Transformation optics and cloaking

Invisibility, a long sought-for speculation in science fiction, has been turned into reality in the laboratory through the use of a theoretical technique called Transformation Optics. The principles of transformation optics show that any desired smooth deformation of the electromagnetic field can be implemented exactly by an appropriately engineered metamaterial. All demonstrations of cloaking to date have had limitations, however, reflecting our technological inability to implement the transformation optics algorithm exactly. However, the scientific principles leading to perfect invisibility are now established, and practical improvements on the initial designs are now occurring very rapidly. Most recently, researchers have re-examined transformation optics to include time as well as space, describing and then implementing the concept of a cloak that hides events, a conceptual breakout that promises many new applications. This review describes the general ideas underlying transformation optics, and how the various types of cloak based on these ideas have been implemented practically to date.     

Phase‐preserved macroscopic visible‐light carpet cloaking beyond two dimensions

Transformation optics, a recent geometrical design strategy of light manipulation with both ray trajectories and optical phase controlled simultaneously, promises an invisibility cloaking device that can render a macroscopic object invisible even to a scientific instrument measuring optical phase. Recent “carpet” cloaks have extended their cloaking capability to broadband frequency ranges and macroscopic scales, but they only demonstrated the recovery of ray trajectories after passing through the cloaks, while whether the optical phase would reveal their existence still remains unverified. In this paper, a phase‐preserved macroscopic visible‐light carpet cloak is demonstrated in a geometrical construction beyond two dimensions. As an extension of previous two‐dimensional (2D) macroscopic carpet cloaks, this almost‐three‐dimensional carpet cloak exhibits three‐dimensional (3D) invisibility for illumination near its center (i.e. with a limited field of view), and its ideal wide‐angle invisibility performance is preserved in multiple 2D planes intersecting in the 3D space. Optical path length is measured with a broadband pulsed‐laser interferometer, which provides unique experimental evidence on the geometrical nature of transformation optics.

     

Invisibility cloaking in light‐scattering media

A major aim of researchers working in the field of optics and photonics is to mold the flow of light in optical structures and devices. In the regime of ballistic light propagation, transformation optics has given a certain boost, for which optical invisibility cloaking devices are striking examples. Our capability to mold the flow of light in the regime of diffuse light propagation in light‐scattering media has fallen behind—while diffuse light from clouds, white wallpaper, computer monitors, and light‐emitting diodes is literally all around us every day. In this review, we summarize progress in steering the flow of diffuse light in turbid media which was triggered by the mathematical analogy between electrostatics, magnetostatics, stationary heat conduction, and stationary light diffusion. We give an extensive tutorial introduction to the mathematics of the diffusion equation for light and its solutions, present an overview on the current experimental state‐of‐the‐art of simple core–shell invisibility cloaking, and compare these experiments with diffusion theory as well as with more advanced modelling based on Monte Carlo simulations. The latter approach enables spanning the bridge from diffusive to ballistic light propagation.

     

微波超材料隐形结构及其新型快速实验方案

基于Maxwell方程组在坐标变换下的协变性,利用柱坐标变换以及开口谐振环周期排列而形成的超材料设计了一个工作于10.14GHz频率下的隐形结构.采用一种新型快速实验方案测量微波信号强度.实验结果表明在外围增加了隐形结构之后,圆柱金属的阴影和散射效应都被削弱,得到了预期的隐形效果.该实验系统降低了系统的结构复杂性并缩短了实验周期,为该领域的研究提供了一种新的简单有效的实验方案.     

Nano‐opto‐mechanical effects in plasmonic waveguides

In order to achieve interaction between light beams, a mediating material object is required. Nonlinear materials are commonly used for this purpose. Here a new approach to control light with light, based on a nano‐opto‐mechanical system integrated in a plasmonic waveguide is proposed. Optomechanics of a free‐floating resonant nanoparticle in a subwavelength plasmonic V‐groove waveguide is studied. It is shown that nanoparticle auto‐oscillations in the waveguide induced by a control light result in the periodic modulation of a transmitted plasmonic signal. The modulation depth of 10% per single nanoparticle of 25 nm diameter with the clock frequencies of tens of MHz and the record low energy‐per‐bit energies of 10⁻¹⁸ J is observed. The frequency of auto‐oscillations depends on the intensity of the continuous control light. The efficient modulation and deep‐subwavelength dimensions make this nano‐optomechanical system of significant interest for opto‐electronic and opto‐fluidic technologies.     

Dielectric‐loaded plasmonic waveguide components: Going practical

Surface plasmon propagating modes supported by metal/dielectric interfaces in various configurations can be used for radiation guiding similarly to conventional dielectric waveguides. Plasmonic waveguides offer two attractive features: subdiffraction mode confinement and the presence of conducting elements at the mode‐field maximum. The first feature can be exploited to realize ultrahigh density of nanophotonics components, whereas the second feature enables the development of dynamic components controlling the plasmon propagation with ultralow signals, minimizing heat dissipation in switching elements. While the first feature is yet to be brought close to the domain of practical applications because of high propagation losses, the second one is already being investigated for bringing down power requirements in optical communication systems. In this review, the latest application‐oriented research on radiation modulation and routing using thermo‐optic dielectric‐loaded plasmonic waveguide components integrated with silicon‐based photonic waveguides is overviewed. Their employment under conditions of real telecommunications is addressed, highlighting challenges and perspectives.     

Controllable femtosecond laser‐induced dewetting for plasmonic applications

Dewetting of thin metal films is one of the most widespread method for functional plasmonic nanostructures fabrication. However, simple thermal‐induced dewetting does not allow to control degree of nanostructures order without additional lithographic process steps. Here we propose a novel method for lithography‐free and large‐scale fabrication of plasmonic nanostructures via controllable femtosecond laser‐induced dewetting. The method is based on femtosecond laser surface pattering of a thin film followed by a nanoscale hydrodynamical instability, which is found to be very controllable under specific irradiation conditions. We achieve control over degree of nanostructures order by changing laser irradiation parametrs and film thickness. This allowed us to exploit the method for the broad range of applications: resonant light absorbtion and scattering, sensing, and potential improving of thin‐film solar cells.

     

Searching for better plasmonic materials

Plasmonics is a research area merging the fields of optics and nanoelectronics by confining light with relatively large free‐space wavelength to the nanometer scale ‐ thereby enabling a family of novel devices. Current plasmonic devices at telecommunication and optical frequencies face significant challenges due to losses encountered in the constituent plasmonic materials. These large losses seriously limit the practicality of these metals for many novel applications. This paper provides an overview of alternative plasmonic materials along with motivation for each material choice and important aspects of fabrication. A comparative study of various materials including metals, metal alloys and heavily doped semiconductors is presented. The performance of each material is evaluated based on quality factors defined for each class of plasmonic devices. Most importantly, this paper outlines an approach for realizing optimal plasmonic material properties for specific frequencies and applications, thereby providing a reference for those searching for better plasmonic materials.     

Large‐area,polarisation‐sensitive plasmonic materials from colloidal lithography

The development of simple to prepare, polarization‐sensitive plasmonic apertures with two plasmonic modes, is described. To achieve these results, monocrystalline nanosphere lithography masks of 438 nm polystyrene spheres are modified with reactive ion etching before silver is subsequently evaporated through the mask at varied angles. As the angle of evaporation increases, round apertures, elliptical apertures or lines with bow‐tie like features between two lines are produced. A primary plasmon mode is shown at 570 nm, while a tunable plasmon mode is demonstrated between 700 nm and 900 nm. Finite‐difference time‐domain calculations agree with the observed results and predict that this method of fabrication can produce tunable plasmonic features throughout the NIR optical telecommunication wavelength range. Lastly, the excitation polarization angle is compared with that of plasmonic nanorods and asymmetric nano‐apertures systems to describe why the excitation polarization of the low energy mode is orthogonal to the long axis of the apertures. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Levitation of finite‐size electric dipole over epsilon‐near‐zero metamaterial

Following the suggestion of Rodriguez‐Fortuno et al. [Phys. Rev. Lett. 112 , 033902 (2014)], we study the repulsive force acting on a electric dipole placed over a surface of epsilon‐near‐zero (ENZ) metamaterial. The dependence of the repulsive force value on the dipole size has been studied. We show that the effect of finite size drastically affects the values of the repulsive force as compared to the point‐dipole case. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

隐身材料

由于超构材料(metamaterials)的发现,隐身(cloaking)科学近年来取得了长足的发展.与军事上的隐形(stealth)所不同的是,隐身指的是将电磁波的散射在各个方向上都完全消除,从而实现神话故事或科幻小说中那种真正的全方位、全角度的"消失".隐身也因其重要的科学价值以及将对人类生活产生跨越式的改变而成为国际上的科研热点.除了隐身之外,科学家还证明了可以将物体变成任意的幻像,从而在光学上实现了孙悟空的"七十二变",这被称为幻像光学(illusion optics).未来,隐身和幻像光学的发展有望将人类的极限想象变为现实.文章介绍了作者在隐身和幻像领域里的一些工作.     

隐声衣结构设计和实验研究新进展

隐声衣是一项使物体隐藏于声场的新技术,与传统吸声方式相比,隐声衣消除回波时不会在目标背后留下声影区。隐声衣的物理实现是重点研究方向之一,利用具有特殊性质的材料或结构消除散射是获得隐声效果的主要途径。文章综述了隐声衣研究在结构设计和实验方面的几项新发展。包括基于超常材料的隐声衣、基于温度递度的隐声衣、应用反演设计方法的隐声衣、有源隐声衣等,主要介绍隐声衣机理和结构设计方面的新思想,以及新型人工材料和人工结构在隐声衣研究中的应用。     

Dark plasmonic modes in diatomic gratings for plasmoelectronics

Owing to the unique ability of nanostructured metals to confine and enhance light waves along metal‐dielectric interfaces, plasmonics has enabled unprecedented flexibility in manipulating light at the deep‐subwavelength scale. With regard to the spectral behavior of plasmonic resonances, the spectral location of a resonance can be tailored with relative ease while the control over the spectral linewidth represents a more daunting task. In this paper, we present sharp resonance features by introducing dark plasmonic modes in diatomic gratings. The induced asymmetry in the metallic structure facilitates the generation of a dark mode with significantly suppressed radiative loss leading to an ultra‐sharp spectral feature ∼5 nm wide. We further use this metallic structure as an optoelectronic platform for the transduction of light waves to electrical signals via a plasmoelectric effect. The light concentrating ability of dark plasmonic modes, in conjunction with the ultra‐sharp resonance feature at a relatively low loss offers a novel route to enhanced light‐matter interactions with high spectral sensitivity for diverse applications.

     

局域共振型声学超材料机理探讨

本文以二维固体薄板中的弹性波传播为例, 对基于共振子结构的声学超材料带隙机理进行了探讨, 证明在声学超材料中带隙形成既与共振子对波的散射相位有关, 也与波在共振体之间的几何传播相位有关. 通过调节散射相位和几何传播相位均能实现对色散关系的调控. 基于这一理解, 探究了弹性波超材料中的次波长缺陷态和负折射现象的实现条件.     

Broadband cross-circular polarization carpet cloaking based on a phase change material metasurface in the mid-infrared region

In view of the fact that most invisibility devices focus on linear polarization cloaking and that the characteristics of mid-infrared cloaking are rarely studied, we propose a cross-circularly polarized invisibility carpet cloaking device in the mid-infrared band. Based on the Pancharatnam–Berry phase principle, the unit cells with the cross-circular polarization gradient phase were carefully designed and constructed into a metasurface. In order to achieve tunable cross-circular polarization carpet cloaks, a phase change material is introduced into the design of the unit structure. When the phase change material is in amorphous and crystalline states, the proposed metasurface unit cells can achieve high-efficiency cross-polarization conversion, and reflection intensity can be tuned. According to the phase compensation principle of carpet cloaking, we construct a metasurface cloaking device with a phase gradient using the designed unit structure. From the near- and far-field distributions, the cross-circular polarization cloaking property is confirmed in the broadband wavelength range of 9.3–11.4 µm. The proposed cloaking device can effectively resist detection of cross-circular polarization.     

Compressibility-near-zero acoustic metamaterial

This letter theoretically analyzes and experimentally demonstrates a novel class of compressibility-near-zero (CNZ) acoustic metamaterials, achieved by using resonant-type metamaterials, namely the Helmholtz resonator. We first present a closed analytical formula for the effective compressibility of the proposed unit cell and then show that two frequencies exist which may support CNZ propagation. We demonstrate how the choice of the actual operating CNZ frequency depends on the properties of the host and finally experimentally verify CNZ propagation of acoustic waves.     

Ultrathin plasmonic metamaterial for spoof localized surface plasmons

The multipolar spoof localized surface plasmons (LSPs) on a planar textured metallic disk are proposed and experimentally demonstrated at microwave frequencies. Based on ultrathin metal film printed on a thin dielectric substrate, the designed plasmonic metamaterial clearly shows multipolar plasmonic resonances, including the dipole, quadrupole, hexapole, octopole, decapole, dodecapole, and quattuordecpole modes. Both numerical simulations and experiments are in good agreement. It is shown that the spoof LSP resonances are sensitive to the disk's geometry and local dielectric environments. Hence, the ultrathin textured metallic disk may be used as plasmonic sensors and find potential applications in the microwave and terahertz frequencies.     

Mapping the near fields of plasmonic nanoantennas by scattering‐type scanning near‐field optical microscopy

Near‐field optical microscopy techniques provide information on the amplitude and phase of local fields in samples of interest in nanooptics. However, the information on the near field is typically obtained by converting it into propagating far fields where the signal is detected. This is the case, for instance, in polarization‐resolved scattering‐type scanning near‐field optical microscopy (s‐SNOM), where a sharp dielectric tip scatters the local near field off the antenna to the far field. Up to now, basic models have interpreted S‐ and P‐polarized maps obtained in s‐SNOM as directly proportional to the in‐plane ( or ) and out‐of‐plane () near‐field components of the antenna, respectively, at the position of the probing tip. Here, a novel model that includes the multiple‐scattering process of the probing tip and the nanoantenna is developed, with use of the reciprocity theorem of electromagnetism. This novel theoretical framework provides new insights into the interpretation of s‐SNOM near‐field maps: the model reveals that the fields detected by polarization‐resolved interferometric s‐SNOM do not correlate with a single component of the local near field, but rather with a complex combination of the different local near‐field components at each point (, and ). Furthermore, depending on the detection scheme (S‐ or P‐polarization), a different scaling of the scattered fields as a function of the local near‐field enhancement is obtained. The theoretical findings are corroborated by s‐SNOM experiments which map the near field of linear and gap plasmonic antennas. This new interpretation of nanoantenna s‐SNOM maps as a complex‐valued combination of vectorial local near fields is crucial to correctly understand scattering‐type near‐field microscopy measurements as well as to interpret the signals obtained in field‐enhanced spectroscopy.