exotic electromagnetic behavior of metamaterials can be dramatically empowered by dynamic control. Many ferroelectric materials such as BaSrTiO₃ (abbreviated as BST), exhibiting strong response to external electric field, hold great promise in both microwave and terahertz tunable devices. A new active Ba_0.6Sr_0.4TiO₃–silicon hybrid metamaterial device, namely, a SRR (square split‐ring resonator)–BaSrTiO₃ thin film‐silicon three‐layer structure is fabricated and intensively studied. The active Ba_0.6Sr_0.4TiO₃ thin film hybrid metamaterial, with nanoscale thickness, delivers a transmission contrast up to ≈79% due to electrically enabled carrier transport between the ferroelectric thin film and silicon substrate. This work has significantly increased the low modulation rate of ferroelectric based devices in terahertz range, a major problem in this field remaining unresolved for many years. The proposed BST metamaterial is promising in developing high‐performance real world photonic devices for terahertz technology.     

Phase Transition Induced via the Template Enabling Cocoon-like MoS2 an Exceptionally Electromagnetic Absorber

Structural engineering via the template method is efficient for micro-nano assembling. However, only structural design and lack of composition control restrict their advanced application. To overcome this issue, applying a template to simultaneously realize the structural design and fine component control is highly desired, which has been ignored. In this study, a spinel-shaped MoS₂ heterostructure with controlled phase ratios of 1H and 2H phase is developed using the AlOOH template method. This work demonstrates that the MoS₂ phase transition mechanism from 2H to 1T is substantially attributed to the close exposed crystal's surface and approximately accordant surface energy. The superiority and additional proof are provided based on density-functional theory simulation, transmission electron microscope holography, etc. With an effective absorptance region of 6.3 GHz under a thickness of 1.4 mm, the reported samples present outstanding microwave absorption capacity. This is attributed to the beneficial coupled effect between the well-designed structure and phase regulation. This work offers valuable insights into structural engineering and component regulation template methods.     

Photoinduced Phase Transition in Infinite-Layer Nickelates

The quantum phase transition caused by regulating the electronic correlation in strongly correlated quantum materials has been a research hotspot in condensed matter science. Herein, a photon-induced quantum phase transition from the Kondo-Mott insulating state to the low temperature metallic one accompanying with the magnetoresistance changing from negative to positive in the infinite-layer NdNiO₂ films is reported, where the antiferromagnetic coupling among the Ni¹⁺ localized spins and the Kondo effect are effectively suppressed by manipulating the correlation of Ni-3d and Nd-5d electrons under the photoirradiation. Moreover, the critical temperature T_c of the superconducting-like transition exhibits a dome-shaped evolution with the maximum up to ≈42 K, and the electrons dominate the transport process proved by the Hall effect measurements. These findings not only make the photoinduction a promising way to control the quantum phase transition by manipulating the electronic correlation in Mott-like insulators, but also shed some light on the possibility of the superconducting in electron-doped nickelates.     

Specific Heat of a Three-Dimensional Semimetal near a Zero-Temperature Phase Transition

We derive the expression of the specific heat of a three-dimensional semimetal near a zero-temperature phase transition with dynamic exponent z. The result showed the occurrence of a non-Fermi behavior for z = 3, according to the general conjecture that the systems with d = z may present a T ln T contribution in the specific heat, which is typical for the non-Fermi behavior.     

光声效应在研究铁电陶瓷相变中的应用

本文报导了用光声技术研究铁电陶瓷 Pb(Zr,Ti,Sn)O_3(PSZT),透明 PLZT 以及 Li_(1-x)Na_xNbO_3(LNN)的相变,清楚地揭示了这些陶瓷所呈现的不同相变特性以及相变点随组分的变化,与某些常规研究方法相比较,显示了在相变点附近,光声信号的幅度和相位的变化非常灵敏。并用热力学解释了实验结果。     

Modulating Photoluminescence of Monolayer Molybdenum Disulfide by Metal–Insulator Phase Transition in Active Substrates

The atomic thickness and flatness allow properties of 2D semiconductors to be modulated with influence from the substrate. Reversible modulation of these properties requires an “active,” reconfigurable substrate, i.e., a substrate with switchable functionalities that interacts strongly with the 2D overlayer. In this work, the photoluminescence (PL) of monolayer molybdenum disulfide (MoS₂) is modulated by interfacing it with a phase transition material, vanadium dioxide (VO₂). The MoS₂ PL intensity is enhanced by a factor of up to three when the underlying VO₂ undergoes the thermally driven phase transition from the insulating to metallic phase. A nonvolatile, reversible way to rewrite the PL pattern is also demonstrated. The enhancement effect is attributed to constructive optical interference when the VO₂ turns metallic. This modulation method requires no chemical or mechanical processes, potentially finding applications in new switches and sensors.     

All‐Dielectric Active Terahertz Photonics Driven by Bound States in the Continuum

The remarkable emergence of all‐dielectric meta‐photonics governed by the physics of high‐index dielectric materials offers a low‐loss platform for efficient manipulation and subwavelength control of electromagnetic waves from microwaves to visible frequencies. Dielectric metasurfaces can focus electromagnetic waves, generate structured beams and vortices, enhance local fields for advanced sensing, and provide novel functionalities for classical and quantum technologies. Recent advances in meta‐photonics are associated with the exploration of exotic electromagnetic modes called the bound states in the continuum (BICs), which offer a simple interference mechanism to achieve large quality factors (Q) through excitation of supercavity modes in dielectric nanostructures and resonant metasurfaces. Here, a BIC‐driven terahertz metasurface with dynamic control of high‐Q silicon supercavities that are reconfigurable at a nanosecond timescale is experimentally demonstrated. It is revealed that such supercavities enable low‐power, optically induced terahertz switching and modulation of sharp resonances for potential applications in lasing, mode multiplexing, and biosensing.     

CoCrFeNi-M系高熵合金的结构与相变

高熵合金作为金属材料领域近年来的三大突破之一,其开拓性地打破了传统合金设计理念的思想囚笼,适当配比的高熵合金可制得具有高强度、高耐磨性及耐蚀性等优异性能的合金材料。Fe、Co、Cr、Ni四种元素在高熵合金体系中研究得最为广泛,并得到一定的研究成果。从CoCrFeNi-M系高熵合金的结构与相变特点切入,介绍了高熵合金的结构分类特点,分析了高熵合金相形成及其规律,阐述了合金元素对铸态高熵合金相结构的影响,探讨了高熵合金的热处理过程。最后,总结了高熵合金的研究现状及其存在的问题。     

铌酸锶钡晶体铁电相变的喇曼光谱研究

在20～350℃内研究了 Ba_xSr_(1-x)Nb_2O_(?)(x=0.25,T_c～67.5℃)晶体的喇曼光谱。没有观察到软模现象。发现在居里点上下很大温度范围内,铁电相和顺电相的喇曼光谱相似,低频扩展峰随温度升高而增强。认为 SBN铁电相变属于有序—无序型。     

MEMS显微干涉测量系统中相位提取算法的选择与分析

描述了一种基于相移显微干涉术的MEMS测试方法,达到了纳米级分辨力．从理论上分析了4种常用相移算法对测量过程主要噪声（相移器的移相误差和探测器的非线性响应误差）的抑制作用,并选定了适合本系统的Hariharan算法．通过对经过美国国家标准研究院（NIST）认证的一个台阶高度的测量,验证了各种算法的测量精度,说明Hariharan算法对噪声有更强的抑制作用,其测量重复性在亚纳米量级．     

Non-Fermi Behavior Near a Quantum Phase Transition Driven by Disorder

We showed that in a d-wave two-dimensional superconductor the disorder given by non-magnetic impurities at low temperature leads to a non-Fermi behavior for the normal state. The transition is similar to the superconductor–insulator transition in a model with a dissipative term.     

现代连续相变理论在玻璃分相中的应用

玻璃分相区中存在一临界点,在此临界点的转变属于连续相变范畴。通过本文的讨论表明。对二组元玻璃系统的分相,可以用现代连续相变理论描述。在临界点附近区域内,其分相曲线为以临界点为界,以两曲线方程:(x~l-x_c)/x_c=A[(T_c-T)/T_c]~(1/3),(x~(l′)-x_c)/x_c=A′[(T_c-T)/T_c]~(1/3)来表达。这里 A、A′为二组元系统分相曲线的普适常数。本文中对 Li_2O-SiO_2,Na_2O-SiO_2,BaO-SiO_2系统分相实验值的拟合结果是 A=-1.8305;A′=2.3551。相应的计算曲线与实验值符合较好。     

复合相变陶瓷材料的FRL-FRH相变研究

制备了低温铁电菱方相（ＦＲＬ）－高温铁电菱方相（ＦＲＨ）相变复合陶瓷材料，利用计算横模拟了复合陶瓷材料的相变行为，结果表明，在复相陶瓷材料中，存在着离子扩散，离子扩散对于宏观热释电效应的影响取决地相关材料的相变特性，计算机模拟的方法可以有效地用于分析复合的离子扩散情形，为新型功能材料的设计与研究提供帮助。