Nanostructured sapphire optical fiber embedded with Au nanorods for high-temperature plasmonics in harsh environments

Sensors for harsh environments must exhibit robust sensing response and considerable thermal and chemical stability. We report the exploration of a novel all-alumina nanostructured sapphire optical fiber (NSOF) embedded with Au nanorods (Au NRs) for plasmonics-based sensing at high temperatures. Temperature dependence of the localized surface plasmon resonance (LSPR) of Au NRs was studied in conjunction with numerical calculations using the Drude model. It was found that LSPR of Au NRs changes markedly with temperature, red shifting and increasing in transmission amplitude as the temperature increases. Furthermore, this variation is highly localized through tunneling by overlapping the near-field of thin cladding and sapphire optical fiber. The NSOF embedded with Au NRs has the potential for sensing in advanced energy generation systems.

Hyperbolic Sound Propagation over Nonlocal Acoustic Metasurfaces

Hyperbolic metasurfaces, supporting extreme anisotropy of the surface impedance tensor, have recently been explored in nanophotonic systems for robust diffractionless propagation over a surface, offering interesting opportunities for subdiffraction imaging and enhanced Purcell emission. In acoustics, due to the longitudinal nature of sound transport in fluids, these phenomena are forbidden by symmetry, requiring the acoustic surface impedance to be inherently isotropic. Here we show that nonlocalities produced by strong coupling between neighboring impedance elements enable extreme anisotropic responses for sound traveling over a surface, supporting negative phase and energy velocities, as well as hyperbolic propagation for acoustic surface waves.

Tunable broadband hyperbolic light dispersion in metal diborides

The naturally hyperbolic materials that conquer the limitations of artificially structured hyperbolic metamaterials are promising candidates for the emerging devices based on light. However, the variety of natural hyperbolic materials and their hyperbolic frequency regime are presently limited. Here, on the basis of first-principles calculations, we demonstrated a family of natural hyperbolic materials, graphite-like metal diborides, with a broadband hyperbolic region from near-IR (∼2.5µm) to the ultraviolet regime (∼248 nm). The operating hyperbolic window and negative refraction can be effectively modulated by extracting electrons from the materials, offering a promising strategy for regulating the optical properties.

Integration of Hybrid Plasmonic Au-BaTiO3 Metamaterial on Silicon Substrates

Silicon integration of nanoscale metamaterials is a crucial step toward low-cost and scalable optical-based integrated circuits. Here, a self-assembled epitaxial Au-BaTiO₃ (Au-BTO) hybrid metamaterial with highly anisotropic optical properties has been integrated on Si substrates. A thin buffer layer stack (<20 nm) of TiN and SrTiO₃ (STO) was applied on Si substrates to ensure the epitaxial growth of the Au-BTO hybrid films. Detailed phase composition and microstructural analyses show excellent crystallinity and epitaxial quality of the Au-BTO films. By varying the film growth conditions, the density and dimension of the Au nanopillars can be tuned effectively, leading to highly tailorable optical properties including tunable localized surface plasmon resonance (LSPR) peak and hyperbolic dispersion shift in the visible and near-infrared regimes. The work highlights the feasibility of integrating epitaxial hybrid oxide-metal plasmonic metamaterials on Si toward future complex Si-based integrated photonics.

Plasmonic Metamaterials for Nanochemistry and Sensing

Plasmonic nanostructures were initially developed for sensing and nanophotonic applications but, recently, have shown great promise in chemistry, optoelectronics, and nonlinear optics. While smooth plasmonic films, supporting surface plasmon polaritons, and individual nanostructures, featuring localized surface plasmons, are easy to fabricate and use, the assemblies of nanostructures in optical antennas and metamaterials provide many additional advantages related to the engineering of the mode structure (and thus, optical resonances in the given spectral range), field enhancement, and local density of optical states required to control electronic and photonic interactions. Focusing on two of the many applications of plasmonic metamaterials, in this Account, we review our work on the sensing and nanochemistry applications of metamaterials based on the assemblies of plasmonic nanorods under optical, as well as electronic interrogation. Sensors are widely employed in modern technology for the detection of events or changes in their local environment. Compared to their electronic counterparts, optical sensors offer a combination of high sensitivity, fast response, immunity to electromagnetic interference, and provide additional options for signal retrieval, such as optical intensity, spectrum, phase, and polarization. Owing to the ability to confine and enhance electromagnetic fields on subwavelength scales, plasmonics has been attracting increasing attention for the development of optical sensors with advantages including both nanometer-scale spatial resolution and single-molecule sensitivity. Inherent hot-electron generation in plasmonic nanostructures under illumination or during electron tunneling in the electrically biased nanostructures provides further opportunities for sensing and stimulation of chemical reactions, which would otherwise not be energetically possible. We first provide a brief introduction to a metamaterial sensing platform based on arrays of strongly coupled plasmonic nanorods. Several prototypical sensing examples based on this versatile metamaterial platform are presented. Record-high refractive index sensitivity of gold nanorod arrays in biosensing based on the functionalization of the nanorod surface for selective absorption arises because of the modification of the electromagnetic coupling between the nanorods in the array. The capabilities of nanorod metamaterials for ultrasound and hydrogen sensing were demonstrated by precision coating of the nanorods with functional materials to create core-shell nanostructures. The extension of this metamaterial platform to nanotube and nanocavity arrays, and metaparticles provides additional flexibility and removes restrictions on the illumination configurations for the optical interrogation. We then discuss a nanochemical platform based on the electrically driven metamaterials to stimulate and detect chemical reactions in the tunnel junctions constructed with the nanorods by exploiting elastic tunneling for the activation of chemical reactions via generated hot-electrons and inelastic tunneling for the excitation of plasmons facilitating optical monitoring of the process. This represents a new paradigm merging electronics, plasmonics, photonics and chemistry at the nanoscale, and creates opportunities for a variety of practical applications, such as hot-electron-driven nanoreactors and high-sensitivity sensors, as well as nanoscale light sources and modulators. With a combination of merits, such as the ability to simultaneously support both localized and propagating modes, nanoporous texture, rapid and facile functionalization, and low cost and scalability, plasmonic nanorod metamaterials provide an attractive and versatile platform for the development of optical sensors and nanochemical platforms using hot-electrons with high performance for applications in fundamental research and chemical and pharmaceutical industries.

Strong Modulations of Optical Reflectance in Tapered Core-Shell Nanowires

Random assemblies of vertically aligned core–shell GaAs–AlGaAs nanowires displayed an optical response dominated by strong oscillations of the reflected light as a function of the incident angle. In particular, angle-resolved specular reflectance measurements showed the occurrence of periodic modulations in the polarization-resolved spectra of reflected light for a surprisingly wide range of incident angles. Numerical simulations allowed for identifying the geometrical features of the core–shell nanowires leading to the observed oscillatory effects in terms of core and shell thickness as well as the tapering of the nanostructure. The present results indicate that randomly displaced ensembles of nanoscale heterostructures made of III–V semiconductors can operate as optical metamirrors, with potential for sensing applications.

Highly anisotropic titanium nitride nanowire arrays for low-loss hyperbolic metamaterials fabricated via dynamic oblique deposition

Hyperbolic metamaterials (HMMs) with highly anisotropic metal nanowires exhibit unique optical properties arising from their extraordinary optical anisotropy. Although metal nanowires are often fabricated by embedding noble metals such as silver in an anodic alumina membrane dielectric host, the low melting point of noble metals limits their utilization in high-temperature applications, and there are fabrication issues such as overfilling or discontinuous islands within the host pores. Thus, metal nanowires with a high melting point for HMMs and alternative fabrication techniques are desired. In this study, we fabricated a highly anisotropic nanowire array (NWA) using titanium nitride, which has a high melting point, via dynamic oblique deposition of titanium and subsequent thermal treatment in ammonia. Spectra of ellipsometric parameters were well-fitted by a Fresnel reflection theoretical model considering the optical anisotropy, in which the effective permittivity was described using effective medium theory and the Drude-Lorentz model. The out-of-plane component of the effective permittivity was negative at λ > 850 nm, whereas the in-plane component was positive, indicating that the fabricated NWA behaves as a HMM. The figure of merit in the near-infrared range was higher than that of conventional multilayer TiN HMMs. The NWA presented here is a promising candidate for HMMs in high-temperature applications due to the high melting point of titanium nitride.

Enhanced reflective dichroism from periodic graphene ribbons via total internal reflection

A rigorous homogenization theory is developed to characterize the effective conductivity tensor of periodic graphene ribbons. This way, the obtained conductivity simplifies the study of the exotic scattering properties of periodic graphene ribbons. As a typical example, we find that the performance of reflective dichroism from the designed graphene ribbons can be enhanced (up to a maximum linear dichroism of 0.98) when the total internal reflection happens. Moreover, by rotating its optical axis, the functionality of the periodic graphene ribbon can change from an absorber for linearly polarized waves to another absorber for circularly polarized waves (maximum circular dichroism of 0.93). The revealed indispensable property of graphene ribbons in controlling the reflective dichroism indicates their promising wide applications including energy harvesting and optical sensing.

Tunable optical angular selectivity in hyperbolic metamaterial via photonic topological transitions

An ultra-narrow angular optical transparency window based on photonic topological transition (PTT) is theoretically and numerically investigated in a low-loss hyperbolic metamaterial (HMM) platform, which consists of aligned metallic nanowires embedded indielectric host matrices. Our results indicate that, near the transition point of PTT, the designed system exhibits high-efficiency optical angular selectivity close to normal incidence by tailoring the topology of metamaterial's equi-frequency surface (EFS). Moreover, the operating wavelength (λ₀) is flexibly tunable by selecting appropriate material and geometrical parameters, which provides significant guidance for the later experimental design. Our method is further applied to super-resolution imaging, with a resolution of at least λ₀/4 and over a significant distances (>12λ₀). The HMM-supported angularly selective system could find promising applications for high-efficiency light manipulation and lensless on-chip imaging.

Frequency-doubling effect in acoustic reflection by a nonlinear, architected rotating-square metasurface

Nonlinear acoustic metamaterials offer the potential to enhance wave control opportunities beyond those already demonstrated via dispersion engineering in linear metamaterials. Managing the nonlinearities of a dynamic elastic system, however, remains a challenge, and the need now exists for new strategies to model and design these wave nonlinearities. Inspired by recent research on soft architected rotating-square structures, we propose herein a design for a nonlinear elastic metasurface with the capability to achieve nonlinear acoustic wave reflection control. The designed metasurface is composed of a single layer of rotating squares connected to thin and highly deformable ligaments placed between a rigid plate and a wall. It is shown that during the process of reflection at normal incidence, most of the incoming fundamental wave energy can be converted into the second harmonic wave. A conversion coefficient of approximately 0.8 towards the second harmonic is derived with a reflection coefficient of <0.05 at the incoming fundamental frequency. The theoretical results obtained using the harmonic balance method for a monochromatic pump source are confirmed by time-domain simulations for wave packets. The reported design of a nonlinear acoustic metasurface can be extended to a large family of architected structures, thus opening new avenues for realistic metasurface designs that provide for nonlinear or amplitude-dependent wave tailoring.

Dual-Band Light Absorption Enhancement in Hyperbolic Rectangular Array

The effect of dual-band light absorption enhancement in a hyperbolic rectangular array (HRA) is presented. The enhanced light absorption of the HRA results from the propagating surface plasmon (PSP) resonance, and a dual-band absorption with low and flat sideband level can be realized. The impedance theory is used to evaluate the absorption properties of the HRA, and shows that the input impedances of the HRA varied abruptly around the absorption bands to meet the impedance matching. The absorption spectra of the HRA can be estimated using the effective medium theory (EMT), and its accuracy can be improved as the number of film stacks is increased. The dual-band absorptions of the HRA are very robust to the variations of the width and the number of film stack. Potential application in refractive index sensing can be achieved by utilizing the two absorption bands.

Tunable Hyperbolic Metamaterials Based on Self-Assembled Carbon Nanotubes

We show that packed, horizontally aligned films of single-walled carbon nanotubes are hyperbolic metamaterials with ultrasubwavelength unit cells and dynamic tunability. Using Mueller matrix ellipsometry, we characterize the films' optical properties, which are doping level dependent, and find a broadband hyperbolic region tunable in the mid-infrared. To characterize the dispersion of in-plane hyperbolic plasmon modes, we etch the nanotube films into nanoribbons with differing widths and orientations relative to the nanotube axis, and we observe that the hyperbolic modes support strong light localization. An agreement between the experiments and theoretical models using the ellipsometry data indicates that the packed carbon nanotubes support bulk anisotropic responses at the nanoscale. Self-assembled films of carbon nanotubes are well-suited for applications in thermal emission and photodetection, and they serve as model systems for studying light-matter interactions in the deep subwavelength regime.

Giant Unruh effect in hyperbolic metamaterial waveguides

The Unruh effect is the prediction that an accelerating object perceives its surroundings as a bath of thermal radiation, even if it accelerates in vacuum. The Unruh effect is believed to be very difficult to observe in an experiment, since an observer accelerating at g=9.8  m/s² should see a vacuum temperature of only 4×10^-20  K. Here we demonstrate that photons in metamaterial waveguides may behave as massive quasi-particles accelerating at up to 10²⁴  g, which is about 12 orders of magnitude larger than the surface acceleration near a stellar black hole. These record high accelerations may enable experimental studies of the Unruh effect and the loss of quantum entanglement in strongly accelerated reference frames.

Magnetic Hyperbolic Metasurface: Concept, Design, and Applications

A fundamental cornerstone in nanophotonics is the ability to achieve hyperbolic dispersion of surface plasmons, which shows excellent potentials in many unique applications, such as near-field heat transport, planar hyperlens, strongly enhanced spontaneous emission, and so forth. The hyperbolic metasurfaces with such an ability, however, are currently restricted to electric hyperbolic metasurface paradigm, and realization of magnetic hyperbolic metasurfaces remains elusive despite the importance of manipulating magnetic surface plasmons (MSPs) at subwavelength scale. Here, magnetic hyperbolic metasurfaces are proposed and designed, on which diffraction-free propagation, anomalous diffraction, negative refraction, and frequency-dependent strong spatial distributions of the MSPs in the hyperbolic regime are experimentally observed at microwave frequencies. The findings can be applied to manipulate MSPs and design planarized devices for near-field focusing, imaging, and spatial multiplexers. This concept is also generalizable to terahertz and optical frequencies and inspires novel quantum optical apparatuses with strong magnetic light-matter interactions.

Chiral metamaterials via Moiré stacking

Chiral metamaterials have attracted strong interest due to their versatile capabilities in spin-dependent light manipulation. Benefiting from advancements in nanofabrication and mechanistic understanding of chiroptical effects, chiral metamaterials have shown potential in a variety of applications including circular polarizers, chiral sensors, and chiroptical detectors. Recently, chiral metamaterials made by moiré stacking, superimposing two or more periodic patterns with different lattice constants or relative spatial displacement, have shown promise for chiroptical applications. The moiré chiral metamaterials (MCMs) take advantage of lattice-dependent chirality, giving cost-effective fabrication, flexible tunability, and reconfigurability superior to conventional chiral metamaterials. This feature article focuses on recent progress of MCMs. We discuss optical mechanisms, structural design, fabrication, and applications of the MCMs. We conclude with our perspectives on the future opportunities for the MCMs.

Nanoporous Anodic Alumina Photonic Crystals for Optical Chemo- and Biosensing: Fundamentals, Advances, and Perspectives

Optical sensors are a class of devices that enable the identification and/or quantification of analyte molecules across multiple fields and disciplines such as environmental protection, medical diagnosis, security, food technology, biotechnology, and animal welfare. Nanoporous photonic crystal (PC) structures provide excellent platforms to develop such systems for a plethora of applications since these engineered materials enable precise and versatile control of light⁻matter interactions at the nanoscale. Nanoporous PCs provide both high sensitivity to monitor in real-time molecular binding events and a nanoporous matrix for selective immobilization of molecules of interest over increased surface areas. Nanoporous anodic alumina (NAA), a nanomaterial long envisaged as a PC, is an outstanding platform material to develop optical sensing systems in combination with multiple photonic technologies. Nanoporous anodic alumina photonic crystals (NAA-PCs) provide a versatile nanoporous structure that can be engineered in a multidimensional fashion to create unique PC sensing platforms such as Fabry⁻Pérot interferometers, distributed Bragg reflectors, gradient-index filters, optical microcavities, and others. The effective medium of NAA-PCs undergoes changes upon interactions with analyte molecules. These changes modify the NAA-PCs' spectral fingerprints, which can be readily quantified to develop different sensing systems. This review introduces the fundamental development of NAA-PCs, compiling the most significant advances in the use of these optical materials for chemo- and biosensing applications, with a final prospective outlook about this exciting and dynamic field.

Self-assembled vertically aligned Ni nanopillars in CeO2 with anisotropic magnetic and transport properties for energy applications

Self-assembled vertically aligned metal-oxide (Ni-CeO2) nanocomposite thin films with novel multifunctionalities have been successfully deposited by a one-step growth method. The novel nanocomposite structures presents high-density Ni-nanopillars vertically aligned in a CeO2 matrix. Strong and anisotropic magnetic properties have been demonstrated, with a saturation magnetization (Ms) of ∼175 emu cm-3 and ∼135 emu cm-3 for out-of-plane and in-plane directions, respectively. Such unique vertically aligned ferromagnetic Ni nanopillars in the CeO2 matrix have been successfully incorporated in high temperature superconductor YBa2Cu3O7 (YBCO) coated conductors as effective magnetic flux pinning centers. The highly anisotropic nanostructures with high density vertical interfaces between the Ni nanopillars and CeO2 matrix also promote the mixed electrical and ionic conductivities out-of-plane and thus demonstrate great potential as nanocomposite anode materials for solid oxide fuel cells and other potential applications requiring anisotropic ionic transport properties.

Topological negative refraction of surface acoustic waves in a Weyl phononic crystal

Reflection and refraction of waves occur at the interface between two different media. These two fundamental interfacial wave phenomena form the basis of fabricating various wave components, such as optical lenses. Classical refraction-now referred to as positive refraction-causes the transmitted wave to appear on the opposite side of the interface normal compared to the incident wave. By contrast, negative refraction results in the transmitted wave emerging on the same side of the interface normal. It has been observed in artificial materials^1-5, following its theoretical prediction⁶, and has stimulated many applications including super-resolution imaging⁷. In general, reflection is inevitable during the refraction process, but this is often undesirable in designing wave functional devices. Here we report negative refraction of topological surface waves hosted by a Weyl phononic crystal-an acoustic analogue of the recently discovered Weyl semimetals^8-12. The interfaces at which this topological negative refraction occurs are one-dimensional edges separating different facets of the crystal. By tailoring the surface terminations of the Weyl phononic crystal, constant-frequency contours of surface acoustic waves can be designed to produce negative refraction at certain interfaces, while positive refraction is realized at different interfaces within the same sample. In contrast to the more familiar behaviour of waves at interfaces, unwanted reflection can be prevented in our crystal, owing to the open nature of the constant-frequency contours, which is a hallmark of the topologically protected  surface states in Weyl crystals^8-12.

Controlling Disorder by Electric-Field-Directed Reconfiguration of Nanowires To Tune Random Lasing

Top-down fabrication is commonly used to provide positioning control of optical structures; yet, it places stringent limitations on component materials, and oftentimes, dynamic reconfigurability is challenging to realize. Here, we present a reconfigurable nanoparticle platform that can integrate heterogeneous particle assembly of different shapes, sizes, and chemical compositions. We demonstrate dynamic control of disorder in this platform and use it to tune random laser emission characteristics for a suspension of titanium dioxide nanowires in a dye solution. Using an alternating current electric field, we control the nanowire orientation to dynamically engineer the collective scattering of the sample. Our theoretical model indicates that a change of up to 22% in scattering coefficient can be achieved for the experimentally determined nanowire length distribution upon alignment. Dependence of light confinement on anisotropic particle alignment provides a means to reversibly tune random laser characteristics; a nearly 20-fold increase in lasing intensity was observed with aligned particle orientation. We illustrate the generality of the approach by demonstrating enhanced lasing for aligned nanowires of other materials including gold, mixed gold/dielectric, and vanadium oxide.

Nanoscale Artificial Plasmonic Lattice in Self-Assembled Vertically Aligned Nitride-Metal Hybrid Metamaterials

Nanoscale metamaterials exhibit extraordinary optical properties and are proposed for various technological applications. Here, a new class of novel nanoscale two-phase hybrid metamaterials is achieved by combining two major classes of traditional plasmonic materials, metals (e.g., Au) and transition metal nitrides (e.g., TaN, TiN, and ZrN) in an epitaxial thin film form via the vertically aligned nanocomposite platform. By properly controlling the nucleation of the two phases, the nanoscale artificial plasmonic lattices (APLs) consisting of highly ordered hexagonal close packed Au nanopillars in a TaN matrix are demonstrated. More specifically, uniform Au nanopillars with an average diameter of 3 nm are embedded in epitaxial TaN platform and thus form highly 3D ordered APL nanoscale metamaterials. Novel optical properties include highly anisotropic reflectance, obvious nonlinear optical properties indicating inversion symmetry breaking of the hybrid material, large permittivity tuning and negative permittivity response over a broad wavelength regime, and superior mechanical strength and ductility. The study demonstrates the novelty of the new hybrid plasmonic scheme with great potentials in versatile material selection, and, tunable APL spacing and pillar dimension, all important steps toward future designable hybrid plasmonic materials.

Orbital angular momentum sidebands of vortex beams transmitted through a thin metamaterial slab

A pure vortex beam carrying m-order orbital angular momentum (OAM) will be deformed when transmitting through a thin slab, and "neighboring" sideband {m + 1} and {m-1} modes will emerge. The emergence of the OAM sideband is accompanied with OAM-dependent Goos-Hänchen (GH) shift. When the energies carried by the {m} mode of the transmitted beam and by the sideband modes are identical, the OAM-dependent shifts reach their upper limits, |m|w₀/2(|m| + 1)1/2, where w₀ is the incident beam waist. The epsilon-near-zero metamaterial is found to be suitable to achieve the upper-limited OAM-dependent GH shifts. These findings provide a deeper insight into the beam shifts of vortex beams and have potential applications in the optical sensing, detection of OAM, and other OAM-based applications.

Low-frequency nonlocal and hyperbolic modes in corrugated wire metamaterials

Metamaterials based on arrays of aligned plasmonic nanowires have recently attracted significant attention due to their unique optical properties that combine tunable strong anisotropy and nonlocality. These optical responses provide a platform for implementation of novel sensing, imaging, and quantum optics applications. Basic building blocks, used for construction of those peculiar composites, are plasmonic metals, such as gold and silver, which have moderate negative values of permittivities at the optical spectral range. Scaling the plasmonic behavior to lower frequencies remains a longstanding challenge also owing to the emergence of strong spatial dispersion in homogenized artificial composites. At lower THz and GHz frequencies, the electromagnetic response of noble metals approaches that of perfect electric conductors, preventing straightforward scaling of visible-frequency plasmonics to the frequency domains that are important for a vast range of applications, including wireless communications, microwave technologies and many others. Here we demonstrate that both extreme anisotropy (so-called hyperbolicity) and nonlocality of artificial composites can be achieved and designed in arrays of corrugated perfectly conducting wires at relatively low GHz frequencies. The key concept is based on hybridization of spoof plasmon polariton modes that in turn emulate surface polariton waves in systems with corrugated interfaces. The method makes it possible to map the recent developments in the field of plasmonics and metamaterials to the domain of THz and RF photonics.

Plasmonic Surface Lattice Resonances: A Review of Properties and Applications

When metal nanoparticles are arranged in an ordered array, they may scatter light to produce diffracted waves. If one of the diffracted waves then propagates in the plane of the array, it may couple the localized plasmon resonances associated with individual nanoparticles together, leading to an exciting phenomenon, the drastic narrowing of plasmon resonances, down to 1–2 nm in spectral width. This presents a dramatic improvement compared to a typical single particle resonance line width of >80 nm. The very high quality factors of these diffractively coupled plasmon resonances, often referred to as plasmonic surface lattice resonances, and related effects have made this topic a very active and exciting field for fundamental research, and increasingly, these resonances have been investigated for their potential in the development of practical devices for communications, optoelectronics, photovoltaics, data storage, biosensing, and other applications. In the present review article, we describe the basic physical principles and properties of plasmonic surface lattice resonances: the width and quality of the resonances, singularities of the light phase, electric field enhancement, etc. We pay special attention to the conditions of their excitation in different experimental architectures by considering the following: in-plane and out-of-plane polarizations of the incident light, symmetric and asymmetric optical (refractive index) environments, the presence of substrate conductivity, and the presence of an active or magnetic medium. Finally, we review recent progress in applications of plasmonic surface lattice resonances in various fields.

Metamaterials with index ellipsoids at arbitrary k-points

Propagation behaviors of electromagnetic waves are governed by the equifrequency surface of the medium. Up to now, ordinary materials, including the medium exist in nature and the man-made metamaterials, always have an equifrequency surface (ellipsoid or hyperboloid) centered at zero k-point. Here we propose a new type of metamaterial possessing multiple index ellipsoids centered at arbitrary nonzero k-points. Their locations in momentum space are determined by the connectivity of a set of interpenetrating metallic scaffolds, whereas the group velocities of the modes are determined by the geometrical details. Such system is a new class of metamaterial whose properties arise from global connectivity and hence can have broadband functionality in applications such as negative refraction, orientation-dependent coupling effect, and cavity without walls, and they are fundamentally different from ordinary resonant metamaterials that are inherently bandwidth limited. We perform microwave experiments to confirm our findings.

Growth of vertically aligned nanowires in metal-oxide nanocomposites: kinetic Monte-Carlo modeling versus experiments

We employ kinetic Monte-Carlo simulations to study the growth process of metal-oxide nanocomposites obtained via sequential pulsed laser deposition. Using Ni-SrTiO3 (Ni-STO) as a model system, we reduce the complexity of the computational problem by choosing a coarse-grained approach mapping Sr, Ti and O atoms onto a single effective STO pseudo-atom species. With this ansatz, we scrutinize the kinetics of the sequential synthesis process, governed by alternating deposition and relaxation steps, and analyze the self-organization propensity of Ni atoms into straight vertically aligned nanowires embedded in the surrounding STO matrix. We finally compare the predictions of our binary toy model with experiments and demonstrate that our computational approach captures fundamental aspects of self-assembled nanowire synthesis. Despite its simplicity, our modeling strategy successfully describes the impact of relevant parameters like the concentration or laser frequency on the final nanoarchitecture of metal-oxide thin films grown via pulsed laser deposition.

Multi-nanolayered VO2/Sapphire Thin Film via Spinodal Decomposition

Coating of VO₂-based thin film has been extensively studied for fabricating energy-saving smart windows. One of the most efficient ways for fabricating high performance films is to create multi-nanolayered structure. However, it has been highly challenge to make such layers in the VO₂-based films using conventional methods. In this work, a facile two-step approach is established to fabricate multilayered VO₂-TiO₂ thin films. We first deposited the amorphous thin films upon sputtering, and then anneal them to transform the amorphous phase into alternating Ti- and V-rich multilayered nanostructure via a spinodal decomposition mechanism. In particular, we take advantage of different sapphire substrate planes (A-plane (11-20), R-plane (1-102), C-plane (0001), and M-plane (10-10)) to achieve different decomposition modes. The new approach has made it possible to tailoring the microstructure of the thin films for optimized performances by controlling the disorder-order transition in terms of both kinetic and thermodynamic aspects. The derived thin films exhibit superior optical modulation upon phase transition, significantly reduced transition temperature and hysteresis loop width, and high degradation resistance, these improvements indicate a high potential to be used for fabricating the next generation of energy saving smart windows.

Inverse-Designed Broadband All-Dielectric Electromagnetic Metadevices

This paper presents a platform combining an inverse electromagnetic design computational method with additive manufacturing to design and fabricate all-dielectric metadevices. As opposed to conventional flat metasurface-based devices that are composed of resonant building blocks resulting in narrow band operation, the proposed design approach creates non-resonant, broadband (Δλ/λ up to >50%) metadevices based on low-index dielectric materials. High-efficiency (transmission >60%), thin (≤2λ) metadevices capable of polarization splitting, beam bending, and focusing are proposed. Experimental demonstrations are performed at millimeter-wave frequencies using 3D-printed devices. The proposed platform can be readily applied to the design and fabrication of electromagnetic and photonic metadevices spanning microwave to optical frequencies.

Low thermal emissivity surfaces using AgNW thin films

The properties of silver nanowire (AgNW) films in the optical and infrared spectral regime offer an interesting opportunity for a broad range of applications that require low-emissivity coatings. This work reports a method to reduce the thermal emissivity of substrates by the formation of low-emissivity AgNW coating films from solution. The spectral emissivity was characterized by thermal imaging with an FLIR camera, followed by Fourier transform infrared spectroscopy. In a combined experimental and simulation study, we provide fundamental data of the transmittance, reflectance, haze, and emissivity of AgNW thin films. Emissivity values were finely tuned by modifying the concentration of the metal nanowires in the films. The simulation models based on the transfer matrix method developed for the AgNW thin films provided optical values that show a good agreement with the measurements.

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.

Realization of III-V Semiconductor Periodic Nanostructures by Laser Direct Writing Technique

In this paper, we demonstrated the fabrication of one-dimensional (1D) and two-dimensional (2D) periodic nanostructures on III-V GaAs substrates utilizing laser direct writing (LDW) technique. Metal thin films (Ti) and phase change materials (Ge₂Sb₂Te₅ (GST) and Ge₂Sb_1.8Bi_0.2Te₅ (GSBT)) were chosen as photoresists to achieve small feature sizes of semiconductor nanostructures. A minimum feature size of about 50 nm about a quarter of the optical diffraction limit was obtained on the photoresists, and 1D III-V semiconductor nanolines with a minimum width of 150 nm were successfully acquired on the GaAs substrate which was smaller than the best results acquired on Si substrate ever reported. 2D nanosquare holes were fabricated as well by using Ti thin film as the photoresist, with a side width of about 200 nm, but the square holes changed to a rectangle shape when GST or GSBT was employed as the photoresist, which mainly resulted from the interaction of two cross-temperature fields induced by two scanning laser beams. The interacting mechanism of different photoresists in preparing periodic nanostructures with the LDW technique was discussed in detail.

Self-Assembled InAs Nanowires as Optical Reflectors

Subwavelength nanostructured surfaces are realized with self-assembled vertically-aligned InAs nanowires, and their functionalities as optical reflectors are investigated. In our system, polarization-resolved specular reflectance displays strong modulations as a function of incident photon energy and angle. An effective-medium model allows one to rationalize the experimental findings in the long wavelength regime, whereas numerical simulations fully reproduce the experimental outcomes in the entire frequency range. The impact of the refractive index of the medium surrounding the nanostructure assembly on the reflectance was estimated. In view of the present results, sensing schemes compatible with microfluidic technologies and routes to innovative nanowire-based optical elements are discussed.

Plasmonic resonances in hybrid systems of aluminum nanostructured arrays and few layer graphene within the UV-IR spectral range

The size-controllable and ordered Al nanocavities and nanodomes arrays were synthesized by electrochemical anodization of aluminum using phosphoric acid, citric acid and mixture both acids. Few layer graphene (FLG) was transferred directly on top of Al nanostructures and their morphology were evaluated by scanning electron microscopy. The interaction between FLG and the plasmonic properties of Al nanostructures arrays were investigated based on specular reflectivity in the ultraviolet-visible-infrared range and Raman spectroscopy. We found that their optical reflectivity was dramatically reduced as compared with unstructured Al. At the same time pronounced reflectivity dips were detectable in the 200-896 nm wavelength range, which were ascribed to plasmonic resonances. The plasmonic properties of these nanostructures do not exhibit evident changes by the presence of FLG in the UV-vis range of the electromagnetic spectrum. By contrast, the surface-enhanced Raman spectroscopy of FLG was observed in nanocavities and nanodomes structures that result in an intensity increase of the characteristic G and 2D bands of FLG induced by the plasmonic properties of Al nanostructures.

Launching Phonon Polaritons by Natural Boron Nitride Wrinkles with Modifiable Dispersion by Dielectric Environments

Interference-free hyperbolic phonon polaritons (HPPs) excited by natural wrinkles in a hexagonal boron nitride (hBN) microcrystal are reported both experimentally and theoretically. Although their geometries are off-resonant with the excitation wavelength, the wrinkles compensate for the large momentum mismatch between photon and phonon polariton, and launch the HPPs without interference. The spatial feature of wrinkles is about 200 nm, which is an order of magnitude smaller than resonant metal antennas at the same excitation wavelength. Compared with phonon polaritons launched by an atomic force microscopy tip, the phonon polaritons launched by wrinkles are interference-free, independent of the launcher geometry, and exhibit a smaller damping rate (γ ≈ 0.028). On the same hBN microcrystal, in situ nanoinfrared imaging of HPPs launched by different mechanisms is performed. In addition, the dispersion of HPPs is modified by changing the dielectric environments of hBN crystals. The wavelength of HPPs is compressed twofold when the substrate is changed from SiO₂ to gold. The findings provide insights into the intrinsic properties of hBN-HPPs and demonstrate a new way to launch and control polaritons in van der Waals materials.

Spiraling Light with Magnetic Metamaterial Quarter-Wave Turbines

Miniaturized quarter-wave plate devices empower spin to orbital angular momentum conversion and vector polarization formation, which serve as bridges connecting conventional optical beam and structured light. Enabling the manipulability of additional dimensions as the complex polarization and phase of light, quarter-wave plate devices are essential for exploring a plethora of applications based on orbital angular momentum or vector polarization, such as optical sensing, holography, and communication. Here we propose and demonstrate the magnetic metamaterial quarter-wave turbines at visible wavelength to produce radially and azimuthally polarized vector vortices from circularly polarized incident beam. The magnetic metamaterials function excellently as quarter-wave plates at single wavelength and maintain the quarter-wave phase retardation in broadband, while the turbine blades consist of multiple polar sections, each of which contains homogeneously oriented magnetic metamaterial gratings near azimuthal or radial directions to effectively convert circular polarization to linear polarization and induce phase shift under Pancharatnum-Berry's phase principle. The perspective concept of multiple polar sections of magnetic metamaterials can extend to other analogous designs in the strongly coupled nanostructures to accomplish many types of light phase-polarization manipulation and structured light conversion in the desired manner.

Broadband polarization conversion with anisotropic plasmonic metasurfaces

Metasurfaces offer exciting opportunities that enable precise control of light propagation, optical intensity, phase and polarization. Plasmonic metasurface based quarter-wave plates have been recently studied to realize the conversion between linear polarization and circular polarization. However, it is still quite challenging to directly measure the birefringent phase retardation introduced by metasurface wave plates with a reliable technique. Here, we report a high-performance broadband metasurface quarter-wave plate made of anisotropic T-shaped plasmonic antennas in near-infrared wavelength range, where the achromatic nearly 90° transmitted phase retardation through the metasurface is precisely characterized with an optical vortex based interferometric approach. Based on the measured transmission amplitude and phase of two orthogonal linear polarization components, nearly unit degree of linear polarization is extracted from the Stokes parameters, indicating excellent broadband polarization conversion between linearly and circularly polarized light through the metasurface. Our results will be an important step forward in the advancement of integrated metasurface devices for polarization conversion and beam manipulation, structured light control, as well as new spectroscopic and interferometric techniques for metasurface characterization.

Broadband room temperature strong coupling between quantum dots and metamaterials

Herein, we report the first demonstration of room temperature enhanced light-matter coupling in the visible regime for metamaterials using cooperative coupled quasi two dimensional quantum dot assemblies located at precise distances from the hyperbolic metamaterial (HMM) templates. The non-monotonic variation of the magnitude of strong coupling, manifested in terms of strong splitting of the photoluminescence of quantum dots, can be explained in terms of enhanced LDOS near the surface of such metamaterials as well as the plasmon mediated super-radiance of closely spaced quantum dots (QDs). Our methodology of enhancing broadband, room temperature, light-matter coupling in the visible regime for metamaterials opens up new possibilities of utilising these materials for a wide range of applications including QD based thresholdless nanolasers and novel metamaterial based integrated photonic devices.

Hyperbolic-polaritons-enabled dark-field lens for sensitive detection

Sensitive detection of features in a nanostructure may sometimes be puzzled in the presence of significant background noise. In this regard, background suppression and super-resolution are substantively important for detecting weakly scattering nanoscale features. Here, we present a lens design, termed hyperbolic-polaritons-enabled dark-field lens (HPEDL), which has the ability to accomplish straightforward sensitive detection. This HPEDL structure consists of type I and type II hyperbolic media that support high-k field waves via hyperbolic polaritons (HPs). We show that the cone-like characteristics of the HPs could be manipulated while the influence of the low-k field waves would be removed. Numerical simulations demonstrate that this proposed structure can successfully realize straightforward sensitive detection by modifying its thickness under the phase compensation condition. Besides, the minimum resolvable length and angular-dependent performance for sensitive detection are also demonstrated by simulations. Remarkably, these findings are very promising for propelling nanophotonics technologies and constitute a further important step towards practical applications of optical microscopy.

Fabrication of large area flexible nanoplasmonic templates with flow coating

We describe the development of a custom-built two-axis flow coater for the deposition of polymeric nanosphere monolayers that could be used in the fabrication of large area nanoplasmonic films. The technique described here has the capability of depositing large areas (up to 7 in. × 10 in.) of self-assembled monolayers of polymeric nanospheres onto polyethylene terephthalate (PET) films. Here, three sets of films consisting of different diameters (ranging from 100 to 300 nm) of polymeric nanospheres were used to demonstrate the capabilities of this instrument. To improve the surface wettability of the PET substrates during wet-deposition, we enhanced the wettability by using a forced air blown-arc plasma treatment system. Both the local microstructure, as confirmed by scanning electron microscopy, describing monolayer and multilayer coverage, and the overall macroscopic uniformity of the resultant nanostructured film were optimized by controlling the relative stage to blade speed and nanosphere concentration. We also show using a smaller nanoparticle template that such monolayers can be used to form nanoplasmonic films. As this flow-coating approach is a scalable technique, large area films such as the ones described here have a variety of crucial emerging applications in areas such as energy, catalysis, and chemical sensing.

Self-assembled Co-BaZrO3 nanocomposite thin films with ultra-fine vertically aligned Co nanopillars

A simple one-step pulsed laser deposition (PLD) method has been applied to grow self-assembled metal-oxide nanocomposite thin films. The as-deposited Co-BaZrO₃ films show high epitaxial quality with ultra-fine vertically aligned Co nanopillars (diameter <5 nm) embedded in a BZO matrix. The diameter of the nanopillars can be further tuned by varying the deposition frequency. The metal and oxide phases grow separately without inter-diffusion or mixing. Taking advantage of this unique structure, a high saturation magnetization of ∼1375 emu cm^-3 in the Co-BaZrO₃ nanocomposites has been achieved and further confirmed by Lorentz microscopy imaging in TEM. Furthermore, the coercivity values of this nanocomposite thin films range from 600 Oe (20 Hz) to 1020 Oe (2 Hz), which makes the nanocomposite an ideal candidate for high-density perpendicular recording media.

Switchable reflection/transmission utilizing polarization on a plasmonic structure consisting of self-assembly polystyrene spheres with silver patches

We report a plasmonic structure for switchable reflection and transmission by polarization. The structure is composed of a hexagonal-packed polystyrene sphere array with silver patches on them. Simulations and experiments demonstrated that the conversions between reflected beams and transmitted ones can be performed when the polarization directions of incident beams vary from 0° to 90°. A switchable reflection and transmission at a given wavelength can be obtained, as long as sizes of PS spheres and azimuthal angles are properly chosen. Such a patchy plasmonic structure serving as a switch between reflection and transmission have potential applications in photoelectric control devices.

Optical wave parameters for spatially dispersive and anisotropic nanomaterials

Spatial dispersion is an intriguing property of essentially all nanostructured optical media. In particular, it makes optical waves with equal frequencies and polarizations have different wavelengths, if they propagate in different directions. This can offer new approaches to control light radiation and propagation. Spatially dispersive nanomaterials, such as metamaterials, are often treated in terms of wave parameters, such as refractive index and impedance retrieved from reflection and transmission coefficients of the material at each incidence angle. Usually, however, the waves are approximated as transverse, which simplifies the description, but yields wrong results, if spatial dispersion or optical anisotropy is significant. In this work, we present a method to calculate the wave parameters of a general spatially dispersive and optically anisotropic medium without applying such an approximation. The method allows one to evaluate the true impedances and field vectors of the effective waves, obtaining thus the true light intensity and energy propagation direction in the medium. The equations are applied to several examples of spatially dispersive and anisotropic materials. The method introduces new insights into optics of nanostructured media and extends the design of such media towards optical phenomena involving significant spatial dispersion.

Near-perfect broadband absorption from hyperbolic metamaterial nanoparticles

Broadband absorbers are essential components of many light detection, energy harvesting, and camouflage schemes. Current designs are either bulky or use planar films that cause problems in cracking and delamination during flexing or heating. In addition, transferring planar materials to flexible, thin, or low-cost substrates poses a significant challenge. On the other hand, particle-based materials are highly flexible and can be transferred and assembled onto a more desirable substrate but have not shown high performance as an absorber in a standalone system. Here, we introduce a class of particle absorbers called transferable hyperbolic metamaterial particles (THMMP) that display selective, omnidirectional, tunable, broadband absorption when closely packed. This is demonstrated with vertically aligned hyperbolic nanotube (HNT) arrays composed of alternating layers of aluminum-doped zinc oxide and zinc oxide. The broadband absorption measures >87% from 1,200 nm to over 2,200 nm with a maximum absorption of 98.1% at 1,550 nm and remains large for high angles. Furthermore, we show the advantages of particle-based absorbers by transferring the HNTs to a polymer substrate that shows excellent mechanical flexibility and visible transparency while maintaining near-perfect absorption in the telecommunications region. In addition, other material systems and geometries are proposed for a wider range of applications.

Hydrophilicity Reinforced Adhesion of Anodic Alumina Oxide Template Films to Conducting Substrates for Facile Fabrication of Highly Ordered Nanorod Arrays

Arrays of ordered nanorods are of special interest in many fields. However, it remains challenging to obtain such arrays on conducting substrates in a facile manner. In this article, we report the fabrication of highly ordered and vertically standing nanorod arrays of both metals and semiconductors on Au films and indium tin oxide glass substrates without an additional layering. In this approach, following the simple hydrophilic treatment of an anodic aluminum oxide (AAO) membrane and conducting substrates, the AAO membrane was transferred onto the modified substrates with excellent adhesion. Subsequently, nanorod arrays of various materials were electrodeposited on the conducting substrates directly. This method avoids any expensive and tedious lithographic and ion milling process, which provides a simple yet robust route to the fabrication of arrays of 1D materials with high aspect ratio on conducting substrates, which shall pave the way for many practical applications in a range of fields.

Electrically tunable metasurface perfect absorber for infrared frequencies

We theoretically investigate a metasurface perfect absorber based on indium-tin-oxide as active material. Our design scheme relies on conventional metal-oxide-semiconductor model and the Drude model. Inducing a voltage into the device causes a blue-shift of 50 nm in the reflectance spectrum in the infrared region. The total thickness of the device is only 3.5% of the working wavelength λ = 2.56 μm, and the rate of reflectance change reaches 5.16 at λ = 2.56 μm. Because the material that we use has advantages of easy fabrication and fast response, our design approach can be used for numerous applications on active plasmonic sensors and filters.

Propagation characteristics of an extremely anisotropic metamaterial loaded helical guide

In this study, we report slow wave propagation characteristics of an extremely anisotropic metamaterial loaded helical guide (EAMLHG). An analytical expression has been theoretically derived and numerically computed to get exact solutions of all possible guided modes of propagation. Anisotropy is defined in terms of positive longitudinal permittivity (ϵz > 0) and negatives transverse permittivity value (ϵt < 0). The waveguide supports hybrid (HE) mode propagation and possesses characteristics of backward wave (BW) mode, forward wave (FW) mode, zero-group velocity and mode-degeneracy. The large value of effective index of BW mode and mode-degeneracy mechanism leads to slowing and trapping of electromagnetic (EM) wave. Closed-form guided mode energy propagation expressions has been also derived and computed which exhibits zero power flow at mode degeneracy point. A comparative study is also carried out between extremely anisotropic metamaterial helical waveguide (EAMLHG) and conventional extremely anisotropic metamaterial cylindrical guide (EAMCG), which reveals enhanced slow wave behaviour. Engineering feasible design and analysis is also presented by combining alternate disks of silver and glass as an extremely anisotropic medium which exhibits lossy and dispersive properties. This type of waveguide can find applications as a filter, phase shifter, and delay lines in microwave to THz applications and, as an optical buffer in optoelectronics applications.

Deep sub-wavelength nanofocusing of UV-visible light by hyperbolic metamaterials

Confining light into a sub-wavelength area has been challenging due to the natural phenomenon of diffraction. In this paper, we report deep sub-wavelength focusing via dispersion engineering based on hyperbolic metamaterials. Hyperbolic metamaterials, which can be realized by alternating layers of metal and dielectric, are materials showing opposite signs of effective permittivity along the radial and the tangential direction. They can be designed to exhibit a nearly-flat open isofrequency curve originated from the large-negative permittivity in the radial direction and small-positive one in the tangential direction. Thanks to the ultraflat dispersion relation and curved geometry of the multilayer stack, hyperlens can magnify or demagnify an incident beam without diffraction depending on the incident direction. We numerically show that hyperlens-based nanofocusing device can compress a Gaussian beam down to tens-of-nanometers of spot size in the ultraviolet (UV) and visible frequency range. We also report four types of hyperlenses using different material combinations to span the entire range of visible frequencies. The nanofocusing device based on the hyperlens, unlike conventional lithography, works under ordinary light source without complex optics system, giving rise to practical applications including truly nanoscale lithography and deep sub-wavelength scale confinement.

Self-Assembled, Nanostructured, Tunable Metamaterials via Spinodal Decomposition

Self-assembly via nanoscale phase separation offers an elegant route to fabricate nanocomposites with physical properties unattainable in single-component systems. One important class of nanocomposites are optical metamaterials which exhibit exotic properties and lead to opportunities for agile control of light propagation. Such metamaterials are typically fabricated via expensive and hard-to-scale top-down processes requiring precise integration of dissimilar materials. In turn, there is a need for alternative, more efficient routes to fabricate large-scale metamaterials for practical applications with deep-subwavelength resolution. Here, we demonstrate a bottom-up approach to fabricate scalable nanostructured metamaterials via spinodal decomposition. To demonstrate the potential of such an approach, we leverage the innate spinodal decomposition of the VO₂-TiO₂ system, the metal-to-insulator transition in VO₂, and thin-film epitaxy, to produce self-organized nanostructures with coherent interfaces and a structural unit cell down to 15 nm (tunable between horizontally and vertically aligned lamellae) wherein the iso-frequency surface is temperature-tunable from elliptic to hyperbolic dispersion producing metamaterial behavior. These results provide an efficient route for the fabrication of nanostructured metamaterials and other nanocomposites for desired functionalities.