Search

Tunable transmission filters are essential for various key terahertz applications including miniaturized spectrometers, hyperspectral imagers and channel selectors in high-speed wireless communication systems. However, tunable terahertz transmission filters have remained elusive, so far. Here, we report an electromechanically reconfigurable microcantilever integrated complementary metamaterial to function as a frequency-agile bandpass transmission filter at terahertz spectral region. A large tunable range of 70% at central frequency of 0.455 THz is designed and the experimental results show switching of terahertz transmission frequency range of 54% at central frequency of 0.5 THz. The proposed electromechanically reconfigurable metadevice provides large tunable range, uses electrical control, and is fabricated using CMOS compatible materials and process, thus making it highly attractive for the realization of plethora of miniaturized high-performance terahertz components and devices.

Rapid advances in metamaterial technology are enabling the engineering of wave-matter interactions heretofore not realized and functionalities with potentially far-reaching implications for major challenges in the fields of energy conservation and radio frequency (RF) communication. We propose a visibly and RF transparent composite metasurface utilizing dielectric-metal spectrally selective coatings with high NIR control and low thermal emissivity, thus achieving a multi-functional metasurface capable of enhancing 5G communication efficiency and exhibiting energy conservation features. The proposed meta-glass yields 92% peak RF transmission at 30 GHz which corresponds to 20% and 90% enhancement when compared to plain glass and low-emissive glass substrates. This meta-glass possesses 86% peak optical transparency at $\lambda=550 nm$, $>$60% near-IR reflection, and $>$ 80% mid-IR reflection which corresponds to $\approx$ 0.2 thermal emissivity. The proposed metasurface design is highly flexible and can be tuned to operate over different frequency ranges owing to its frequency scalability. This study provides a better alternative using earth-abundant materials compared to low-emissive glass based on indium tin oxide (ITO) while boosting the efficiency of 5G communication amenable to window systems demanding simultaneous functionalities for emergent smart/energy-efficient buildings/cities and autonomous transportation applications.

Strong electromagnetic field confinement and enhancement can be readily achieved in plasmonic nanoantennas, however, this is considerably more difficult to realize over large areas, which is essential for many applications. Here, dispersion engineering in plasmonic metamaterials is applied to successfully develop and demonstrate a coupled array of plasmonic–dielectric nanoantennas offering an ultrahigh density of electromagnetic hot spots (10 cm ) over macroscopic, centimeter scale areas. The hetero-metamaterial is formed by a highly ordered array of vertically standing plasmonic dipolar antennas with a ZnO gap and fabricated using a scalable electrodeposition technique. It supports a complex modal structure, including guided, surface and gap modes, which offers rich opportunities, frequently beyond the local effective medium theory, with optical properties that can be easily controlled and defined at the fabrication stage. This metamaterial platform can be used in a wide variety of applications, including hot-electron generation, nanoscale light sources, sensors, as well as nonlinear and memristive devices. 11 −2

Radiative losses are crucial in optimizing the performance of metamaterial-based devices across the electromagnetic spectrum. Introducing structural asymmetry in meta-atom design leads to the excitation of sharp Fano resonances with reduced radiative losses. However, at larger asymmetries, the Fano resonance becomes highly radiative which results in the broadening of the asymmetric line shaped resonances. Here, the authors experimentally demonstrate a scheme to couple mirrored asymmetric Fano resonators through interaction of anti-aligned magnetic dipoles which results in a strong toroidal resonance in 2D planar metasurface. The quality (Q) factor and the figure of merit of the toroidal dipolar mode are significantly higher than the Fano resonance. Moreover, the authors discover that the exponential decay of the Q factor of toroidal resonance mode occurs at half the rate of that in the Fano resonance as the asymmetry of the system is enhanced which indicates significant tailoring and the suppression of the radiative loss channel in the toroidal configuration. The weakly radiative toroidal resonance in planar metamaterials offers the potential for applications in terahertz and optical sensing, spectroscopy, narrow-band filtering, and large modulation.

CDW would like to acknowledge funding via EPSRC grants EP/M015173/1 and EP/M015130/1. TJO acknowledges funding from the EPSRC Centre for Doctoral Training in Metamaterials, grant number EP/L015331/1

Surface plasmon resonance (SPR) sensors operate mainly on prism and grating coupling techniques, with spectral and angular scans being the two major interrogation schemes. Among them, the angular scan technique has several advantages including higher measurement precision owing to its higher signal-to-noise ratio. The currently available SPR sensor arrangements provide a maximum angular sensitivity of 500°–600° per RIU. Here, we report the study of grating coupled-hyperbolic metamaterial (GC-HMM) sensors with high angular sensitivity. The experimental studies show extraordinary angular sensitivities from visible to near infrared (NIR) wavelengths by exciting bulk plasmon polaritons associated with hyperbolic metamaterials, with a maximum of 7000° per RIU. This angular-scan plasmonic biosensor has been used for the detection of low molecular weight biomolecules such as biotin (244 Da) and high molecular weight macromolecules such as Cowpea mosaic virus (CPMV, 5.6 × 106 Da) at ultralow concentrations. The miniaturized sensing device can be integrated with microfluidic systems for the development of next-generation biosensors for lab-on-a-chip and point-of-care applications.

Goos–Hanchen (G–H) effect is of great interest in the manipulation of optical beams. However, it is still fairly challenging to attain efficient controls of the G–H shift for diverse applications. Here, a mechanism to realize tunable G–H shift in the terahertz regime with electrically controllable graphene is proposed. Taking monolayer graphene covered epsilon-near-zero metamaterial as a planar model system, it is found that the G–H shifts for the orthogonal s-polarized and p-polarized terahertz beams at oblique incidence are positive and negative, respectively. The G–H shift can be modified substantially by electrically controlling the Fermi energy of the monolayer graphene. Reversely, the Fermi energy dependent G–H effect can also be used as a strategy for measuring the doping level of graphene. In addition, the G–H shifts of the system are of strong frequency-dependence at oblique angles of incidence, therefore the proposed graphene hybrid system can potentially be used for the generation of terahertz “rainbow,” a flat analog of the dispersive prism in optics. The proposed scheme of hybrid system involving graphene for dynamic control of G–H shift will have potential applications in the manipulation of terahertz waves.

We demonstrate 2D and multilayer dielectric metamaterials made from III–V semiconductors using a monolithic fabrication process. The resulting structures could be used to recompress chirped femtosecond optical pulses and in a variety of other optical applications requiring low loss. Moreover, these III–V all-dielectric metamaterials could enable novel active applications such as efficient nonlinear frequency converters, light emitters, detectors, and modulators.

A precise and scalable method is introduced for realizing deeply subwavelength nanostructures through laser-induced dewetting. This fabrication method is used to realize a three layer planar absorber device that allows for highly tunable selective visible-IR light absorption. This large area laser dewetting method uses a 532 nm laser with a μm ranged focal spot on a translation stage allowing for fine pattern control. A range of laser powers from 60 to 130 mW is explored and it is found that three dramatically different structures, all aperiodic and random in nature are developed. An interconnected metallic nanowire network, partially connected nanowire networks, and spherical or elliptically shaped nanoparticles are observed for low, intermediate, and high laser powers, respectively. Optical absorptance spectra demonstrate a noticeable variation in response to the structures formed. A tunable absorption in the visible range is demonstrated and the partially connected nanowires show the greatest enhancement in broadband absorption compared to the other structures. Finite element simulations uncover plasmonic and magnetic resonances as the underlying mechanisms for the absorbing behavior. This high degree of control over the dewetting area makes this technique a suitable candidate for many photonic and solar applications allowing for precise engineering of the absorbing nanostructure design.

Terahertz metamaterials (THz MMs) have been proven to be good candidates for chemical, biological, temperature, strain, and position sensing. However, currently developed thin-metal-film-based split ring resonator (SRR) MMs have relatively low quality factor (Q-factors), leading to a poor sensitivity, which is one of the obstacles for development of sensors. In order to enhance the Q-factor, novel THz MMs, nanopillar-based MMs, are designed, fabricated, and characterized. The nanopillar-based MMs excite the inductive-capacitive resonance via desplacement currents, showing a significantly enhanced Q-factor around 450, which is about 30 times higher than typical thin-metal-film-based MMs. Nanopillar-based MMs also show 17 times larger frequency shift compared to the metal-film-based MMs when the permittivity of the ambient dielectric properties of the MMs changes. Due to high Q-factor and large frequency shift, the nanopillar-based THz MMs utilizing displacement current have great potential for highly sensitive chemical and biomaterial detection as well as frequency-agile THz devices.

Metasurfaces, two-dimensional versions of metamaterials, retain the great capabilities of three-dimensional counterparts in manipulating electromagnetic wave behaviors, while reducing the challenges in fabrication. By judiciously engineering parameters of individual building blocks (such as geometry, size, and material) and selecting specific design algorithms, metasurfaces are promising to replace conventional electromagnetic elements in nanoplasmonic/photonic devices. Significantly, such concept can be readily promoted to other disciplines, such as acoustics, thermal physics, and seismology. In this article, the latest advances in full control of electromagnetic waves with metasurfaces are briefly reviewed from a functionality perspective. A broad avenue towards real-life applications of metamaterials has been opened up, although they are still at their infant stage. At the end, several promising approaches are suggested to extend the applications of metasurfaces.

A large-area metamaterial thermal emitter is fabricated using facile, lithography-free techniques. The device is composed of conductive oxides, refractory ceramics, and noble metals and shows stable, selective emission after exposure to 1173 K for 22 h in oxidizing and inert atmospheres. Lastly, the results indicate that the metamaterial can be used to achieve high-performance thermophotovoltaic devices for applications such as portable power generation.

Coherent control of the reflection and refraction in semicontinuous metasurfaces is realized. Both the reflection and refraction efficiency of a single-layered metasurface can be greatly improved with this method. By controlling the relative phase between the signal beam and the control beam, the Snell's law can be dynamically manipulated. This method may open possibilities to the practical application of metasurfaces.

In the past years, many researchers have tried to realize the electromagnetically induced transparency (EIT) phenomena from the metamaterial arrays for the attractable applications such as slow light devices, novel optical communication systems, and nonlinear optical devices. Most of metamaterial enabled EIT analogues reported so far are based on the destructive interference between the bright and dark meta-atoms. Whether they are located on the same plane or off the plane, EIT-like properties can be achieved only by breaking the symmetry between the bright and dark meta-atoms. In this study, a novel self-complimentary terahertz meta-atom is developed by combining cut-wire resonators and its pseudo-complimentary pattern within a single meta-atom. From the measurement results, it is verified that the proposed meta-atom can exhibit EIT-like phenomenon within a single unit cell by controlling the focus of the electric field induced by the incident terahertz waves within the self-complimentary meta-atom cell as predicted by the numerical simulation as well as by the theoretical forced oscillation model.

1 wileyonlinelibrary.com C o m m u n iC a io n and their possible applications. The ultimate form of active manipulation of EIT phenomenon will be when all three primary parameters are controlled independently. The independent control of individual resonators demands for the controllability at unit cell level, and conventional approaches such as optical pumping of photoconductive elements or thermally controlled superconductor are restricted to provide only global control. Recently, microelectromechanical systems (MEMS) based tunable metamaterials have been reported to achieve controllability at unit cell level, along with the added advantage of being electrically controlled, miniaturized size and enhanced electrooptic performance. The versatility of MEMS design has enabled active manipulation of numerous THz properties such as magnetic resonance,[19–22] electrical resonance,[23–25] anisotropy,[26] broadband response,[27] isotropic resonance switching[28] multiresonance switching,[29–31] and coupling strength between resonators.[32] The enhanced controllability and direct integration of MEMS actuators into metamaterial unit cell geometry is an ideal fit for the realization of selective control of coupled mode resonators. In this Communication, reconfigurable metamaterial with independently controlled bright and dark mode resonators is proposed for advanced manipulation of the classical analog of EIT and slow light effects in THz spectral region. The active control of bright mode resonator enables modulation of EIT intensity, while the tuning of dark mode resonance causes the EIT peak to tune in frequency. Furthermore, simultaneous switching of bright and dark mode resonators results in dynamic switching of the system between coupled and uncoupled states. The proposed approach of selective reconfiguration can be scaled for multiresonator systems, which can be coupled either through inductive, capacitive, or conductive means. The metamaterial consists of 80 × 80 periodic array of cut wire resonator (CWR) with closely placed split ring resonators (SRRs), as shown in Figure 1 and Figure 2. The periodicity of unit cell is 100 μm along both axial directions. The CWR has length, lC = 60 μm and width, wC = 5 μm, respectively. The SRRs have a base length, bS = 30 μm, side length, lS = 20 μm, and split gap, gS = 4 μm. The SRRs are placed at a distance of S = 2 μm from the CWR. When the polarization of the excitation field is along the CWR arm, the dipole mode resonance of the CWR will be the bright mode and the inductive-capacitive (LC) mode of SRR resonance acts as the dark mode. Thus for the incident THz polarization, the direct excitation of the bright mode induces image charges on the nearby SRRs through nearfield inductive coupling, thereby exciting the LC resonance of the SRRs. These bright-dark resonances have contrasting line widths with identical resonance frequencies and under a strong coupling regime they experience an EIT-type of interference that gives rise to a sharp transmission peak. Thus, through Active Control of Electromagnetically Induced Transparency Analog in Terahertz MEMS Metamaterial

The natural world exhibits numerous examples of efficient optical designs with novel hierarchical microstructures and specialized functionality after millions of years of evolution. Materials scientists have long been deriving understanding and inspiration from nature's optical ingenuity, such as vivid structural colors, light antireflection, light focusing, and chirality. Progress in engineering bioinspired optical functional materials has been exciting in the past years. In this review, the focus is on the state-of-the-art achievements of bioinspired optical materials with applications in various areas including efficient light manipulation, optical sensors, light–energy conversion, plasmonic materials with ultrahigh surface plasmon resonance (SPR) efficiency and metamaterials. The major challenges and perspectives for bioinspired designs of optical functional materials in the future are also briefly addressed.

A microelectromechanically reconfigurable digital metamaterial for dynamic switching of terahertz anisotropy is experimentally demonstrated. Out-of-plane reconfigurable microcantilevers placed in orthogonal direction with isolated control allows active making and breaking of unit cell symmetry. The electrical control, miniaturized size, and ease of process integration make the reported microelectromechanical system metamaterial ideal for the realization of THz polarimetric devices. ASTAR (Agency for Sci., Tech. and Research, S’pore) MOE (Min. of Education, S’pore) Accepted version

THz waves transmission amplitude at room temperature. Notably, we found that the amount of the trapped charge carriers within the fully randomized, complex molecular defects (i.e., localized impurity states (LIS)) of GO can be precisely adjusted by the DC biasing with versatile and simple device architecture (i.e., a two-terminal, interdigitated electrode device); the excitation of the electrically trapped charge carrier in turn leads to a large change in broadband THz transmission (e.g., ≈ 30% at 1.0 THz). This dramatic change in broadband THz transmission (i.e., ≈0.3–2 THz) was achieved with a deepsubwavelength (< λ /10,000) thick GO paper (≈ 500 nm) without any assistance of the foreign structural motifs (e.g., metamaterials, plasmonic structures, and Fabry–Perot architectures). Systematic THz spectroscopy of the chemically reduced GO further revealed the critical role of GO’s complex molecular defects in the active control of broadband THz transmission. Also, we observed the trapped charge carriers within LIS of GO can additionally provide interesting hysteretic behavior in the control of broadband THz transmission, which could be applied to the THz-based photonic memory devices. Figure 1 a–c presents schematic and macro/microscopic images of the two-terminal GO devices used in the present work. The GO device mainly consists of two functional parts: (i) GO paper (i.e., multistacked GO fl akes) and (ii) two-terminal, interdigitated gold (Au) electrodes (Au microwire array) patterned on the polyimide (PI) fl exible substrate (2.6 μm in thickness). High-quality monolayer GO fl akes, prepared by acidic chemical exfoliation of graphite, [ 23 ] were dispersed in deionized water; then, GO aqueous solutions were vacuumfi ltrated in order to obtain mechanically robust, freestanding paper platform (≈500 nm in averaged thickness). The transmission electron microscope image and UV–vis spectroscopy of the obtained GO fl akes are summarized in Figures S1 and S2 in the Supporting Information. In particular, UV–vis spectroscopy and its Tauc plot analysis verify the presence of both energy bandgap (3.4−4.0 eV) and LIS, which makes GO contrast to graphene with zero bandgap. The mechanical robustness of the paper-type fi lm can facilitate the transfer of GO onto any desired substrate; herein, by using solvent-assisted transfer printing (see Figure S3 in the Supporting Information), GO paper was directly integrated onto the surface of the interdigitated, two-terminal electrodes (100 nm thick Au micro wire array with 10 nm Ti adhesion layer), as shown in Figure 1 b–d. Scanning electron microscope image in Figure 1 d shows the large-area GO paper (1 cm × 2 cm), conformably printed onto the predeveloped electrodes. The average roughness of GO (i.e., root-mean-square) was measured about 180 nm, which is still far below the wavelength of interest (Figure S4 in the Two-Terminal Graphene Oxide Devices for Electrical Modulation of Broadband Terahertz Waves

Fano resonances in metasurfaces are important due to their low loss subradiant behavior that allows excitation of high-quality (Q) factor resonances extending from the microwave to the optical regime. High-Q Fano resonances have recently enabled applications in the areas of sensing, modulation, filtering, and efficient cavities for lasing spasers. Highly conducting metals are the most commonly used materials for fabricating the metasurfaces, especially at the low-frequency terahertz region where the DC, Drude, and perfect electric conductivity show similar resonant behavior of the subwavelength meta-atoms. Here, it is experimentally and theoretically demontrated that the Q factor of a low asymmetry Fano resonance is extremely sensitive to the conducting properties of the metal at terahertz frequencies. Large differences in the Q factor and figure of merit of the Fano resonance is observed for perfect electric conductors, Drude metal, and a DC-conducting metal, which is in sharp contrast to the behavior of the inductive–capacitive resonance of meta-atoms at terahertz frequency. Identification of such a low asymmetry regime in Fano resonances is the key to engineer the radiative and nonradiative losses in plasmonic and metamaterial-based devices that have potential applications in the microwave, terahertz, infrared, and the optical regimes.

Mirrors are widely used for redirection of electromagnetic waves in optical systems, making them arguably the most irreplaceable optical component. Metamaterial-based chiral mirrors, composed of a 2D-chiral planar metallic structure backed by a conventional mirror, reflect one circular polarization without changing its handedness, while absorbing the other. Here, three types of switchable chiral mirror are demonstrated. Switching from a chiral mirror to either a conventional mirror, a handedness-preserving mirror, or a chiral mirror of opposite handedness, is realized. These advances are underpinned by switching the handedness of 2D-chiral metamaterial and the associated effect of circular conversion dichroism, which is reported here for the first time. Switching is achieved by exploiting the temperature-activated dielectric-to-metal phase transition of vanadium dioxide to modify the symmetry and chirality of the metamaterial's resonators. Current distributions explain the temperature-controlled optical properties by handedness-selective excitation of reflective electric dipole and absorbing magnetic dipole modes.

Layered metal/dielectric hyperbolic metamaterials (HMMs) support a wide landscape of plasmon polariton excitations. In addition to surface plasmon polaritons, coupled Bloch‐like gap‐plasmon polaritons with high modal confinement inside the multilayer are supported. Photons can excite only a subset of these polaritonic modes, typically with a limited energy and momentum range in respect to the wide set of high‐K modes supported by hyperbolic dispersion media, and coupling with gratings or local excitation is necessary. Strikingly, electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope allows nm‐scale local excitation and mapping of the spatial field distribution of all the modes supported by a photonic or plasmonic structure, both bright and dark, and also all other inelastic interactions of the beam, including phonons and interband transitions. Herein, experimental evidence of the spatial distribution of plasmon polaritons in multilayered type II HMM nanostructures is acquired with an aloof electron beam adjacent to structures of current interest. HMM pillars are useful for their separation and adjustability of optical scattering and absorption, while HMM slot cavities can be used as waveguides with high field confinement. The nature of the modes is confirmed with corresponding simulations of EEL and optical spectra and near‐field intensities.