Fano Metamaterials Research Articles

Full manipulation of transparency and absorption through direct tuning of dark modes in high-Q Fano metamaterials.

Controlling the line shape of Fano resonance has continued to attract significant research attention in recent years owing to its practical applications such as lasing, biosensing, and slow-light devices. However, controllable Fano resonances always require stringent alignment of complex symmetry-breaking structures; therefore, the manipulation can only be performed with limited degrees of freedom and a narrow tuning range. This work demonstrates dark-mode excitation tuning independent of the bright mode for the first time, to the authors' knowledge, in asymmetric Fano metamaterials. Metallic subwavelength slits are arranged to form asymmetric unit cells and generate a broad and bright (radiative) Fabry-Perot mode and a sharp and dark (non-radiative) surface mode. The introduction of the independent radial and angular asymmetries realizes independent control of the Fano phase (q) and quality factor (Q). This tunability provides a dynamic phase shift while maintaining a high-quality factor, enabling switching between nearly perfect transmission and absorption, which is confirmed both numerically and experimentally. The proposed scheme for fully controlled Fano systems can aid practical applications such as phase-sensitive switching devices.

Fano resonance in terahertz parallel plate waveguide

Metamaterial-based Fano resonances are very promising for the design of sensors, lasers, and slow light devices, which are particularly important for the terahertz band where functional devices have long been lacking. However, most of the reported Fano metamaterials are designed to manipulate free-space terahertz waves. Here, we present a Fano resonance metamaterial design to manipulate the waveguide mode in a parallel plate waveguide. Different from the free-space cases, the interaction length between the waveguide mode and the Fano metamaterials can be adjusted flexibly, allowing a manipulation of the Fano resonance intensity. Moreover, we also demonstrate thin-film sensing of a monolayer graphene film with the proposed Fano resonance metamaterials. The proposed design scheme may motivate the design and fabrication of future practical devices in waveguide systems, and promote a wider application of Fano resonances.

High sensitive refractive index sensor based on spatial symmetry breaking Fano metamaterials fabricated by ion beam irradiation

The sensitivity of plasmonic nanostructures based refractive index sensor can be improved by free-standing three-dimensional (3D) configuration. Here, by means of focused-ion-beam (FIB) induced folding technique, we fabricated arbitrary spatially oriented double-flake 3D metamaterials (MMs). The experimental measured sensitivity can reach 2703 nm/RIU, and based on the symmetry breaking configuration, the figure of merit (FoM) reaches 37.2 RIU −1 in simulation results. What's more, the different coupling mechanisms in our MMs enable the independent modulation of each resonance. This work opens up new avenues for near-field coupling modulation, holding great potential applications in multi-resonance differential sensing, switch devices and optical communication. • The sensitivity of plasmonic nanostructures based refractive index sensor can be improved by free-standing three-dimensional configuration. • The measured sensitivity can reach 2703 nm/RIU. • Based on the symmetry breaking configuration, the figure of merit reaches 37.2 RIU-1. • Each resonance can be modulated independently.

Strong terahertz matter interaction induced ultrasensitive sensing in Fano cavity based stacked metamaterials

Strong interaction between terahertz (THz) and matter is a topic of paramount importance, considering continuously enhanced interest in THz photonics as well as condensed matter physics, which can lead to the observation of many linear and nonlinear phenomena in the THz regime. Here, we demonstrate a unique and novel metamaterial-based technique of strong THz matter interaction towards thin film sensing, where the analyte is sensed in between the stacked metasurfaces forming the Fano cavity. Sub-wavelength structures typically overcome the diffraction limit of any optical system, which also possess very high confinement of electromagnetic energy. Fano resonance possesses a sharp asymmetric line shape, low radiation loss and large tuning capability. In addition to them, the material under test is placed in between the metasurfaces to utilize the substantial energy confinement leading to strong light matter interaction, a scheme never explored before. By intelligently exploiting the above characteristics, we have demonstrated a novel way to detect both the refractive index (dielectric constant), thickness and loss factor of the material under test when placed between the array of the meta-resonators forming the Fano metamaterials. Our study revealed that the sensitivity and figure of merit (FOM) are strikingly different for dipole and Fano modes. A maximum sensitivity of >1 THz RIU−1 (1.76 × 105 nm RIU−1) and FOM of around 14.05 are achieved at the Fano mode. Additionally, our sensor shows better performance with decreasing spacer thickness (lesser the material, more the sensitivity). Moreover, the proposed device is passive towards typical ambience temperature variation, and is highly compact because of its stacked configuration. The demonstrated device can be extremely beneficial towards realizing ultra-sensitive meta-sensors and other miniaturized THz meta-photonic devices, and bio-chemical sensing where strong light matter interaction is mandatory.

Active Switching of Extremely High-Q Fano Resonances Using Vanadium Oxide-Implanted Terahertz Metamaterials

In this paper, we demonstrate an active switching of extremely high Q-factor Fano resonances using vanadium oxide (VO2)-implanted THz asymmetric double C-shaped metamaterial (MM) structures. The simulation results indicate the highly temperature-sensitive nature of the double Fano resonances that can be switched at very low external thermal pumping (68 °C), which is only slightly higher than room temperature. We employ the surface current and electric field distributions of the structure to analyze the physical mechanism of the observed switching behavior in the thermally excited Fano MMs. More importantly, by optimizing the asymmetric parameter (offset length), the linewidth of the Fano resonance can reach only 0.015 THz and the Q-factor is as high as 98, which is one order of magnitude higher than that of the traditional MMs. The findings of this work would enable a thermally-induced high-Q Fano resonance MMs for ultra-sensitive sensors, modulators, low threshold switching in metadevices.

A multiple mode integrated biosensor based on higher order Fano metamaterials.

A multiple mode integrated biosensor based on higher order Fano metamaterials (FRMMs) is proposed. The frequency shifts (Δf) of x-polarized quadrupolar (Qx), octupolar (Ox), hexadecapolar (Hx), y-polarized quadrupolar (Qy) and octupolar (Oy) Fano resonance modes are integrated to detect the concentration of lung cancer cells. In experiments, the concentrations of lung cancer cells can be distinguished by the shape and distribution of integrated graphics. In addition, an anomalous response in Δf in resonant mode is surprisingly observed. As the cell concentration increases, the Δf at the Qx-dip, Qy-dip and Oy-dip successively experiences an increasing frequency shift stage (IFSS), decreasing frequency shift stage (DFSS) and re-increasing frequency shift stage (RIFSS). The extraordinary DFSS confirmed by single-factor analysis of variance (ANOVA) means an unusual physical phenomenon in metamaterial biosensors. By introducing a new dielectric constant εf, we amend perturbation theory to explain the unusual phenomenon in Δf. With the change of the mode order from Qx to Hx, the εf increases from -2.78 to 0.75, which implies that the negative εf leads to the appearance of the DFSS. As a platform for biosensing, this study opens a new window from the perspective of multiple mode integration.

Fano Metamaterials on Nanopedestals for Plasmon-Enhanced Infrared Spectroscopy

We report a sensing platform for surface-enhanced infrared absorption (SEIRA) spectroscopy, based on Fano metamaterials (FMMs) on dielectric nanopedestals. FMMs consist of two parallel gold (Au) nanorod antennas, with a small horizontal coupler attached to one of the nanorod antenna. When placed on SiO2 dielectric nanopedestals, which exhibit strong field enhancements caused by the interference between subradiant and superradiant plasmonic resonances, they provide the highly enhanced E-field intensities formed near the Au nanoantenna, which can provide more enhanced molecular detection signals. Here, the sensing characteristics of FMMs on nanopedestals structure was confirmed by comparison with FMMs on an unetched SiO2 substrate as a control sample. The control FMMs and the FMMs on nanopedestals were carefully designed to excite Fano resonance near the target 1-octadecanethiol (ODT) fingerprint vibrations. The FMMs were fabricated by using nanoimprint lithography and the nanopedestal structures were formed by isotropic dry-etching. The experimental reflection spectra containing the enhanced absorption signals of the ODT monolayer molecules was analyzed using temporal coupled-mode theory. The FMMs on nanopedestals achieved over 7% of reflection difference signal, which was 1.7 times higher signal than the one from the control FMMs. Based on the FMMs on nanopedestal structures proposed in this study, it may be widely applied to future spectroscopy and sensor applications requiring ultrasensitive detection capability.

Dual-wavelength terahertz sensing based on anisotropic Fano resonance metamaterials.

Higher order Fano resonance metamaterials provide a desirable platform for biosensing applications. In this work we exhibit higher order Fano modes by designing an elliptical metallic ring. The simulation results show that for Fano resonant metamaterials, the higher order modes lead to improved sensitivity to refractive index change and larger frequency shifts. Numerically, the sensitivity of dip A (quadrupole mode) is 112GHz/RIU, whereas the sensitivity of dip B (octupole mode) is 234GHz/RIU, over 2 times that of dip A. According to our experimental results, the Fano resonant frequencies of dip A and dip B exhibit redshift as the concentration of the anti-cancer drug methotrexate decreases from 120 to 90μM, with the cell analyte concentration of A549 cells at 5×105 cell/ml. For dip A (quadrupole mode), there is a frequency shift of 0.84GHz for drug concentration of 120μM and a frequency shift of 22GHz for a 90μM drug-treated sample. For dip B (octupole mode), there is a frequency shift of 6.767GHz for the drug concentration of 120μM treated sample and a frequency shift of 51.815GHz for the drug concentration of 90μM treated sample. Furthermore, the frequency shift of dip A is always smaller than that of dip B for both 90μM and 120μM drug concentrations. Such phenomena indicate that dip B is much more sensitive than dip A. The enhanced sensitivity of higher order Fano metamaterials makes it possible to realize high-performance terahertz sensing for biomedical applications.

Terahertz Modulator Based on Vertically Coupled Fano Metamaterial

We introduce a vertical coupling concept to Fano resonance based terahertz (THz) metamaterials, in order to enhance their modulation characteristics. The approach is demonstrated for an asymmetric split-ring structure fabricated from gold with standard micromachining processes on a fused silica substrate. The realized samples are characterized with THz time-domain spectroscopy and the results are in close agreement with the simulation model. Due to the enhanced field concentration facilitated by vertical coupling, we achieve a 95% modulation depth and 35% resonance frequency tunability at 1.23 THz with less than 1-dB inherent loss. The exemplary device shows vertical coupling as an attractive concept for designing efficient modulators for terahertz imaging applications.

Bi-anisotropic Fano resonance in three-dimensional metamaterials

We experimentally investigated the bi-anisotropic properties of Fano resonance in three-dimensional (3D) metamaterials. Fano resonance in 3D metamaterials arises from the interference of in-phase and anti-phase modes that originate from mode hybridization in coupled 3D split ring resonators (SRRs) with detuned resonant wavelengths. At Fano resonance, not only permittivity and permeability but also the bi-anisotropic parameter show doubly dispersive response. Manipulation of the bi-anisotropic response at Fano resonance was demonstrated through controlling the inversion symmetry of the 3D-SRRs. Improvement of inversion symmetry due to rotation of 3D-SRRs results in enhancement of magnetic response and inhibition of electric and bi-anisotropy responses at Fano resonance. Negligible electric and bi-anisotropic responses at Fano resonance were achieved due to the small radiative nature of the anti-phase mode. This bi-anisotropic Fano metamaterials with rich and tunable bi-anisotropy will extend the capabilities of new optical phenomena and broaden the applications of bi-anisotropic metamaterials.

Metallic Fano Metamaterials: High‐Q Metallic Fano Metamaterial for Highly Efficient Cerenkov Lasing (Advanced Optical Materials 12/2018)

Metallic Fano Metamaterials: High‐Q Metallic Fano Metamaterial for Highly Efficient Cerenkov Lasing (Advanced Optical Materials 12/2018)

High‐Q Metallic Fano Metamaterial for Highly Efficient Cerenkov Lasing

AbstractA high‐quality‐factor (high‐Q) metallic Fano metamaterial is demonstrated both experimentally and theoretically. This material is suitable for highly efficient Cerenkov lasing in which subwavelength metallic slits are arranged to form asymmetric unit cells. In contrast to conventional dielectric Cerenkov or Smith–Purcell devices, in the proposed device, convection electrons traverse the high‐Q metallic metamaterial. The interplay between an extremely low group velocity from the infinite transverse permittivity of the anisotropic metamaterial and the subradiant damping from a Fano‐type slit mode is shown to be responsible for the high‐Q value, which is measured to be unprecedented at 700. The resulting improvement in the Cerenkov lasing efficiency is estimated to be more than two orders of magnitude using a particle‐in‐cell simulation.

Higher order Fano graphene metamaterials for nanoscale optical sensing.

Plasmonic Fano metamaterials provide a unique platform for optical sensing applications due to their sharp spectral response and the ability to confine light to nanoscale regions that make them a strong prospect for refractive-index sensing. Higher order Fano resonance modes in noble metal plasmonic structures can further improve the sensitivity, but their applications are heavily limited by crosstalk between different modes due to the large damping rates and broadband spectral responses of the metal plasmon modes. Here, we create pure higher order Fano modes by designing asymmetric metamaterials comprised of a split-ring resonator and disk with a low-loss graphene plasmon. These higher order modes are highly sensitive to the nanoscale analyte (8 nm thick) both in refractive-index and in infrared vibrational fingerprint sensing, as demonstrated by the numerical calculation. The frequency sensitivity and figure-of-merit of the hexacontatetrapolar mode can reach 289 cm-1 per RIU and 29, respectively, and it can probe the weak infrared vibrational modes of the analyte with more than 400 times enhancement. The enhanced sensitivity and tunability of higher order Fano graphene metamaterials promise a high-performance nanoscale optical sensor.

Dynamically Tunable Fano Metamaterials through the Coupling of Graphene Grating and Square Closed Ring Resonator

We present the numerical studies of a novel hybrid graphene-metal Fano metamaterial, which is composed of a graphene grating (graphene ribbon array) and a square closed ring resonator (SCRR) separated by a dielectric substrate. The destructive interference between the narrow and broad electrical dipolar surface plasmons induced respectively on the surface of the graphene ribbon and the SCRR leads to the classical analog of electromagnetically induced transparency (EIT). By decreasing the thickness of the substrate spacer (enhancing the coupling between the two components), a double EIT system could be achieved. More importantly, the transparency windows in the hybrid structures can be actively controlled by varying the applied gate voltage on the graphene ribbon. Large effective group index and small loss within the transparency windows suggest the promising slow-light applications.

Fano line shape and phase reversal in a split-ring resonator based metamaterial

We introduce a universal scheme for invoking a full line shape and phase reversal of Fano resonances in plasmonic metamaterials. The effect is based on opening a new excitation channel through minor structural displacements which enable continuous tunability. At the example of a metamaterial designed for terahertz frequencies, consisting of two coupled split-ring resonators, it is demonstrated by simulation and experiment that displacements by only a fraction of the wavelength are sufficient to invoke a full line shape reversal, in combination with the change from inducing a phase delay to a phase advance for a transmitted wave. Our approach provides unprecedented control of Fano asymmetry and phase, potentially enabling the implementation of dynamically tuneable Fano metamaterials and Fano-plasmonic devices such as dynamic filters or phase shifters.

Low-loss ultra-high-Q dark mode plasmonic Fano metamaterials

We experimentally demonstrate a planar terahertz Fano metamaterial with an ultrahigh quality (Q) factor of 227. This is achieved by the excitation of the nonradiative dark modes by introducing a tiny asymmetry in the metamaterial structure. The extremely sharp quadrupole and Fano resonances are excited at normal incidence for orthogonal polarizations of the electric field. In order to capture the narrow linewidth of the dark resonance modes, we perform high resolution terahertz time-domain measurements with a scan length of 200 picoseconds and frequency resolution of 5 GHz. These high-Q metamaterials can be used in ultrasensitive label-free terahertz sensing, dense photonic integration, and narrowband filtering.