衍射极限尺度下的亚波长电磁学

作为波的本性之一,衍射是现代物理学的重要研究内容.衍射导致自由空间中波的能量不能被无限小地聚集,从而为成像、光刻、光存储、光波导等技术设定了一个原理性的障碍——衍射极限.对于电磁波和光波而言,尽管通过提高介质的折射率可以压缩衍射效应,但由于自然界中材料的折射率有限,该方法存在很大限制.近年来,随着表面等离子体光学的兴起,表面等离子体在超越传统衍射极限方面的能力和应用前景受到了学术界的关注.本文从亚波长电磁学的角度出发,介绍衍射极限研究的历史,综述了突破衍射极限的理论方法.首先,利用金属介质表面等离子体激元的短波长特性,可将等效波长压缩一个数量级以上,在纳米尺度实现光波的聚焦或定向传输;更进一步,通过人为设计超构材料和超构表面,利用结构化金属和介质中的局域谐振、耦合等特殊电磁响应,可实现亚波长局域相位调制、超宽带色散调控、近完美吸收、光子自旋轨道耦合等,从而突破传统理论的诸多局限,为下一代电磁学和光学功能器件奠定重要基础.     

Optical beam splitting and asymmetric transmission in bi-layer metagratings

In this work, inspired by advances in twisted two-dimensional materials, we design and study a new type of optical bi-layer metasurface system, which is based on subwavelength metal slit arrays with phase-gradient modulation, referred to as metagratings(MGs). It is shown that due to the found reversed diffraction law, the interlayer interaction that can be simply adjusted by the gap size can produce a transition from optical beam splitting to high-efficiency asymmetric transmission of incident light from two opposite directions. Our results provide new physics and some advantages for designing subwavelength optical devices to realize efficient wavefront manipulation and one-way propagation.     

Principal waves in multiply connected waveguide light concentration on subwavelength areas

The use of multiply connected waveguides for localizing and concentrating optical electromagnetic radiation onto areas of subwavelength transverse dimension is substantiated. In optical microscopes, this makes it possible to reach subwavelength resolution along all three spatial coordinates. In experiments where interaction of laser radiation with matter is studied, the subwavelength multiply connected waveguides allow the field intensity in the optical wave to be significantly increased. Applications of subwavelength waveguides in submicrometer photolithograpy, submicrosecond microelectronics, and photogrammetry is discussed, as well as for optical information recording and readout.     

Subwavelength-sized beam spot confinement using an optical thin core dielectric ridge-type waveguide in leaky mode

Optical near-field plasmon-enhancement techniques have made marked contributions to the field of subwavelength optical beam spot generation for photonic devices. However, the metal material used is not sufficiently heat resistant for high-temperature operating requirements, such as laser-assisted magnetic recording storage. Therefore, it is preferable to use a dielectric material as a heat source for this application. To realize a subwavelength optical beam spot size using a dielectric material, we extensively analyze a leaky mode dielectric waveguide with a thin core. Finally, we show a new ridge-type dielectric waveguide using a leaky mode operation for a subwavelength beam spot.     

Extraordinary acoustic transmission through a 1D grating with very narrow apertures

Recently, there has been an increased interest in studying extraordinary optical transmission (EOT) through subwavelength aperture arrays perforated in a metallic film. In this Letter, we report that the transmission of an incident acoustic wave through a one-dimensional acoustic grating can also be drastically enhanced. This extraordinary acoustic transmission (EAT) has been investigated both theoretically and experimentally, showing that the coupling between the diffractive wave and the wave-guide mode plays an important role in EAT. This phenomenon can have potential applications in acoustics and also might provide a better understanding of EOT in optical subwavelength systems.     

THz波段亚波长半导体球形阵列光学特性的数值模拟

利用T-matrix方法对太赫兹波段亚波长半导体球形阵列进行了数值模拟并在数值模拟结果的基础上讨论了其光学特性。在太赫兹波段可以通过掺杂等手段调节半导体的表面等离子体特性。以半导体InSb为例并采用Drude模型,对单个亚波长球及两个或多个亚波长球组成的阵列进行了数值模拟,主要以归一化消光截面为参数,讨论了不同阵元半径、不同球形单元间距、不同单元数目及入射波不同极化方向对阵列特性的影响。     

Near-field observation of subwavelength confinement of photoluminescence by a photonic crystal microcavity

We present a direct, room-temperature near-field optical study of light confinement by a subwavelength defect microcavity in a photonic crystal slab containing quantum-well sources. The observations are compared with three-dimensional finite-difference time-domain calculations, and excellent agreement is found. Moreover, we use a subwavelength cavity to study the influence of a near-field probe on the imaging of localized optical modes.     

Optical spin-dependent angular shift in structured metamaterials

The physics behind the spin (polarization)-dependent electromagnetic hot-spot phenomenon is due to the presence of the geometric phase resulting from the optical spin-orbit interaction in the interaction of light with the subwavelength microstructures. Unlike the tiny spin-dependent shift of light associated with the usual spin-Hall effect of light, here we present the distinct polarization-dependent angular shift by employing an array of subwavelength metallic apertures. More importantly, this novel electromagnetic precession is accompanied by the extraordinary optical transmission phenomenon and offers the exciting possibilities for novel applications for subwavelength structured metallic systems.     

Enhancement of light extraction from freestanding GaN nanocolumn slab with bottom subwavelength nanostructures

It is of great interest to investigate a freestanding GaN nanocolumn slab with bottom subwavelength nanostructures. A low-index buffer layer offers more paths accessible to emit light in air, and the nanostructures break the total internal reflection condition at the bottom surface to improve the light-extraction efficiency. The GaN nanocolumns and subwavelength nanostructures can also effectively suppress optical reflection over a broad wavelength range. In this work, the freestanding GaN nanocolumn slabs with bottom subwavelength nanostructures are implemented with a diameter of 1200 μm by a combination of self-assembly technique, silicon-on-insulator (SOI) technology, manufacturing of silicon, and epitaxial growth of GaN. Reflectance results experimentally show that optical reflection is greatly reduced by introducing the GaN nanocolumns and subwavelength nanostructures, and photoluminescence measurements demonstrate that the extracted light is significantly enhanced with the assistance of the low-index buffer layer and bottom subwavelength nanostructures.     

Subwavelength imaging enhancement through a three-dimensional plasmon superlens with rough surface

In this Letter we investigate the subwavelength imaging of a three-dimensional plasmon superlens based on the full vector wave simulations of optical wave propagation and transmission. The optical transfer functions are computed. Comparisons are made between the results of lenses with flat and periodic/random rough surfaces. We also study the problem of practical imaging system geometry using laser as an illumination source. Results show that the lens with periodic or random roughness can reduce the field interference effects, and provide improved focus on the transmission field and the Poynting flux. We illustrate that the subwavelength roughness in a plasmon lens can enhance the image resolution over a flat lens for both matched and unmatched permittivity conditions. The enhancement of resolution occurs because the introduced subwavelength roughness can amplify the evanescent wave components and suppress the surface plasmon resonance peaks.     

基于一维金属光子晶体平凹镜的柱矢量光束亚波长聚焦

柱矢量光束具有柱对称性的偏振分布,其独特的光场分布和聚焦特性被广泛应用于光学微操纵及光学成像等领域,并迅速向亚波长尺度拓展.通常,亚波长尺度聚焦采用等离激元透镜实现,但存在光场调控的偏振态局限性.而借助光子晶体的负折射效应,不仅能够实现亚波长聚焦或成像,而且应对正交偏振态同时有效.采用对电磁波具有更强调控能力的一维金属光子晶体结构,计算得到的能带结构和等频曲线表明其负折射效应在特定波段对正交偏振态同时有效.在此基础上设计出一维金属光子晶体柱对称平凹镜结构,通过有限元算法模拟显示了可见光波段的径向和旋向偏振光的同时亚波长聚焦行为.进一步的结果表明,改变柱矢量光束的偏振组分能够直接有效地调节焦场空间分布及偏振分布特性.所提出的平凹镜结构能够实现对任意偏振组分的柱矢量光束的亚波长尺度聚焦,且该结构的设计对于各波段情况均有参考意义.该研究结果对小尺度粒子的光学微操纵、超分辨率成像等相关领域具有潜在的应用价值.     

Subwavelength propagation and localization of light using surface plasmons: A brief perspective

Surface plasmons at the metal–dielectric interface have emerged as an important candidate to propagate and localize light at subwavelength scales. By tailoring the geometry and arrangement of metallic nanoarchitectures, propagating and localized surface plasmons can be obtained. In this brief perspective, we discuss: (1) how surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs) can be optically excited in metallic nanoarchitectures by employing a variety of optical microscopy methods; (2) how SPPs and LSPs in plasmonic nanowires can be utilized for subwavelength polarization optics and single-molecule surface-enhanced Raman scattering (SERS) on a photonic chip; and (3) how individual plasmonic nanowire can be optically manipulated using optical trapping methods.     

Lessons from nature: biomimetic subwavelength structures for high‐performance optics

In nature, optical structures in the subwavelength range have been evolved over millions of years. For example, in the form of ‘moth‐eye’ structures they show a strong anti‐reflective effect on the compound eyes of night‐active insects and therefore offer a successful protection over predators. In this contribution the advantages and challenges to transfer this natural concept of subwavelength structured optical interfaces to high‐end optical systems are discussed. Here, in comparison to alternative conventional multilayer systems, the bioinspired anti‐reflective structures offer a wide wavelength range and a broad angle dependency. Additionally, adhesion problems are reduced drastically. Simultaneously to the theoretical consideration of the best profile form of the subwavelength structures, appropriate realization technologies have been developed in recent years, where both top‐down and bottom‐up approaches have been investigated. Depending on the choice of the structuring technique, anti‐reflective subwavelength structures are applicable to a wide spectrum of optical elements ranging from micro‐optical components to aspheres for applications in imaging and also illumination setups of high‐end optical instruments.     

Theoretical near-field optical properties of branched plasmonic nanoparticle networks

Extended branched networks of single-nanoparticle chains have recently been self-assembled from colloidal suspensions. In particular, gold nanoparticle linear chains and complex chain networks have revealed unique signatures of plasmon modes in their extinction spectra. In this Letter, we investigate theoretically their near-field optical properties and show that a real space mapping of these modes can be achieved with a photon scanning tunneling microscope setup. A distinct subwavelength patterning of the optical near-field gives rise to well-resolved photon scanning tunneling microscope images that can be used to identify the network segments able to efficiently carry optical energy.     

Control amplitude and phase of light by plasmonic meta-hologram with T-shaped nano-cavity

Controlling both amplitude and phase of light in the subwavelength scale is a challenge for traditional optical devices. Here, we propose and numerically investigate a novel plasmonic meta-hologram, demonstrating broadband manipulation of both phase and amplitude in the subwavelength scale. In the meta-hologram, phase modulation is achieved by the detour phase distribution of unit cells, and amplitude is continuously modulated by a T-shaped nano-cavity with tunable plasmonic resonance. Compared to phase-only holograms, such a metahologram could reconstruct three-dimensional(3D) images with higher signal-to-noise ratio and better image quality, thus offering great potential in applications such as 3D displays, optical communications, and beam shaping.     

Tapered plasmonic waveguides with efficient and broadband field transmission

In this paper we suggest, numerically simulate and discuss a tapered plasmonic nanostructure which allows efficient transmission of the wave beam energy into a weakly subwavelength (λ/4 … λ/2) spatial region within a broad frequency range. The frequency stable location of this region becomes possible due to unusual plasmonic modes we have theoretically revealed and used. Our structure is promising for prospective transducers between conventional optical waveguides and subwavelength nanoguides or nanocircuits.     

基于超材料的中波红外宽带偏振转换研究

基于硅纳米块阵列和亚波长金属光栅,硅纳米块长轴与金属光栅夹角为45°,本文设计了一种高效、宽带偏振转换结构.模拟计算表明该结构实现了线偏振光90°旋转,在3.4—4.5μm波段偏振转换率大于60%,在3—5μm光谱范围内的转换对比率大于10~4.由于该结构光学性能优异,制备难度低,可以应用于光传输控制.     

新兴的纳米结构亚波长光学器件及其发展

介绍了新颖的纳米结构亚波长光学器件的特点,从器件的基本功能特性和工作机理两个方面详细讨论了其应用状况,并阐述了纳米制造技术对亚波长光学器件发展的重要推动作用及这一领域今后的研究热点。     

Sub-diffraction-limited far-field imaging in infrared

We investigated a far-field superlens operating at mid-infrared wavelength that allows resolving subwavelength features in the far-field. By utilizing evanescent enhancement provided by surface plasmon excitation of silver nanorods and Moiré effect, we numerically demonstrated that subwavelength information of an object can be converted to propagating information. This information can then be captured by conventional optical components. A simple image reconstruction algorithm can restore the subwavelength object. A sub-diffraction-limited resolution of 2.5 μm at 6-μm wavelength is demonstrated.