Two-photon reduction: a cost-effective method for fabrication of functional metallic nanostructures

Metallic nanostructures have underpinned plasmonic-based advanced photonic devices in a broad range of research fields over the last decade including physics, engineering, material science and bioscience. The key to realizing functional plasmonic resonances that can manipulate light at the optical frequencies relies on the creation of conductive metallic structures at the nanoscale with low structural defects. Currently, most plasmonic nanostructures are fabricated either by electron beam lithography (EBL) or by focused ion beam (FIB) milling, which are expensive, complicated and time-consuming. In comparison, the direct laser writing (DLW) technique has demonstrated its high spatial resolution and cost-effectiveness in three-dimensional fabrication of micro/nanostructures. Furthermore, the recent breakthroughs in superresolution nanofabrication and parallel writing have significantly advanced the fabrication resolution and throughput of the DLW method and made it one of the promising future nanofabrication technologies with low-cost and scalability. In this review, we provide a comprehensive summary of the state-of-the-art DLW fabrication technology for nanometer scale metallic structures. The fabrication mechanisms, different material choices, fabrication capability, including resolution, conductivity and structure surface smoothness, as well as the characterization methods and achievable devices for different applications are presented. In particular, the development trends of the field and the perspectives for future opportunities and challenges are provided at the end of the review. It has been demonstrated that the quality of the metallic structures fabricated using the DLW method is excellent compared with other methods providing a new and enabling platform for functional nanophotonic device fabrication.     

Nanolasers Enabled by Metallic Nanoparticles: From Spasers to Random Lasers

Owing to exotic optical responses, metallic nanoparticles and nanostructures are finding broad applications in laser science, leading to numerous design variations of plasmonic nanolasers. Nowadays, two of the most intriguing plasmonic nanolasing devices are spasers and random lasers. While a spaser is based on a single metallic nanoparticle resonator with the optical feedback provided by the localized surface plasmon resonance, the operation of a random laser relies on multiple light scattering within randomly distributed metallic nanoparticles. In this paper, an up‐to‐date review on the applications of metallic nanoparticles in spasers and random lasers is provided. Principles of a random spaser, a device combining the features of a spaser and a random laser, are briefly discussed as well. The paper is focused on major theoretical and experimental approaches to control the core metrics of lasing performance, including threshold, resonant wavelength, and emission directionality. The applications of spasers and random lasers in the fields of sensing and imaging are also mentioned. Finally, the challenges and future perspectives in this area of research are discussed.     

金属微结构纳米线中等离激元传播和分光特性

本文从理论和实验两方面探讨了具有微结构的金属纳米线系统中表面等离激元传播规律和分光特性. 我们由麦克斯韦方程组出发, 利用严格耦合波近似和有限元差分等方法首先从理论上给出了金属纳米线系统中等离激元的色散关系和能带特征, 然后基于微结构的银纳米线及其等离激元能带结构, 设计并制备出等离激元分光原型器件, 实验展示其将不同频率的光在微小空间分离的特性. 该研究结果是我们前期相关工作的延续和补充, 可应用于构造多功能集成的光子芯片和新型亚波长光电材料和器件.     

Influence of cavity geometry towards plasmonic gap tolerance and respective near-field in nanoparticle-on-mirror

In this work, we emphasize the importance of cavity geometry along with nanoparticle shape and plasmonic nanogap (based on a nanoparticle on a metallic film (NPOM) design) which plays significant role in understanding the complex plasmonic mode characteristics involving nanoparticle and gap mode resonances. The cross-section imprint of planar cavity on metallic film plays decisive role in near field enhancement properties at similar NP size and plasmonic nanogap conditions for spherical and cubical NPOM systems. By mimicking the NPOM structure to metal-insulator-metal design, we understand the resonant emission differences for the respective plasmonic modes. Influence of dominant and weaker gap mode resonances resulted in an interesting optical behavior (fluctuations in near field enhancement strength) in NP mode in case of cubical nanostructures. By such extensive investigation and interpretation of sub-wavelength complex plasmonic mode characteristics, various practical applications in plasmonics field can be accomplished.     

Fano resonance in novel plasmonic nanostructures

Recently, a large number of experimental and theoretical works have revealed a variety of plasmonic nanostructures with the capabilities of Fano resonance (FR) generation. Among these structures, plasmonic oligomers consisting of packed metallic nanoelements with certain configurations have been of significant interest. Oligomers can exhibit FR independently of the polarization direction based on dipole–dipole antiparallel modes without the need to excite challenging high‐order modes. The purpose of this review article is to provide an overview of recent achievements on FR of plasmonic nanostructures in recent years. Meanwhile, more attention is given to the optical properties of plasmonic oligomers due to the high potential of such structures in optical spectra engineering.     

Nanowire plasmonic waveguides,circuits and devices

As typical one‐dimensional nanostructures for waveguiding tightly confined optical fields beyond the diffraction limit, metal nanowires have been used as versatile nanoscale building blocks for functional plasmonic and photonic structures and devices. Metal nanowires, especially those fabricated by bottom‐up synthesis such as Ag and Au nanowires, usually exhibit excellent diameter uniformity and surface smoothness with diameters down to tens of nanometers, which offers great opportunities for plasmonic waveguiding of optical fields with deep‐subwavelength confinement, coherence maintenance and low scattering losses. Based on nanowire plasmonic waveguides, a variety of applications ranging from plasmonic couplers, interferometers, resonators to photon emitters have been reported in recent years. In this article, significant progresses in these nanowire plasmonic waveguides, circuits and devices are reviewed. Future outlook and challenges are also discussed.     

Oblique angle deposition and its applications in plasmonics

Plasmonics based on localized surface plasmon resonance （LSPR） has found many exciting appli- cations recently. Those applications usually require a good morphological and structural control of metallic nanostructures. Oblique angle deposition （OAD） has been demonstrated as a powerful technique for various plasmonic applications due to its advantages in controlling the size, shape, and composition of metallic nanostructures. In this review, we focus on the fabrication of metallic nanostructures by OAD and their applications in plasmonics. After a brief introduction to OAD technique, recent progress of applying OAD in fabricating noble metallic nanostructures for LSPR sensing, surface-enhanced Raman scattering, surface-enhanced infrared absorption, metal-enhanced fluorescence, and metamaterials, and their corresponding properties are reviewed. The future requirements for OAD plasmonics applications are also discussed.     

表面等离激元结构光照明显微成像技术研究进展

结构光照明显微成像技术(SIM)因其高分辨、宽场、快速成像的优势,在生物医学成像领域发挥了不可估量的作用.结构光照明显微成像技术与动态可控的亚波长表面等离激元条纹相结合,可以在不借助非线性效应的情况下,将传统SIM的分辨率从2倍于衍射极限频率提升到3 4倍,此外还有抑制背景噪声、提升信噪比的能力,在近表面的生物医学成像应用中有重要价值.本文介绍了表面等离激元结构光照明显微成像技术的原理,并总结了近几年国内外的相关研究进展.     

Deviating from the nanorod shape: Shape-dependent plasmonic properties of silver nanorice and nanocarrot structures

Noble metallic nanostructures exhibit special optical properties resulting from excitation of surface plasmons. Among the various metallic nanostructures, nanorods have attracted particular attention because of their unique and intriguing shape-dependent plasmonic properties. Nanorods can support transverse and longitudinal plasmon modes, the latter ones depending strongly on the aspect ratio of the nanorod. These modes can be routinely tuned from the visible to the near-infrared spectral regions. Although nanorods have been investigated extensively, there are few studies devoted to nanostructures deviating from the nanorod shape. This review provides an overview of recent progress in the development of two kinds of novel quasi-one-dimensional silver nanostructures, nanorice and nanocarrot, including their syntheses, crystalline characterizations, plasmonic property analyses, and performance in plasmonic sensing applications.     

Design and fabrication of structural color by local surface plasmonic meta-molecules

In this paper, we propose a new form of nanostructures with Al film deposited on a patterned dielectric material for generating structural color, which is induced by local surface plasmonic resonant(LSPR) absorption in sub-wavelengthindented hole/ring arrays. Unlike other reported results obtained by using focus ion beam(FIB) to create metallic nanostructures, the nano-sized hole/ring arrays in Al film in this work are replicated by high resolution electron beam lithography(EBL) combined with self-aligned metallization. Clear structural color is observed and systematically studied by numerical simulations as well as optical characterizations. The central color is strongly related to the geometric size, which provides us with good opportunities to dye the colorless Al surface by controlling the hole/ring dimensions(both diameter and radius), and to open up broad applications in display, jewelry decoration, green production of packing papers, security code,and counterfeits prevention.     

Semi-Ellipsoid Nanoarray for Angle-Independent Plasmonic Color Printing

Employing a silver nano semi-ellipsoid nanoarray with high symmetry into applications in plasmonic color printing, we fulfill printing images with colors independent of observing angles. Also, by decreasing the period of a nano semi-ellipsoid array into deep-subwavelength scales, we obtain high reflectivity over 50%, promising high efficiency for imaging generations. A facile technique based on the transfer of anodized aluminum oxide template is developed to fabricate the silver nano semi-ellipsoid nanoarray, realizing plasmonic color printing with features of low cost, scalable, full color and high flexibility. Our approach provides a feasible way to address the angledependent issue in the previous practice of plasmonic color printing, and boosts this field on its way to real-world commercial applications.     

Optical response of small-diameter semiconducting carbon nanotubes under exciton-surface-plasmon coupling

We analyze the optical response of small-diameter (?1 nm) semiconducting carbon nanotubes under the exciton-surface-plasmon coupling. Calculated optical absorption lineshapes exhibit the significant line (Rabi) splitting ∼0.1-0.3 eV as the exciton energy is tuned to the nearest interband surface plasmon resonance of the nanotube so that the mixed strongly coupled surface plasmon-exciton excitations are formed. We discuss possible ways to bring the exciton in resonance with the surface plasmon. The exciton-plasmon Rabi splitting effect we predict here for an individual carbon nanotube is close in its magnitude to that previously reported for hybrid plasmonic nanostructures artificially fabricated of organic semiconductors deposited on metallic films. We believe this effect may be used for the development of carbon nanotube based tunable optoelectronic device applications in areas such as nanophotonics and cavity quantum electrodynamics.     

表面等离子体平面金属透镜及其应用

由于衍射极限的存在,传统光学透镜成像分辨率理论上只能达到入射光波长的一半。通过恢复和增强携带物体细部特征信息的高频倏逝波,基于表面等离子体的平面金属透镜有望突破这种光学衍射极限,实现超分辨成像。本文对平面薄膜式与纳米结构式两类平面金属透镜进行了综述,详细介绍了若干典型平面金属透镜的结构设计、工作机理及其聚焦性能,并对其特点与存在的问题进行了分析与讨论。由于光波在金属中传播时存在一定损耗,如何更高效地增强高频倏逝波信号并转换成传播波,使其参与成像,以更好地实现远场超分辨成像,以及如何进一步增大近场超高分辨率聚焦光斑焦深以及减小远场聚焦光斑尺寸,是表面等离子体平面金属透镜进一步研究的重点。     

Nano‐imaging through tip‐enhanced Raman spectroscopy: Stepping beyond the classical limits

The spatial resolution in optical imaging is restricted by so‐called diffraction limit, which prevents it to be better than about half of the wavelength of the probing light. Tip‐enhanced Raman spectroscopy (TERS), which is based on the SPP‐induced plasmonic enhancement and confinement of light near a metallic nanostructure, can however, overcome this barrier and produce optical images far beyond the diffraction limit. Here in this article, the basic phenomenon involved in TERS is reviewed, and the high spatial resolution achieved in optical imaging through this technique is discussed. Further, it is shown that when TERS is combined with some other physical phenomena, the spatial resolution can be dramatically improved. Particularly, by including tip‐applied extremely localized pressure in TERS process, it has been demonstrated that a spatial resolution as high as 4 nm could be achieved.     

Theoretical study of photon emission from single quantum dot emitter coupled to surface plasmons

Motivated by the recent pioneering advances on nanoscale plasmonics and also nanophotonics technology based on the surface plasmons (SPs), in this work, we give a master equation model in the Lindblad form and investigate the quantum optical properties of single quantum dot (QD) emitter coupled to the SPs of a metallic nanowire. Our main results demonstrate the QD luminescence results of photon emission show three distinctive regimes depending on the distance between QD and metallic nanowire, which elucidates a crossover passing from being metallic dissipative for much smaller emitter-nanowire distances to surface plasmon (SP) emission for larger separations at the vicinity of plasmonic metallic nanowire. Besides, our results also indicate that, for both the resonant case and the detuning case, through measuring QD emitter luminescence spectra and second-order correlation functions, the information about the QD emitter coupling to the SPs of the dissipative metallic nanowire can be extracted. This theoretical study will serve as an introduction to understanding the nanoplasmonic imaging spectroscopy and pave a new way to realize the quantum information devices.     

阴极荧光在表面等离激元研究领域的应用

表面等离激元以其独特的光学性质广泛应用于纳米尺度的局域电磁场增强、超高分辨成像及微弱光电探测.阴极荧光是电子与物质相互作用而产生的光学响应,利用电子束激发金属纳米结构能够实现局域等离激元共振,并在亚波长尺度实现对共振模式的调控,具有超高空间分辨的成像特点.阴极荧光探测通常结合扫描电子显微镜或透射电子显微镜而实现,目前己被应用于表面等离激元的探测及共振模式的分析.本文从阴极荧光物理机理出发,综述了单一金属纳米结构和金属耦合结构的等离激元共振模式阴极荧光研究进展,并总结了阴极荧光与角分辨、时间分辨以及电子能量损失谱等关键技术相结合的应用,进一步分析了其面临的关键问题,最后展望了阴极荧光等离激元研究方向.     

New progress of plasmonics in complex metal nanostructures

Noble metal nanostructures possess novel optical properties because of their collective electronic oscillations,known as surface plasmons(SPs).The resonance of SPs strongly depends on the material,surrounding environment,as well as the geometry of the nanostructures.Complex metal nanostructures have attracted research interest because of the degree of freedom in tailoring the plasmonic properties for more advanced applications that are unattainable by simple ones.In this review,we discuss the plasmonic properties of several typical types of complex metal nanostructures,that is,electromagnetically coupled nanoparticles(NPs),NPs/metal films,NPs/nanowires(NWs),NWs/NWs,and metal nanostructures supported or coated by dielectrics.The electromagnetic field enhancement and surface-enhanced Raman scattering applications are mainly discussed in the NPs systems where localized SPs have a key role.Propagating surface plasmon polaritons and relevant applications in plasmonic routers and logic gates using NWs network are also reviewed.The effect of dielectric substrates and surroundings of metal nanostructures to the plasmonic properties is also discussed.     

Recent progress on photoluminescence from plasmonic nanostructures: Phenomenon,mechanism, and application

Photoluminescence(PL) from bulk noble metals arises from the interband transition of bound electrons. Plasmonic nanostructures can greatly enhance the quantum yield of noble metals through the localized surface plasmon. In this work,we briefly review recent progress on the phenomenon, mechanism, and application of one-photon PL from plasmonic nanostructures. Particularly, our recent efforts in the study of the PL peak position, partial depolarization, and mode selection from plasmonic nanostructures can bring about a relatively complete and deep understanding of the physical mechanism of one-photon PL from plasmonic nanostructures, paving the way for future applications in plasmonic imaging,plasmonic nanolasing, and surface enhanced fluorescence spectra.     

Hotspot decorations map plasmonic patterns with the resolution of scanning probe techniques

In high definition mapping of the plasmonic patterns on the surfaces of nanostructures, the diffraction limit of light remains an important obstacle. Here we demonstrate that this diffraction limit can be completely circumvented. We show that upon illuminating nanostructures made of nickel and palladium, the resulting surface-plasmon pattern is imprinted on the structures themselves; the hotspots (regions of local field enhancement) are decorated with overgrowths, allowing for their subsequent imaging with scanning-probe techniques. The resulting resolution of plasmon pattern imaging is correspondingly improved.     

Two‐Dimensionally Ordered Plasmonic and Magnetic Nanostructures on Transferable Electron‐Transparent Substrates

Discovery of new plasmonic behaviors from nanostructured materials can be greatly accelerated by the ability to prepare and characterize their near‐field behaviors with high resolution in a rapid manner. Here, an efficient and cost‐effective way is reported to make 2D periodic nanostructures on electron‐transparent substrates for rapid characterization by transmission electron microscopy. By combining nanosphere lithography with a substrate float‐off technique, large areas of electron‐transparent periodic nanostructures can be achieved. For this study, the synthesis of plasmonic nanostructures of Ag, magnetic nanostructures of Co, and bimetallic nanostructures of Ag–Co are investigated. Characterization of the materials by a combination of transmission electron microscopy, far‐field optical spectroscopy, and magnetization measurements reveals that this new approach can yield useful nanostructures on transparent, flexible, and transferable substrates with desirable plasmonic and/or magnetic properties.