Large-area flexible 3D optical negative index metamaterial formed by nanotransfer printing

Negative-index metamaterials (NIMs) are engineered structures with optical properties that cannot be obtained in naturally occurring materials. Recent work has demonstrated that focused ion beam and layer-by-layer electron-beam lithography can be used to pattern the necessary nanoscale features over small areas (hundreds of μm(2)) for metamaterials with three-dimensional layouts and interesting characteristics, including negative-index behaviour in the optical regime. A key challenge is in the fabrication of such three-dimensional NIMs with sizes and at throughputs necessary for many realistic applications (including lenses, resonators and other photonic components). We report a simple printing approach capable of forming large-area, high-quality NIMs with three-dimensional, multilayer formats. Here, a silicon wafer with deep, nanoscale patterns of surface relief serves as a reusable stamp. Blanket deposition of alternating layers of silver and magnesium fluoride onto such a stamp represents a process for 'inking' it with thick, multilayer assemblies. Transfer printing this ink material onto rigid or flexible substrates completes the fabrication in a high-throughput manner. Experimental measurements and simulation results show that macroscale, three-dimensional NIMs (>75?cm(2)) nano-manufactured in this way exhibit a strong, negative index of refraction in the near-infrared spectral range, with excellent figures of merit.     

Exploiting Colloidal Metamaterials for Achieving Unnatural Optical Refractions

The scaling down of meta-atoms or metamolecules (collectively denoted as metaunits) is a long-lasting issue from the time when the concept of metamaterials was first suggested. According to the effective medium theory, which is the foundational concept of metamaterials, the structural sizes of meta-units should be much smaller than the working wavelengths (e.g., << 1/5 wavelength). At relatively low frequency regimes (e.g., microwave and terahertz), the conventional monolithic lithography can readily address the materialization of metamaterials. However, it is still challenging to fabricate optical metamaterials (metamaterials working at optical frequencies such as the visible and near-infrared regimes) through the lithographic approaches. This serves as the rationale for using colloidal self-assembly as a strategy for the realization of optical metamaterials. Colloidal self-assembly can address various critical issues associated with the materialization of optical metamaterials, such as achieving nanogaps over a large area, increasing true 3D structural complexities, and cost-effective processing, which all are difficult to attain through monolithic lithography. Nevertheless, colloidal self-assembly is still a toolset underutilized by optical engineers. Here, the design principle of the colloidally self-assembled optical metamaterials exhibiting unnatural refractions, the practical challenge of relevant experiments, and the future opportunities are critically reviewed.     

Tailoring Optical and Plasmon Resonances in Core-shell and Core-multishell Nanowires for Visible Range Negative Refraction and Plasmonic Light Harvesting:A Review

Semiconductor nanowires(NWs) are sub-wavelength structures which exhibit strong optical(Mie)resonances in the visible range.In addition to such optical resonances,the localized surface plasmon resonances(LSPRs) in metal-semiconductor core-shell(CS) and core-multishell(CMS) NWs can be tailored to achieve novel negative-index metamaterials(N1M),extreme absorbers,invisibility cloaks and sensors.Particularly,in this review,we focus on our recent theoretical studies which highlight the versatility of CS and CMS NWs for:1) the design of negative-index metamaterials in the visible range and2) plasmonic light harvesting in ultrathin photocatalyst layers for water splitting.Utilizing the LSPR in the metal layer and the magnetic dipole(Mie) resonance in the semiconductor shell under transverse electric(TE) polarization,semiconductor-metal-semiconductor CMS NWs can be designed to exhibit spectrally overlapping electric and magnetic resonances in the visible range.NWs exhibiting such double resonances can be considered as meta-atoms and arrayed to form polarization dependent,low-loss NIM.Alternatively,by tuning the LSPR in the TE polarization and the optical resonance in the transverse magnetic(TM) polarization of metal-photocatalyst CS and semiconductor-metal-photocatalyst CMS NWs,the absorption within ultrathin(sub-50 nm) photocatalyst layers can be substantially enhanced.Notably,aluminum and copper based NWs provide absorption enhancement remarkably close to silver and gold based NWs,respectively.Further,such absorption is polarization independent and remains high over a large range of incidence angles and permittivity of the medium.Therefore,due to the tunability of their optical properties,CS and CMS NWs are expected to be vital components for the design of nanophotonic devices.     

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.     

Alternative Plasmonic Materials: Beyond Gold and Silver

Materials research plays a vital role in transforming breakthrough scientific ideas into next‐generation technology. Similar to the way silicon revolutionized the microelectronics industry, the proper materials can greatly impact the field of plasmonics and metamaterials. Currently, research in plasmonics and metamaterials lacks good material building blocks in order to realize useful devices. Such devices suffer from many drawbacks arising from the undesirable properties of their material building blocks, especially metals. There are many materials, other than conventional metallic components such as gold and silver, that exhibit metallic properties and provide advantages in device performance, design flexibility, fabrication, integration, and tunability. This review explores different material classes for plasmonic and metamaterial applications, such as conventional semiconductors, transparent conducting oxides, perovskite oxides, metal nitrides, silicides, germanides, and 2D materials such as graphene. This review provides a summary of the recent developments in the search for better plasmonic materials and an outlook of further research directions.     

Negative‐Index Materials: New Frontiers in Optics

A lot of recent interest has been focused on a new class of materials, the so‐called left‐handed materials (LHMs) or negative‐index materials, which exhibit highly unusual electromagnetic properties and hold promise for new device applications. These materials do not exist in nature and can only be fabricated artificially; for this reason, they are called metamaterials. Their unique properties are not determined by the fundamental physical properties of their constituents, but rather by the shape and distribution of the specific patterns included in them. Metamaterials can be designed to exhibit both electric and magnetic resonances that can be separately tuned to occur in frequency bands from megahertz to terahertz frequencies, and hopefully to the visible region of the electromagnetic spectrum. This article presents a short history of the field, describes the underlying physics, and reviews the experimental and theoretical status of the field at present. Many interesting questions on how to fabricate more isotropic LHMs, on how to push the operational frequency to optical wavelengths, how to reduce the losses, and how to incorporate active or nonlinear materials in LHMs remain to be explored further.     

New Twists of 3D Chiral Metamaterials

Rationally designed artificial materials, called metamaterials, allow for tailoring effective material properties beyond (“meta”) the properties of their bulk ingredient materials. This statement is especially true for chiral metamaterials, as unlocking certain degrees of freedom necessarily requires broken centrosymmetry. While the field of chiral electromagnetic/optical metamaterials has become rather mature, the field of elastic/mechanical metamaterials is just emerging and wide open. This research news reviews recent theoretical and experimental progress concerning 3D chiral mechanical and optical metamaterials, with special emphasis on work performed at KIT.     

Design, fabrication and characterization of indefinite metamaterials of nanowires

Indefinite optical properties, which are typically characterized by hyperbolic dispersion relations, have not been observed in naturally occurring materials, but can be realized through a metamaterial approach. We present here the design, fabrication and characterization of nanowire metamaterials with indefinite permittivity, in which all-angle negative refraction of light is observed. The bottom-up fabrication technique, which applies electrochemical plating of nanowires in porous alumina template, is developed and demonstrated in achieving uniform hyperbolic optical properties at a large scale. We developed techniques to improve the uniformity and to reduce the defect density in the sample. The non-magnetic design and the off-resonance operation of the nanowire metamaterials significantly reduce the energy loss of electromagnetic waves and make the broad-band negative refraction of light possible.     

Chiromagnetic Plasmonic Nanoassemblies with Magnetic Field Modulated Chiral Activity

Chiral plasmonic nanoassemblies, which exhibit outstanding chiroptical activity in the visible or near‐infrared region, are popular candidates in molecular sensing, polarized nanophotonics, and biomedical applications. Their optical chirality can be modulated by manipulating chemical molecule stimuli or replacing the building blocks. However, instead of irreversible chemical or material changes, real‐time control of optical activity is desired for reversible and noninvasive physical regulating methods, which is a challenging research field. Here, the directionally and reversibly switching optical chirality of magneto‐plasmonic nanoassemblies is demonstrated by the application of an external magnetic field. The gold‐magnetic nanoparticles core–satellite (Au@Fe₃O₄) nanostructures exhibit chiral activity in the UV–visible range, and the circular dichroism signal is 12 times greater under the magnetic field. Significantly, the chiral signal can be reversed by regulating the direction of the applied magnetic field. The attained magnetic field‐regulated chirality is attributed to the large contributions of the magnetic dipole moments to polarization rotation. This magnetic field‐modulated optical activity may be pivotal for photonic devices, information communication, as well as chiral metamaterials.     

平面手征超常介质研究进展

平面手征超常介质(Planar chiral metamaterials, PCM)是近年迅速发展起来的能获得大旋光性进而控制光偏振的新颖介质.综述了PCM的研究进展,包括金属结构、全介质结构以及单层和双层结构等几种不同情况,这些PCM都能产生非常强的旋光性,在光子学中具有潜在的应用前景,并且有可能成为手征负折射光学材料中有前途的候选者.     

Designed ultrafast optical nonlinearity in a plasmonic nanorod metamaterial enhanced by nonlocality

All-optical signal processing enables modulation and transmission speeds not achievable using electronics alone. However, its practical applications are limited by the inherently weak nonlinear effects that govern photon-photon interactions in conventional materials, particularly at high switching rates. Here, we show that the recently discovered nonlocal optical behaviour of plasmonic nanorod metamaterials enables an enhanced, ultrafast, nonlinear optical response. We observe a large (80%) change of transmission through a subwavelength thick slab of metamaterial subjected to a low control light fluence of 7 mJ cm(-2), with switching frequencies in the terahertz range. We show that both the response time and the nonlinearity can be engineered by appropriate design of the metamaterial nanostructure. The use of nonlocality to enhance the nonlinear optical response of metamaterials, demonstrated here in plasmonic nanorod composites, could lead to ultrafast, low-power all-optical information processing in subwavelength-scale devices.     

Metamaterials based on polymer dispersions of nanoparticles and particles of copper obtained by cathodic current pulse electrolysis

In the present paper a novel group of electromagnetic metamaterials as well as the method of their fabrication is presented. The studied metamaterials are polymer composites and nanocomposites made of polymer matrix/host (ethylene-vinyl acetate (EVA), polyethylene, polypropylene etc.) filled with copper flakes, of micrometer and/or nanometer size, as the conducting inclusions. The copper filler flakes were obtained by cathodic current pulse electrolysis from copper sulfate electrolytes at the stainless steel electrodes. SEM analysis showed that the morphology and structure of the copper deposit can be precisely controlled by applying different kind of current pulse and reversed current pulsed electrolysis. The polymer composite metamaterials formed by extrusion of small beads of polymer mixed with the copper flakes consisted of polymer matrix and copper flakes, ranging in length from 1 to 500 micrometers, and ranging in thickness from 80 nm to 2000 nm. The concentration of the copper flakes ranged from 0.5 wt% to 40 wt%, depending on the applications and required electromagnetic and mechanical properties. The studied materials were found to exhibit effective magnetic permeability that was smaller than unity, which is indicative of the typical properties of metamaterials. Present development solves technological and economical problems related to modern microelectronics methods which are currently mainly used for metamaterial fabrication.     

Optical negative refraction by four-wave mixing in thin metallic nanostructures

The law of refraction first derived by Snellius and later introduced as the Huygens-Fermat principle, states that the incidence and refracted angles of a light wave at the interface of two different materials are related to the ratio of the refractive indices in each medium. Whereas all natural materials have a positive refractive index and therefore exhibit refraction in the positive direction, artificially engineered negative index metamaterials have been shown capable of bending light waves negatively. Such a negative refractive index is the key to achieving a perfect lens that is capable of imaging well below the diffraction limit. However, negative index metamaterials are typically lossy, narrow band, and require complicated fabrication processes. Recently, an alternative approach to obtain negative refraction from a very thin nonlinear film has been proposed and experimentally demonstrated in the microwave region. However, such approaches use phase conjugation, which makes optical implementations difficult. Here, we report a simple but different scheme to demonstrate experimentally nonlinear negative refraction at optical frequencies using four-wave mixing in nanostructured metal films. The refractive index can be designed at will by simply tuning the wavelengths of the interacting waves, which could have potential impact on many important applications, such as superlens imaging.     

Metasurfaces Atop Metamaterials: Surface Morphology Induces Linear Dichroism in Gyroid Optical Metamaterials

Optical metamaterials offer the tantalizing possibility of creating extraordinary optical properties through the careful design and arrangement of subwavelength structural units. Gyroid‐structured optical metamaterials possess a chiral, cubic, and triply periodic bulk morphology that exhibits a redshifted effective plasma frequency. They also exhibit a strong linear dichroism, the origin of which is not yet understood. Here, the interaction of light with gold gyroid optical metamaterials is studied and a strong correlation between the surface morphology and its linear dichroism is found. The termination of the gyroid surface breaks the cubic symmetry of the bulk lattice and gives rise to the observed wavelength‐ and polarization‐dependent reflection. The results show that light couples into both localized and propagating plasmon modes associated with anisotropic surface protrusions and the gaps between such protrusions. The localized surface modes give rise to the anisotropic optical response, creating the linear dichroism. Simulated reflection spectra are highly sensitive to minute details of these surface terminations, down to the nanometer level, and can be understood with analogy to the optical properties of a 2D anisotropic metasurface atop a 3D isotropic metamaterial. This pronounced sensitivity to the subwavelength surface morphology has significant consequences for both the design and application of optical metamaterials.     

声学超构材料技术实用化的进展

声学超构材料是当前声学和材料学一个热门的研究领域.声学超构材料可定义为:通过对材料在特征物理尺度上进行人工设计制备,使其具有超越常规材料的声学性能的一种人工序构的复合材料.其亚波长特性、超常声学性能以及颠覆性应用的可能吸引了学界和工程界的关注.21世纪以来,随着增材制造技术的发展,声学超构材料的实验室加工与制备问题得以...     

Composite material made of plasmonic nanoshells with quantum dot cores: loss-compensation and ε-near-zero physical properties

A theoretical investigation of loss-compensation capabilities in composite materials made of plasmonic nanoshells is carried out by considering quantum dots (QDs) as the nanoshells' cores. The QD and metal permittivities are modeled according to published experimental data. We determine the modes with real or complex wavenumber able to propagate in a 3D periodic lattice of nanoshells. Mode analysis is also used to assess that only one propagating mode is dominant in the composite material whose optical properties can hence be described via homogenization theory. Therefore, the material effective permittivity is found by comparing different techniques: (i)?the mentioned mode analysis, (ii)?Maxwell Garnett mixing rule and (iii)?the Nicolson-Ross-Weir method based on transmission and reflection when considering a metamaterial of finite thickness. The three methods are in excellent agreement, because the nanoshells considered in this paper are very subwavelength, thus justifying the parameter homogenization. We show that QDs are able to provide loss-compensated ε-near-zero metamaterials and also loss-compensated metamaterials with large negative values of permittivity. Besides compensating for losses, the strong gain via QD can provide optical amplification with particular choices of the nanoshell and lattice dimensions.     

Wire Metamaterials: Physics and Applications

The physics and applications of a broad class of artificial electromagnetic materials composed of lattices of aligned metal rods embedded in a dielectric matrix are reviewed. Such structures are here termed wire metamaterials. They appear in various settings and can operate from microwaves to THz and optical frequencies. An important group of these metamaterials is a wire medium possessing extreme optical anisotropy. The study of wire metamaterials has a long history, however, most of their important and useful properties have been revealed and understood only recently, especially in the THz and optical frequency ranges where the wire media correspond to the lattices of microwires and nanowires, respectively. Another group of wire metamaterials are arrays and lattices of nanorods of noble metals whose unusual properties are driven by plasmonic resonances.     

Self-organized silver nanoparticles for three-dimensional plasmonic crystals

Metal nanostructures that support surface plasmons are compelling as plasmonic circuit elements and as the building blocks for metamaterials. We demonstrate here the spontaneous self-assembly of shaped silver nanoparticles into three-dimensional plasmonic crystals that display a frequency-selective response in the visible wavelengths. Extensive long-range order mediated by exceptional colloid monodispersity gives rise to optical passbands that can be tuned by particle volume fraction. These metallic supercrystals present a new paradigm for the fabrication of plasmonic materials, delivering a functional, tunable, completely bottom-up optical element that can be constructed on a massively parallel scale without lithography.     

Ultrasonic metamaterials with negative modulus

The emergence of artificially designed subwavelength electromagnetic materials, denoted metamaterials, has significantly broadened the range of material responses found in nature. However, the acoustic analogue to electromagnetic metamaterials has, so far, not been investigated. We report a new class of ultrasonic metamaterials consisting of an array of subwavelength Helmholtz resonators with designed acoustic inductance and capacitance. These materials have an effective dynamic modulus with negative values near the resonance frequency. As a result, these ultrasonic metamaterials can convey acoustic waves with a group velocity antiparallel to phase velocity, as observed experimentally. On the basis of homogenized-media theory, we calculated the dispersion and transmission, which agrees well with experiments near 30 kHz. As the negative dynamic modulus leads to a richness of surface states with very large wavevectors, this new class of acoustic metamaterials may offer interesting applications, such as acoustic negative refraction and superlensing below the diffraction limit.     

Active Metamaterials for Modulation and Detection

This paper illustrates some new concepts in the area of hybrid metamaterials, which are metamaterials that are embedded with active circuit elements such as transistors. Such transistor/metamaterial hybrids can exhibit some exotic electromagnetic properties which can be exploited for unusual and exciting functions. Two specific examples are provided. In one application, terahertz (THz) modulator based on embedding of psuedomorphic high electron mobility transistor (pHEMT) within the metamaterial resonator, all implemented monolithically in a commercial gallium arsenide (GaAs) technology is presented. In another application, a detector array based on metamaterial perfect absorber for room-temperature detection of gigahertz (GHz) radiation within each sub wavelength metamaterial unit cell is presented. The latter application utilizes a hybridization of metamaterial on printed circuit board (PCB) with discrete microwave electronic components. Both applications indicate the promise of the approach of integrating electronics or semiconductor devices with metamaterials for new and innovative functions.