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.     

All‐Dielectric Programmable Huygens' Metasurfaces

Low‐loss nanostructured dielectric metasurfaces have emerged as a breakthrough platform for ultrathin optics and cutting‐edge photonic applications, including beam shaping, focusing, and holography. However, the static nature of their constituent materials has traditionally limited them to fixed functionalities. Tunable all‐dielectric infrared Huygens' metasurfaces consisting of multi‐layer Ge disk meta‐units with strategically incorporated non‐volatile phase change material Ge₃Sb₂Te₆ are introduced. Switching the phase‐change material between its amorphous and crystalline structural state enables nearly full dynamic light phase control with high transmittance in the mid‐IR spectrum. The metasurface is realized experimentally, showing post‐fabrication tuning of the light phase within a range of 81% of the full 2π phase shift. Additionally, the versatility of the tunable Huygen's metasurfaces is demonstrated by optically programming the spatial light phase distribution of the metasurface with single meta‐unit precision and retrieving high‐resolution phase‐encoded images using hyperspectral measurements. The programmable metasurface concept overcomes the static limitations of previous dielectric metasurfaces, paving the way for “universal” metasurfaces and highly efficient, ultracompact active optical elements like tunable lenses, dynamic holograms, and spatial light modulators.     

Design Method of Tunable Pixel with Phase‐Change Material for Diffractive Optical Elements

In this paper, we propose a scheme for designing a tunable pixel layer based on a Ge₂Sb₂Te₅ (GST) alloy thin film. We show that the phase change of GST can significantly affect the reflection characteristic when the GST film is embedded into a dielectric encapsulation layer. We investigate the appropriate positions of the GST film within the dielectric layer for high diffraction efficiency, and we prove that they are antinodes of Fabry–Perot resonance inside the dielectric layer. Using the proposed scheme, we can increase the diffraction efficiency by about ten times compared to a bare GST film pixel, and 80 times for the first‐to‐zeroth‐order diffraction power ratio. We show that the proposed scheme can be designed alternatively for a broadband or wavelength‐selective type by tuning the dielectric thickness, and we discuss a multi‐phase example with a double‐stack structure.     

复合鱼鳞型超构材料中光控Fano谐振的设计

为解决传统超构材料存在的结构一旦形成,其谐振特性便无法进行动态可调的问题,本文将鱼鳞型超构材料与光电导材料硅相结合,实现了太赫兹波段Fano电磁响应的动态调控。该复合超构材料由鱼鳞型金属线、硅层以及聚酰亚胺组成。在鱼鳞型结构的金属弧线无、有缝隙两种情况下,研究了电磁波的入射角度和硅的电导率对Fano谐振的影响。当硅的电导率达到1×103S/m时,多频点电磁响应的调制深度都接近1。结果表明,调节缝隙宽度可以成为Fano谐振工作频率调控的有效方式。本文为实现超构材料中Fano谐振的可调谐特性提供了一种可行途径,对实际应用中太赫兹波的主动调控、传感等方面具有重要意义。     

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.     

High‐Temperature Large‐Scale Self‐Assembly of Highly Faceted Monocrystalline Au Metasurfaces

Localized surface plasmon resonance (LSPR) devices based on resonant metallic metasurfaces have shown disruptive potential for many applications including biosensing and photocatalysis. Despite significant progress, highly performing Au plasmonic nanotextures often suffer of suboptimal electric field enhancement, due to damping effects in multicrystalline domains. Fabricating well‐defined Au nanocrystals over large surfaces is very challenging, and usually requires time‐intensive multi‐step processes. Here, presented are first insights on the large‐scale self‐assembly of monocrystalline Au nano‐islands with tunable size and separation, and their application as efficient LSPR surfaces. Highly homogeneous centimeter‐sized Au metasurfaces are fabricated by one‐step deposition and in situ coalescence of hot nanoparticle aerosols into a discontinuous monolayer of highly faceted monocrystals. First insights on the mechanisms driving the high‐temperature synthesis of these highly faceted Au nanotextures are obtained by molecular dynamic and detailed experimental investigation of their growth kinetics. Notably, these metasurfaces demonstrat high‐quality and tunable LSPR, enabling the fabrication of highly performing optical gas molecule sensors detecting down to 3 × 10^?6 variations in refractive index at room temperature. It is believed that these findings provide a rapid, low‐cost nanofabrication tool for the engineering of highly homogenous Au metasurfaces for large‐scale LSPR devices with application ranging from ultrasensitive optical gas sensors to photocatalytic macroreactors.     

A novel proposal for ultra-high optical nonlinearity in GaN/AlGaN spherical centered defect quantum dot (SCDQD)

In this paper a novel idea for enhancement and tunable optical nonlinearity based on spherical centered defect quantum dot (SCDQD) is proposed. The proposed structure is a special quantum dot including a spherical defect inside it. Complete analysis of the proposed structure based on the effective mass equation is done and optical properties (third-order susceptibilities of quadratic electro-optic effect (QEOE) and third harmonic generation (THG) associated with intersubband transition) of the introduced structure using density matrix method are investigated also. Effects of system parameters including defect and dot on energy levels and optical nonlinearity are examined. We observed that the proposed structure has high nonlinear and tunable susceptibilities suitable for implementation of optical active and passive devices. It is shown that the magnitudes of dipole transition matrix element, third-order susceptibilities of THG and QEOE can be increased significantly compared traditional cases (4.5-10 nm, 10⁻¹¹-10⁻⁹ m²/V² and 10⁻¹²-10⁻⁹ m²/V², respectively). Also, the resonance wavelengths are displaced using these parameters that can be used for realization of tunable devices.     

Electrode Materials for Ge2Sb2Te5-Based Memristors

The memristive characteristics of Ge₂Sb₂Te₅ (GST) as a representative chalcogenide material have been verified and show great potential for memory applications. This paper focuses on the influence of different electrode materials on the properties of GST-based memristors. Several electrode materials (Ti₃W₇, Ag, Cu, and Ta) were adopted in devices with a top electrode (TE)/GST/bottom electrode (BE) layer structure. Through different current–voltage (I–V) curves, it was demonstrated that devices with Ag or Cu electrodes are suitable for GST-based memristors, while those utilizing inert electrodes are not. Because of their relatively smaller radius and lower binding energy, it is much easier for Ag and Cu to diffuse into the GST layer and form conductive filaments. The results obtained from memristors annealed at different temperatures further support the conductive filament model. Moreover, an optimized Cu/Ag/GST/Cu device structure different from the traditional TE/GST/BE structure is proposed, showing improved stability with higher R _off/R _on ratio and good endurance.     

Independently tunable Fano resoances based on E-shaped cavity and its high performance sensing application

A novel nanostructure based on a simple metal-dielectric-metal (MDM) bus waveguide with sliver baffle as well as an E-shaped cavity is proposed to generate double Fano resonances arising from interference between broad continuous state and narrow discrete state. The commercial software COMSOL based on finite element methods is used to explore the Fano resonances properties and the senor properties of the system. According to the simulation results, it is demonstrated that E-shaped cavity can generate two discrete states, and the continuous sate is constructed by loading sliver baffle in bus waveguide. Moreover, it can be found that the Fano peak wavelengths and profiles can easily be tuned via structural parameters. Interestingly, the independent control of the left Fano resonance (LFR) can be accomplished by changing horizontal cavity length of E-shaped cavity. Meanwhile, our proposed nanostructure has a higher sensitivity of 1 440 nm/RIU as well as a higher figure of merit (FOM) value approximately 5 244. In conclusion, the proposed independently tunable Fano nanostructure may have a great promising in the fields of nanosensors, filters and other optical devices.     

基于电介质超表面的光场复振幅调制及应用

超表面作为一种人工设计的二维阵列纳米结构,能够在亚波长尺度上实现光场波前振幅、相位和偏振态的灵活调控,为现代光学器件的小型化、集成化提供了全新的实现途径。随着光学成像、显示等应用的发展,在可见光波段具有高工作效率的微型光学器件的需求日益凸显。近年来,由高折射率、低损耗电介质材料制备的光学超表面得到了极大地发展,在消色差光学超透镜、偏振相关全息显示等方面展现出广泛的应用前景。文中围绕电介质超表面的相关研究,首先介绍广义斯涅耳定律及电介质超表面结构调控光场振幅、相位和偏振态的基本原理,在此基础上,重点回顾近年来关于光场波前单一参量调控和多参量联合调控在全息显示、结构光场产生等方面的研究进展,最后讨论电介质超表面发展的可能挑战与前景。     

Hybrid Metasurfaces of Plasmonic Lattices and 2D Materials

Plasmonic metasurfaces can significantly enhance the interaction between light and 2D materials. Hybrid structures of plasmonic lattices and 2D materials show great promise for both fundamental and practical studies because of their unprecedented ability for precise manipulation of light at the nanoscale. This review starts with an overview of the basic concepts of plasmonic lattices and optical properties of 2D materials, as well as fabrication strategies for hybrid metasurfaces. Then, the enhanced photoluminescence, quantum emission, optoelectronic detection, nonlinear process, and valleytronics in hybrid metasurfaces are summarized, and their development for nanophotonic functional devices are reviewed. Further, several compelling topics are also outlined that provide outlooks for future directions of hybrid metasurfaces such as novel structural design and high-quality fabrication, all-dielectric metasurfaces, dynamic metasurfaces, and plasmonic mediation of chemical reactions and physical processes. It is believed that hybrid metasurfaces of plasmonic lattices and 2D materials can open prospects for versatile platforms for light-matter interactions and contribute to the revolutions on nanophotonic devices.     

Growth of Topological Insulator Bi2Se3 Thin Films on SrTiO3 with Large Tunability in Chemical Potential

The growth of high quality, gate‐tunable topological insulator Bi₂Se₃ thin films on SrTiO₃ substrates by molecular beam epitaxy is reported in this paper. The optimized substrate preparation procedures are critical for obtaining undoped Bi₂Se₃ thin films with sufficiently low carrier densities while maintaining the strong dielectric strength of the substrates. The large tunability in chemical potential is manifested in the greatly enhanced longitudinal resistivity and the reversal of the sign of the Hall resistivity at negative back‐gate voltages. These thin films provide a convenient basis for fabrication of hybrid devices consisting of gate‐tunable topological insulators and other materials such as a superconductor and a ferromagnet.     

Highly Sensitive Mechanoresponsive Smart Windows Driven by Shear Strain

Advanced optical materials with rational designs and tunable light transmission have been drawing increasing interest due to their great potential in energy-efficient buildings and on-demand optical devices. Mechanoresponsive smart windows (SWs) can modulate light transmittance by mechanical actuation, showing high energy efficiency, low cost, and chemical stability. However, current research mainly focuses on tensile strain-responsive SWs that typically require a large strain to achieve optical transparency switching-which causes great inconvenience to practical application and fatigue damage to matrix materials. Herein, a novel shear-responsive SW with high strain sensitivity is fabricated by vertically fixing a Fe₃O₄@SiO₂ nanochains (NCs) array in an elastic polyacrylamide matrix. The flexible SW exhibits optically transparent with all NCs standing vertically to the SW surface at initial relaxation state, which enables a good shielding effect, with NCs tilting along the shearing direction as the strain applied. Critically, a rather small shear displacement (1.5 mm) applied on the surface of SW gives rise to tunable optical states varying from the transparency state of 65% transmittance to the opaque state of 10%. The as-prepared SW with novel tuning modulation, high shear strain sensitivity, and optical angle-dependence holds promising potential in smart windows, optical switches, anti-voyeurism, and etc.     

MIM波导内嵌金属板结构Fano共振传感特性

基于法诺(Fano)共振传感特性,提出了一种金属-介质-金属(MIM)波导耦合矩形腔结构,目的是为了实现高灵敏度、高可靠性的折射率传感检测。通过耦合模理论和有限元数值模拟仿真,分析了矩形腔中有无内嵌金属板两种结构的Fano透射光谱特征,并且进一步优化了矩形腔内嵌金属板结构参数,最后阐明了结构参数对其传感特性的内在影响。结果表明,当入射光以TM模式入射到矩形谐振腔时,会形成两个Fano模式共振峰:第一种模式的品质因数(FOM)达9.4×10⁴,灵敏度为700nm/RIU;第二种模式的FOM达8.4×10³,灵敏度为1200nm/RIU。研究结果表明此结构设计实现了双Fano峰检测,同时各个模式品质因数都很高,这为高性能微纳光学折射率传感器的设计提供了一定的理论参考依据。     

Organic/Inorganic Hybrid Nanochannels Based on Polypyrrole‐Embedded Alumina Nanopore Arrays: pH‐ and Light‐Modulated Ion Transport

Inspired by the asymmetric structure and responsive ion transport in biological ion channels, organic/inorganic hybrid artificial nanochannels exhibiting pH‐modulated ion rectification and light‐regulated ion flux have been constructed by introducing conductive polymer into porous nanochannels. The hybrid nanochannels are achieved by partially modifying alumina (Al₂O₃) nanopore arrays with polypyrrole (PPy) layer using electrochemical polymerization, which results in an asymmetric component distribution. The protonation and deprotonation of Al₂O₃ and PPy upon pH variation break the surface charge continuity, which contributes to the pH‐tunable ion rectification. The ionic current rectification ratio is affected substantially by the pH value of electrolyte and the pore size of nanochannels. Furthermore, the holes (positive charges) in PPy layer induced by the cooperative effect of light and protons are used to regulate the ionic flux through the nanochannels, which results in a light‐responsive ion current. The magnitude of responsive ionic current could be amplified by optimizing this cooperation. This new type of stimuli‐responsive PPy/Al₂O₃ hybrid nanochannels features advantages of unique optical and electric properties from conducting PPy and high mechanical performance from porous Al₂O₃ membrane, which provide a platform for creating smart nanochannels system.     

Polarization and Holography Recording in Real- and k-Space Based on Dielectric Metasurface

The diverse design freedom and mechanisms of metasurfaces motivate the manipulation of polarization in an ultrashort distance with subwavelength resolution and make metasurfaces outperform conventional polarization optical elements. However, in order to enhance the information capability and encryption security of metasurface holograms, polarization manipulation together with multiplexing technologies are still highly desired. Here, a birefringent dielectric metasurface with the capability of encoding a grayscale image in real-space based on Malus's law by utilizing the inhomogeneous polarization distribution and realizing the reconstruction of a vectorial holographic image in k-space with the help of the phase profiles of different polarization components of output light is demonstrated. This novel functionality is realized by exploiting the manipulation of polarization and phase of output light simultaneously offered by the dielectric metasurface. The proposed method may enhance the information capability and security level of applications such as the anticounterfeiting and encryption.     

Casimir Force Contrast Between Amorphous and Crystalline Phases of AIST

Phase change materials (PCMs) can be rapidly and reversibly switched between the amorphous and crystalline state. The structural transformation is accompanied by a significant change of optical and electronic properties rendering PCMs suitable for rewritable optical data storage and non‐volatile electronic memories. The phase transformation is also accompanied by an increase of the Casimir force of 20 to 25% between gold and AIST (Ag₅In₅Sb₆₀Te₃₀) upon crystallization. Here the focus is on reproducing and understanding the observed change in Casimir force, which is shown to be related to a change of the dielectric function upon crystallization. The dielectric function changes in two separate frequency ranges: the increase of absorption in the visible range is due to resonance bonding, which is unique for the crystalline phase, while free carrier absorption is responsible for changes in the infrared regime. It is shown that free carriers contribute ≈50% to the force contrast, while the other half comes from resonance bonding. This helps to identify PCMs that maximize force contrast. Finally it is shown that if this concept of force control is to be employed in microelectromechanical devices, then protective capping layers of PCMs must be only a few nanometers thick to minimize reduction of the force contrast.     

Patterning of Ge2Sb2Te5 phase change material using UV nano-imprint lithography

DRAM is the most commonly used memory due to many advantages such as high speed and easy manufacturability owing to its simple structure, but is volatile. On the other hand, flash memory is non-volatile, but has other disadvantages such as slow speed, short lifetime, and low endurance for repetitive data writing. Compared to DRAM and flash memory, PRAM (Phase-change Random Access Memory), which is a non-volatile memory using a reversible phase change between amorphous and crystalline state, has many advantages such as high speed, high sensing margin, low operating voltage, and is being pursed as a next generation memory. Being able to pattern and etch phase change memory in nanometer scale is essential for the integration of PRAM. This study uses the Nano-Imprint Lithography (NIL) for patterning the PRAM in nanometer scale which is believed to be a future lithography technology that will replace the conventional Photo Lithography. Si wafers coated with SiO₂ were used as substrates, and Ge₂Sb₂Te₅ (GST) films with the thicknesses of 100 nm were deposited by RF sputtering. Poly-benzylmethacrylate based polymer patterns were formed using NIL on the surface of GST films, and the GST films were etched using Cl₂/Ar plasma in an Oxford ICP (inductively coupled plasma) etcher.     

波长间隔可调谐多波长光纤光学参量振荡器

提出并实现了一种以高非线性色散位移光纤为增益介质,以光栅对形成谐振腔,简单线形结构的连续光抽运的波长间隔可调谐多波长光纤光学参量振荡器(MW-FOPO)。采用波长可调谐的窄线宽激光器作为抽运种子光源,以伪随机相位调制抽运光来抑制高非线性光纤中的受激布里渊(SBS)散射效应,结合高功率掺铒光纤放大器构成光纤光学参量振荡器的大功率抽运,通过四波混频(FWM)效应获得了室温下稳定的多波长激光输出。MW-FOPO的波长间隔可以通过调节抽运波长进行调谐。在1505~1615 nm光谱范围内,获得了17条消光比大于10 dB的多波长谱线。实验证明了MW-FOPO实现多波长激光光源的优异特性。     

Implementation of Low‐Power Electronic Devices Using Solution‐Processed Tantalum Pentoxide Dielectric

The development of solution‐processed field effect transistors (FETs) based on organic and hybrid materials over the past two decades has demonstrated the incredible potential in these technologies. However, solution processed FETs generally require impracticably high voltages to switch on and off, which precludes their application in low‐power devices and prevent their integration with standard logic circuitry. Here, a universal and environmentally benign solution‐processing method for the preparation of Ta₂O₅, HfO₂ and ZrO₂ amorphous dielectric thin films is demonstrated. High mobility CdS FETs are fabricated on such high‐κ dielectric substrates entirely via solution‐processing. The highest mobility, 2.97 cm² V^?1 s^?1 is achieved in the device with Ta₂O₅ dielectric with a low threshold voltage of 1.00 V, which is higher than the mobility of the reference CdS FET with SiO₂ dielectric with an order of magnitude decrease in threshold voltage as well. Because these FETs can be operated at less than 5 V, they may potentially be integrated with existing logic and display circuitry without significant signal amplification. This report demonstrates high‐mobility FETs using solution‐processed Ta₂O₅ dielectrics with drastically reduced power consumption; ≈95% reduction compared to that of the device with a conventional SiO₂ gate dielectric.