Low-loss multilayered metamaterial exhibiting a negative index of refraction at visible wavelengths

3D Bulk Metamaterials with Engineered Optical Dispersion at Terahertz Frequencies Utilizing Amorphous Multilayered Split-Ring Resonators

A 3D bulk metamaterial (MM) containing amorphous multilayered split-ring resonators is proposed, fabricated, and evaluated. Experimentally, the effective refractive index is engineered via the 3D bulk MM, with a contrast of 0.118 across the frequency span from 0.315 to 0.366 THz and the index changing at a slope of 2.314 per THz within this frequency range. Additionally, the 3D bulk MM exhibits optical isotropy with respect to polarization. Moreover, the peak transmission and optical dispersion are tailored by adjusting the density of the split-ring resonators. Compared to reported conventional approaches for constructing bulk MMs, this approach offers advantages in terms of the potential for large-scale manufacturing, the ability to adopt any shape, optical isotropy, and rapid optical dispersion. These features hold promise for dispersive optical devices operating at THz frequencies, such as high-dispersive prisms for high-resolution spectroscopy.

Enhanced Chiral Sensing at the Few-Molecule Level Using Negative Index Metamaterial Plasmonic Nanocuvettes

Chirality is a fundamental property of biological molecules and some pharmaceutical molecules. Chiral molecules have a pair of chiral isomers (enantiomers) with opposite handedness. Although both enantiomers of the same molecule have identical chemical and physical properties, one enantiomer may be toxic to living organisms while the other one is harmless. The detection of these enantiomers is done using their small differential absorption between right and left circularly polarized light, known as circular dichroism (CD). Considering the macroscopic size of these molecules, combined with their small differential absorption, the obtained CD signal is very small, imposing a severe limitation on the minimal concentration that can be detected. Chiral plasmonic and metamaterial structures have been used to enhance the sensitivity of CD measurements by orders of magnitude through chiral density hot spots (super chiral fields). However, the large background signal due to these structures' intrinsic chirality limits the effectiveness of these methods. Contrary to absorption-based chiral sensing measurements (CD), fluorescence detection circular dichroism (FDCD) sensing can greatly improve chiral measurement sensitivity, down to the ultimate limit of a few and even a single chiral molecule. Like differential absorption, differential fluorescence also produces a weak signal at the few-chiral-molecule limit. However, here we demonstrate a negative-index metamaterial (NIM) cavity that acts as a "plasmonic nanocuvette" with globally enhanced volume super chiral fields. Moreover, the achiral structure of the plasmonic nanocuvette allows for completely background-free chiral sensing. We show that with NIM-cavity-enhanced FDCD, we can detect as low as a few tens of chiral molecules, well within the zeptomole range.

Effective electrical resistivity in a square array of oriented square inclusions

The continuing miniaturization of optoelectronic devices, alongside the rise of electromagnetic metamaterials, poses an ongoing challenge to nanofabrication. With the increasing impracticality of quality control at a single-feature (-device) resolution, there is an increasing demand for array-based metrologies, where compliance to specifications can be monitored via signals arising from a multitude of features (devices). To this end, a square grid with quadratic sub-features is amongst the more common designs in nanotechnology (e.g. nanofishnets, nanoholes, nanopyramids, μLED arrays etc). The electrical resistivity of such a quadratic grid may be essential to its functionality; it can also be used to characterize the critical dimensions of the periodic features. While the problem of the effective electrical resistivity ρ _eff of a thin sheet with resistivity ρ ₁, hosting a doubly-periodic array of oriented square inclusions with resistivity ρ ₂, has been treated before (Obnosov 1999 SIAM J. Appl. Math. 59 1267-87), a closed-form solution has been found for only one case, where the inclusion occupies c = 1/4 of the unit cell. Here we combine first-principle approximations, numerical modeling, and mathematical analysis to generalize ρ _eff for an arbitrary inclusion size (0 < c < 1). We find that in the range 0.01 ≤ c ≤ 0.99, ρ _eff may be approximated (to within <0.3% error with respect to finite element simulations) by: [Formula: see text] [Formula: see text] whereby at the limiting cases of c → 0 and c → 1, α approaches asymptotic values of α = 2.039 and α = 1/c - 1, respectively. The applicability of the approximation to considerably more complex structures, such as recursively-nested inclusions and/or nonplanar topologies, is demonstrated and discussed. While certainly not limited to, the theory is examined from within the scope of micro four-point probe (M4PP) metrology, which currently lacks data reduction schemes for periodic materials whose cell is smaller than the typical μm-scale M4PP footprint.

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.

3D Hybrid Trilayer Heterostructure: Tunable Au Nanorods and Optical Properties

Engineering plasmonic nanostructures from three dimensions (3D) is very attractive toward controllable and tunable nanophotonic components and devices. Herein, Au-based trilayer heterostructures composed of a dielectric spacer sandwiched by hybrid Au-TiN vertically aligned nanocomposite (VAN) nanoplasmonic claddings are demonstrated with a broad range of geometries and property tuning. Two types of spacer layers, that is, a pure dielectric BaTiO₃ layer and a hybrid plasmonic Au-BaTiO₃ VAN layer, contribute to the tuning of the Au nanorod dimension. Such geometrical variations of Au nanostructures originate from the surface energy and lattice strain tuned by the spacer layers. Optical measurements and numerical simulations suggest the change of the localized surface plasmon resonance which is strongly affected by the tailored Au nanorods as either separated or channeled. The uniaxial dielectric tensors suggest a tunable hyperbolic property affected by such a metal-insulator-metal trilayer stack. The complex 3D heterostructures offer additional tuning parameters and design flexibilities in hybrid plasmonic metamaterials toward potential applications in light harvesting, sensing, and nanophotonic devices.

Realization of broadband negative refraction in visible range using vertically stacked hyperbolic metamaterials

Negative refraction has generated much interest recently with its unprecedented optical phenomenon. However, a broadband negative refraction has been challenging because they mainly involve optical resonances. This paper reports the realization of broadband negative refraction in the visible spectrum by using vertically-stacked metal-dielectric multilayer structures. Such structure exploits the characteristics of the constituent metal and dielectric materials, and does not require resonance to achieve negative refraction. Broadband negative refraction (wavelength 270–1300 nm) is numerically demonstrated. Compared to conventional horizontally-stacked multilayer structures, the vertically-stacked multilayer structure has a broader range of working wavelength in the visible range, with higher transmittance. We also report a variety of material combinations with broad working wavelength. The broadband negative refraction metamaterial provides an effective way to manipulate light and may have applications in super-resolution imaging, and invisibility cloaks.

Optical Chirality in Dispersive and Lossy Media

Several dynamical properties of electromagnetic waves such as energy, momentum, angular momentum, and optical helicity have been recently reexamined in dispersive and lossless media. Here, we address an alternative derivation for the optical chirality, extending it so as to include dissipative effects as well. To this end, we first elaborate on the most complete form of the conservation law for the optical chirality, without any restrictions on the nature of the medium. As a result we find a general expression for the optical chirality density both in lossless and lossy dispersive media. Our definition is perfectly consistent with that originally introduced for electromagnetic fields in free space, and is applicable to any material system, including dielectrics, plasmonic nanostructures, and left-handed metamaterials.

Towards three-dimensional optical metamaterials

Metamaterials have opened up the possibility of unprecedented and fascinating concepts and applications in optics and photonics. Examples include negative refraction, perfect lenses, cloaking, perfect absorbers, and so on. Since these metamaterials are man-made materials composed of sub-wavelength structures, their development strongly depends on the advancement of micro- and nano-fabrication technologies. In particular, the realization of three-dimensional metamaterials is one of the big challenges in this research field. In this review, we describe recent progress in the fabrication technologies for three-dimensional metamaterials, as well as proposed applications.

Freestanding optical negative-index metamaterials of green light

A freestanding, multilayered fishnet metamaterial is reported to experimentally exhibit a negative refractive index in the green-light spectral range. The realization of a negative refractive index at such a high frequency range mainly originates from low-loss magnetic resonance and interactions between the neighboring functional layers. Based on a good agreement between the numerically simulated and experimentally measured transmittance and reflectance spectra, a single negative refractive index of -0.76 with a figure-of-merit of 0.5 is achieved for the metamaterial at the wavelength of 532 nm.

Experimental Evidence for Sign Reversal of the Hall Coefficient in Three-Dimensional Metamaterials

Effectively inverting the sign of material parameters is a striking possibility arising from the concept of metamaterials. Here, we show that the electrical properties of a p-type semiconductor can be mimicked by a metamaterial solely made of an n-type semiconductor. By fabricating and characterizing three-dimensional simple-cubic microlattices composed of interlocked hollow semiconducting tori, we demonstrate that sign and magnitude of the effective metamaterial Hall coefficient can be adjusted via a tori separation parameter-in agreement with previous theoretical and numerical predictions.

Self-Assembled Epitaxial Au-Oxide Vertically Aligned Nanocomposites for Nanoscale Metamaterials

Metamaterials made of nanoscale inclusions or artificial unit cells exhibit exotic optical properties that do not exist in natural materials. Promising applications, such as super-resolution imaging, cloaking, hyperbolic propagation, and ultrafast phase velocities have been demonstrated based on mostly micrometer-scale metamaterials and few nanoscale metamaterials. To date, most metamaterials are created using costly and tedious fabrication techniques with limited paths toward reliable large-scale fabrication. In this work, we demonstrate the one-step direct growth of self-assembled epitaxial metal-oxide nanocomposites as a drastically different approach to fabricating large-area nanostructured metamaterials. Using pulsed laser deposition, we fabricated nanocomposite films with vertically aligned gold (Au) nanopillars (∼20 nm in diameter) embedded in various oxide matrices with high epitaxial quality. Strong, broad absorption features in the measured absorbance spectrum are clear signatures of plasmon resonances of Au nanopillars. By tuning their densities on selected substrates, anisotropic optical properties are demonstrated via angular dependent and polarization resolved reflectivity measurements and reproduced by full-wave simulations and effective medium theory. Our model predicts exotic properties, such as zero permittivity responses and topological transitions. Our studies suggest that these self-assembled metal-oxide nanostructures provide an exciting new material platform to control and enhance optical response at nanometer scales.

One-dimensional photonic crystals: fabrication, responsiveness and emerging applications in 3D construction

Classical usages of one-dimensional photonic crystals and emerging applications in 3D construction.

Hyperbolic polaritonic crystals based on nanostructured nanorod metamaterials

Surface plasmon polaritons usually exist on a few suitable plasmonic materials; however, nanostructured plasmonic metamaterials allow a much broader range of optical properties to be designed. Here, bottom-up and top-down nanostructuring are combined, creating hyperbolic metamaterial-based photonic crystals termed hyperbolic polaritonic crystals, allowing free-space access to the high spatial frequency modes supported by these metamaterials.

Tunable terahertz fishnet metamaterials based on thin nematic liquid crystal layers for fast switching

The electrically tunable properties of liquid-crystal fishnet metamaterials are theoretically investigated in the terahertz spectrum. A nematic liquid crystal layer is introduced between two fishnet metallic structures, forming a voltage-controlled metamaterial cavity. Tuning of the nematic molecular orientation is shown to shift the magnetic resonance frequency of the metamaterial and its overall electromagnetic response. A shift higher than 150 GHz is predicted for common dielectric and liquid crystalline materials used in terahertz technology and for low applied voltage values. Owing to the few micron-thick liquid crystal cell, the response speed of the tunable metamaterial is calculated as orders of magnitude faster than in demonstrated liquid-crystal based non-resonant terahertz components. Such tunable metamaterial elements are proposed for the advanced control of electromagnetic wave propagation in terahertz applications.

Plasmon Injection to Compensate and Control Losses in Negative Index Metamaterials

Metamaterials have introduced a whole new world of unusual materials with functionalities that cannot be attained in naturally occurring material systems by mimicking and controlling the natural phenomena at subwavelength scales. However, the inherent absorption losses pose a fundamental challenge to the most fascinating applications of metamaterials. Based on a novel plasmon injection (PI or Π) scheme, we propose a coherent optical amplification technique to compensate losses in metamaterials. Although the proof of concept device here operates under normal incidence only, our proposed scheme can be generalized to an arbitrary form of incident waves. The Π scheme is fundamentally different from major optical amplification schemes. It does not require a gain medium, interaction with phonons, or any nonlinear medium. The Π scheme allows for loss-free metamaterials. It is ideally suited for mitigating losses in metamaterials operating in the visible spectrum and is scalable to other optical frequencies. These findings open the possibility of reviving the early dreams of making "magical" metamaterials from scratch.

Quantum entanglement distillation with metamaterials

We propose a scheme for the distillation of partially entangled two-photon Bell and three-photon W states using metamaterials. The distillation of partially entangled Bell states is achieved by using two metamaterials with polarization dependence, one of which is rotated by π/2 around the direction of propagation of the photons. On the other hand, the distillation of three-photon W states is achieved by using one polarization dependent metamaterial and two polarization independent metamaterials. Upon transmission of the photons of the partially entangled states through the metamaterials the entanglement of the states increases and they become distilled. This work opens up new directions in quantum optical state engineering by showing how metamaterials can be used to carry out a quantum information processing task.

Chiral Light-Matter Interaction in Optical Resonators

The Purcell effect explains the modification of the spontaneous decay rate of quantum emitters in a resonant cavity. For quantum emitters such as chiral molecules, however, the cavity modification of the spontaneous decay rate has been little known. Here we extend Purcell's work to the chiral light-matter interaction in optical resonators and find the differential spontaneous decay rate of chiral molecules coupled to left and right circularly polarized resonator modes. We determine the chiral Purcell factor, which characterizes the ability of optical resonators to enhance chiroptical signals, by the quality factor and the chiral mode volume of a resonator, representing, respectively, the temporal confinement of light and the spatial confinement of the helicity of light. We show that the chiral Purcell effect can be applied to chiroptical spectroscopy. Specifically, we propose a realistic scheme to achieve resonator enhanced chiroptical spectroscopy that uses the double fishnet structure as a nanoscale cuvette supporting the chiral Purcell effect.

Multiband one-way polarization conversion in complementary split-ring resonator based structures by combining chirality and tunneling

Multiband one-way polarization conversion and strong asymmetry in transmission inspired by it are demonstrated in ultrathin sandwiched structures that comprise two twisted aperture-type arrays of complementary split-ring resonators (CSRRs), metallic mesh, and dielectric layers. The basic features of the resulting mechanism originate from the common effect of chirality and tunneling. The emphasis is put on the (nearly) perfect polarization conversion of linear incident polarization into the orthogonal one and related diodelike asymmetric transmission within multiple narrow bands. Desired polarization conversion can be obtained at several resonances for one of the two opposite incidence directions, whereas transmission is fully blocked for the other one. The resonances, at which the (nearly) perfect conversion takes place, are expected to be inherited from similar structures with parallel, i.e., not rotated CSRR arrays that do not enable chirality and, thus, polarization conversion. It is found that the basic transmission and polarization conversion features and, thus, the dominant physics are rather general, enabling efficient engineering of such structures. The lowest-frequency resonance can be obtained in structures made of conventional materials with total thickness less than λ/50 and up to ten such resonances can correspond to thickness less than λ/20.

Split-ball resonator as a three-dimensional analogue of planar split-rings

Split-ring resonators are basic elements of metamaterials, which can induce a magnetic response in metallic nanosctructures. Tunability of such response up to the visible frequency range is still a challenge. Here we introduce the concept of the split-ball resonator and demonstrate the strong magnetic response in the visible for both gold and silver spherical plasmonic nanoparticles with nanometre scale cuts. We realize this concept experimentally by employing the laser-induced transfer method to produce near-perfect metallic spheres and helium ion beam milling to make cuts with the clean straight sidewalls and nanometre resolution. The magnetic resonance is observed at 600 nm in gold and at 565 nm in silver nanoparticles. This method can be applied to the structuring of arbitrary three-dimensional features on the surface of nanoscale resonators. It provides new ways for engineering hybrid resonant modes and ultra-high near-field enhancement.

Optical magnetism and plasmonic Fano resonances in metal-insulator-metal oligomers

The possibility of achieving optical magnetism at visible frequencies using plasmonic nanostructures has recently been a subject of great interest. The concept is based on designing structures that support plasmon modes with electron oscillation patterns that imitate current loops, that is, magnetic dipoles. However, the magnetic resonances are typically spectrally narrow, thereby limiting their applicability in, for example, metamaterial designs. We show that a significantly broader magnetic response can be realized in plasmonic pentamers constructed from metal-insulator-metal (MIM) sandwich particles. Each MIM unit acts as a magnetic meta-atom and the optical magnetism is rendered quasi-broadband through hybridization of the in-plane modes. We demonstrate that scattering spectra of individual MIM pentamers exhibit multiple Fano resonances and a broad subradiant spectral window that signals the magnetic interaction and a hierarchy of coupling effects in these intricate three-dimensional nanoparticle oligomers.

Ultrafast tunable chirped phase-change metamaterial with a low power

We numerically demonstrate an all-optical tunable dual-band double negative (DNG) index chirped metamaterial (MM) in the mid-infrared (M-IR) region. This MM possesses an ultrafast and significant tunability under low pump light power, realized by combining phase change material (PCM). It has a configuration of elliptical nanohole array (ENA) penetrating through metal/PCM/metal (Au-Ge(2)Sb(2)Te(5)-Au) films. Here, we consider the case when the chirp is introduced by displacing the positions of the ENA along the short axis of the elliptical apertures inside the primitive cell, which can achieve multiple internal surface-plasmon polariton (SPP) modes at the inner metal-dielectric interfaces of the structure and thus providing a dual-band negative index with simultaneous negative permittivity and permeability. The influence of amorphous and crystalline states of Ge(2)Sb(2)Te(5) on the effective optical parameters of the structure is analyzed. Switching between these states provides a large wavelength shift of the structure's effective optical parameters. A photothermal model is used to study the temporal variation of the temperature of the Ge(2)Sb(2)Te(5) layer to show a potential to switch the phase of Ge(2)Sb(2)Te(5) by optical heating. Generation of the tunable dual-band DNG index presents clear advantages as it possesses a fast tuning time of 0.4 ns, a low pump light intensity of 7.3μW/μm(2), and a large tunable wavelength range of 978 nm. We expect that our design may have potential applications in actively tunable multi-band nanodevices.

From 1D to 3D: a new route to fabricate tridimensional structures via photo-generation of silver networks

A time-saving and low-cost method is established to construct stacked 3D structures through the combination of bottom-up and top-down techniques which enables us to create building blocks freely and to precisely adjust the matrix feature.

Directive emission of red conjugated polymer embedded within zero index metamaterials

We numerically demonstrate an impedance-matched multilayer stacked fishnet metamaterial that has zero index with flat high transmittance from 600 nm to 620 nm. The effective refractive index(n(eff)) is calculated to be -0.045 + 0.466 i and the normalize effective impedance(Z(eff)/Z(0)) is 0.956-0.368 i at 610 nm. The light emitted by a red conjugated polymer layer embedded in such a zero index metamaterial (ZIM) is concentrated in a narrow cone in the surrounding media, where the half-power beam width (HPBW) of the center lobe of the radiation pattern is around 25° in the wavelength range between 600 nm and 620 nm, giving directive emission in the visible region. This proposed light focusing system can be applied to sensing, beam collimating and filtering functionalities.

Waveguide-mode interference lithography technique for high contrast subwavelength structures in the visible region

We explore possibilities of waveguide-mode interference lithography (WMIL) technique for high contrast subwavelength structures in the visible region. Selecting an appropriate waveguide-mode, we demonstrate high contrast resist mask patterns for the first time. TM1 mode in the waveguide is shown to be useful for providing a three-dimensional structure whose cross section is checkerboard pattern. Applying our WMIL technique, we demonstrate 1D, 2D and 3D subwavelength resist patterns that are widely used for the fabrication of metamteterials in the visible region. In addition to the resist patterns, we demonstrate a resonance at 1.9 eV for a split tube structure experimentally.

Nanoimprinting techniques for large-area three-dimensional negative index metamaterials with operation in the visible and telecom bands

We report advances in materials, designs, and fabrication schemes for large-area negative index metamaterials (NIMs) in multilayer "fishnet" layouts that offer negative index behavior at wavelengths into the visible regime. A simple nanoimprinting scheme capable of implementation using standard, widely available tools followed by a subtractive, physical liftoff step provides an enabling route for the fabrication. Computational analysis of reflection and transmission measurements suggests that the resulting structures offer negative index of refraction that spans both the visible wavelength range (529-720 nm) and the telecommunication band (1.35-1.6 μm). The data reveal that these large (>75 cm(2)) imprinted NIMs have predictable behaviors, good spatial uniformity in properties, and figures of merit as high as 4.3 in the visible range.

A visible metamaterial fabricated by self-assembly method

A visible negative-index metamaterial was fabricated by adopting a template-assisted and self-assembled electrochemical deposition method. Originating from the inherent characteristic of bottom-up fabrication, it has been demonstrated by the effective medium theory that the metamaterial resonance could realize a negative index at visible wavelengths if the degree of asymmetry is kept in a moderate range. This was experimentally substantiated by fabricating an asymmetric Ag–polyvinyl alcohol (PVA)–Ag multilayer nanostructure with two apertured silver films on the opposite sides of PVA film. The extraction of constitutive parameters shows negative index in the wavelength range from 535 to 565 nm, with a minimum value close to −0.5. Simultaneously, an optically active medium Rhodamine B was incorporated into the PVA layer, which readily changed the transmission peak through control experiments. Finally, the enhanced transmission was realized through a flat sample.

Design of fishnet metamaterials with broadband negative refractive index in the visible spectrum

We propose a technique capable of designing fishnet metamaterials that have a negative refractive index (NRI) over a broad range in the visible and infrared. The technique relies on optimizing the shape and scale of the fishnet apertures as well as the depth of different layers of the composite. A metamaterial is obtained that exhibits an unbroken 552 nm bandwidth of NRI, covering the entire red and infrared regions. Moreover, two fishnet structures perforated with star-like holes are found to render refractive index negative in the yellow and green spectra.

Bifunctional plasmonic metamaterials enabled by subwavelength nano-notches for broadband, polarization-independent enhanced optical transmission and passive beam-steering

In this work, we present the design, numerical experiments, and analysis of a plasmonic metamaterial thin film based on subwavelength nano-notch loaded modified fishnet structures. The resulting device offers a simultaneous bandpass filtering functionality with a broad enhanced optical transmission window and a gapless negative-zero-positive index transition to enable polarization-independent passive beam-steering. This unique characteristic is made possible by the introduced subwavelength nano-notches, which provide fine tuning and hybridization of the external and internal surface plasmon polariton modes. This allows tailoring of the dispersive properties of the plasmonic metamaterial for broadband operation. Specifically, a multilayer nanostructured modified fishnet with feature sizes accessible by modern nanofabrication techniques is presented, exhibiting a broad passband at the mid-infrared wavelengths from 3.0 to 3.7 µm and stopbands elsewhere in the 2.5 ~4.5 µm window. The transmittance normalized to area is around 3 dB within the broad 20% bandwidth of the passband. Additionally, the effective index undergoes a smooth transition from negative unity through zero to positive unity with low loss within the passband. The physical mechanism and the angular dispersion of the metamaterial are analyzed in detail. Finally, full-wave simulations of a prism formed from this metamaterial are performed to demonstrate that the proposed structure achieves simultaneous polarization-insensitive passive beam-steering and filtering functionalities.

All-angle collimation of incident light in μ-near-zero metamaterials

We use the theory of inhomogeneous waves to study the transmission of light in μ-near-zero metamaterials. We find the effect of all-angle collimation of incident light, which means that the vector of energy flow in a wave transmitted to a μ-near-zero metamaterial is perpendicular to the interface for any incident angles if an incident wave is s-polarized. This effect is similar to the all-angle collimation of incident light recently found through a different theoretical framework in ε-near-zero metamaterials for a p-polarized incident wave [S. Feng, Phys. Rev. Lett. 108, 193904 (2012)]. To provide a specific example, we consider the transmission of light in a negative-index metamaterial in the spectral region with a permeability resonance, and show that all-angle collimation indeed takes place at the wavelength for which the real part of permeability is vanishingly small.

The ratio of the kinetic inductance to the geometric inductance: a key parameter for the frequency tuning of the THz semiconductor split-ring resonator

By introducing the frequency tuning sensitivity, an analytical model based on equivalent LC circuit is developed for the relative frequency tuning range of THz semiconductor split-ring resonator (SRR). And the model reveals that the relative tuning range is determined by the ratio of the kinetic inductance to the geometric inductance (RKG). The results show that under the same carrier density variation, a larger RKG results in a larger relative tuning range. Based on this model, a stacked SRR-dimer structure with larger RKG compared to the single SRR due to the inductive coupling is proposed, which improves the relative tuning range effectively. And the results obtained by the simple analytical model agree well with the numerical FDTD results. The presented analytical model is robust and can be used to analyze the relative frequency tuning of other tunable THz devices.

Magnetic hot spots in closely spaced thick gold nanorings

Light-matter interaction at optical frequencies is mostly mediated by the electric component of the electromagnetic field, with the magnetic component usually being considered negligible. Recently, it has been shown that properly engineered metallic nanostructures can provide a magnetic response at optical frequencies originated from real or virtual flows of electric current in the structure. In this work, we demonstrate a magnetic plasmonic mode which emerges in closely spaced thick gold nanorings. The plasmonic resonance obtains a magnetic dipole character by sufficiently increasing the height of the nanorings. Numerical simulations show that a virtual current loop appears at resonance for sufficiently thick nanorings, resulting in a strong concentration of the magnetic field in the gap region (magnetic hot spot). We find that there is an optimum thickness that provides the maximum magnetic intensity enhancement (over 200-fold enhancement) and give an explanation of this observation. This strong magnetic resonance, observed both experimentally and theoretically, can be used to build new metamaterials and resonant loop nanoantennas at optical frequencies.

Compensating substrate-induced bianisotropy in optical metamaterials using ultrathin superstrate coatings

In this work, we propose an efficient approach to compensate for the commonly observed substrate-induced bianisotropy that occurs in on-wafer optical metamaterials at normal incidence. First, the consequence of placing a finite thickness substrate underneath a metamaterial is analyzed, indicating that the induced bianisotropy is a near-field effect. The properties of metamaterials sandwiched between an infinitely thick substrate and a finite-thickness superstrate with different permittivity and thickness values are then investigated. It is demonstrated from full-wave simulations that by adding an ultrathin superstrate with a judicious choice of its thickness and permittivity value, the substrate-induced bianisotropy of the system can be suppressed and even eliminated. In addition to the extracted nonlocal effective medium parameters, the induced electric and magnetic dipole moments calculated from the volumetric microscopic fields are also presented, validating that the magnetoelectric coupling compensation is a real physical phenomenon. This study will benefit future optical metamaterial design and implementation strategies as well as the corresponding fabrication and characterization methodologies.

Three-layered metallodielectric nanoshells: plausible meta-atoms for metamaterials with isotropic negative refractive index at visible wavelengths

A three-layered Ag-low-permittivity (LP)-high-permittivity (HP) nanoshell is proposed as a plausible meta-atom for building the three-dimensional isotropic negative refractive index metamaterials (NIMs). The overlap between the electric and magnetic responses of Ag-LP-HP nanoshell can be realized by designing the geometry of the particle, which can lead to the negative electric and magnetic polarizabilities. Then, the negative refractive index is found in the random arrangement of Ag-LP-HP nanoshells. Especially, the modulation of the middle LP layer can move the negative refractive index range into the visible region. Because the responses arise from the each meta-atom, the metamaterial is intrinsically isotropic and polarization independent. It is further found with the increase of the LP layer thickness that the negative refractive index range of the random arrangement shows a large blue-shift and becomes narrow. With the decrease of the filling fraction, the negative refractive index range shows a blue-shift and becomes narrow while the maximum of the negative refractive index decreases.

Photonic metamaterials: a new class of materials for manipulating light waves

A decade of research on metamaterials (MMs) has yielded great progress in artificial electromagnetic materials in a wide frequency range from microwave to optical frequencies. This review outlines the achievements in photonic MMs that can efficiently manipulate light waves from near-ultraviolet to near-infrared in subwavelength dimensions. One of the key concepts of MMs is effective refractive index, realizing values that have not been obtained in ordinary solid materials. In addition to the high and low refractive indices, negative refractive indices have been reported in some photonic MMs. In anisotropic photonic MMs of high-contrast refractive indices, the polarization and phase of plane light waves were efficiently transformed in a well-designed manner, enabling remarkable miniaturization of linear optical devices such as polarizers, wave plates and circular dichroic devices. Another feature of photonic MMs is the possibility of unusual light propagation, paving the way for a new subfield of transfer optics. MM lenses having super-resolution and cloaking effects were introduced by exploiting novel light-propagating modes. Here, we present a new approach to describing photonic MMs definitely by resolving the electromagnetic eigenmodes. Two representative photonic MMs are addressed: the so-called fishnet MM slabs, which are known to have effective negative refractive index, and a three-dimensional MM based on a multilayer of a metal and an insulator. In these photonic MMs, we elucidate the underlying eigenmodes that induce unusual light propagations. Based on the progress of photonic MMs, the future potential and direction are discussed.

Magnetic light

Spherical silicon nanoparticles with sizes of a few hundreds of nanometers represent a unique optical system. According to theoretical predictions based on Mie theory they can exhibit strong magnetic resonances in the visible spectral range. The basic mechanism of excitation of such modes inside the nanoparticles is very similar to that of split-ring resonators, but with one important difference that silicon nanoparticles have much smaller losses and are able to shift the magnetic resonance wavelength down to visible frequencies. We experimentally demonstrate for the first time that these nanoparticles have strong magnetic dipole resonance, which can be continuously tuned throughout the whole visible spectrum varying particle size and visually observed by means of dark-field optical microscopy. These optical systems open up new perspectives for fabrication of low-loss optical metamaterials and nanophotonic devices.

Spatial dispersion of multilayer fishnet metamaterials

We study the anisotropic properties of multilayer fishnet optical metamaterials and describe topological transitions between the elliptic and hyperbolic dispersion regimes. In contrast to other hyperbolic media, multilayer fishnet metamaterials may have negative components not only in the effective permittivity tensor but also in the effective permeability tensor, thus allowing the realization of magnetic hyperbolic and generalized indefinite media.

Asymmetric fishnet metamaterials with strong optical activity

We investigate the optical properties of mono- and double-layer asymmetric fishnet metamaterials with orientated elliptical holes, which exhibit exotic spectral and polarization rotating characteristics in the visible spectral range. Our results show that nontrivial orientations of the holes with respect to the reciprocal lattice vectors of the periodic lattice in both systems produce strong polarization rotation as well as additional enhanced optical transmission peaks. Analysis of the electromagnetic field distribution shows the unusual effect is produced by the spinning localized surface plasmon resonances due to the asymmetric geometry. High sensitivity of the hybridized mode on the dielectric spacing, the aspect ratio of the holes and the embedding media in double-layer structure is also observed. The dependence of spectral and polarization response on the orientation of the holes and the embedding media is useful for design of chiral metamaterials at optical frequencies and tailoring the polarization behavior of the metallic nano-structures.

Oblique incidence ellipsometric characterization and the substrate dependence of visible frequency fishnet metamaterials

We use spectroscopic ellipsometry to investigate the angular-dependent optical modes of fishnet metamaterials fabricated by nanoimprint lithography. Spectroscopic ellipsometry is demonstrated as a fast and efficient method for metamaterial characterization and the measured polarization ratios significantly simplify the calibration procedures compared to reflectance and transmittance measurements. We show that the modes can be well identified by a combination of comparing different substrates and considering the angular dependence of the Wood's anomalies. The lack of angular dispersion of the anti-symmetric gap-modes does not agree with the model and requires further theoretical investigation.

Negative-index metamaterial at visible frequencies based on high order plasmon resonance

A type of negative-index metamaterial composed of periodic arrays of SRRs is proposed and numerically investigated in the visible frequencies. Employing the high order magnetic resonance to induce negative permeability, negative refractive index is obtained between 395 THz and 430 THz with the maximum FOM=4.59. The effective permeability exhibits a rapid convergence with increasing number of metamaterial layers. Different responses from the electric and magnetic resonances to the changing geometric parameters are compared and analyzed in terms of the field distribution. Simulation results show that the high order magnetic resonance can be greatly enhanced at visible frequencies as well as effectively tuned over a wide spectral range without notably altering the coupling between unit cells.

Study of incident angle dependence for dual-band double negative-index material using elliptical nanohole arrays

This work demonstrates the angular dependence of dual-band negative-index materials implemented by elliptical nanohole arrays (ENAs) consisting of an Al2O3 dielectric layer between two Au films. This article, it is believed for the first time, analyzes the scattering coefficients and displacement current of the ENA at different angles of plane-wave incidence to show that the ENA is double negative (showing both a negative effective permeability μ(eff) and a negative effective permittivity ε(eff) at multiple wavelengths (1095 and 1680 nm) for p polarization over a broad range of incident angles.

Dual-band double-negative-index fishnet metamaterial at millimeter-waves

An effective negative refractive index (NRI) is demonstrated and experimentally verified for the first two propagation bands of a fishnet-like metamaterial at millimeter-wave frequencies. The dual-band NRI behavior is achieved by engineering the diffraction order (±1, ±1) associated with the internal mode supported between holey layers to correspond with the second propagation band. In addition to the experimental interferometric technique that accounts for the handedness of the propagation, numerical results are given to predict the dual-band effective NRI and to confirm dual-band negative refraction for a prism composed of the proposed metamaterial.

Controlling the propagation velocity of a femtosecond laser pulse with negative index metamaterials

We theoretically study the propagation of a femtosecond laser pulse in negative-index metamaterials, and show that its propagation velocity can be easily controlled at a certain wavelength range simply by changing the initial chirp. This phenomenon may be used as an extremely simple way to control the propagation velocity of a femtosecond laser pulse.