Subwavelength Micro-Antenna for Achieving Slow Light at Microwave Wavelengths via Electromagnetically Induced Transparency in 2D Metamaterials

In this paper, a tunable slow light 2D metamaterial is presented and investigated. The metamaterial unit cell is composed of three metallic strips as radiative and non-radiative modes. Once introducing asymmetry, a transparency window induced by coupling between the dark and bright modes is observed. The transmission characteristics and the slow light properties of the metamaterial are verified by numerical simulation, which is in a good agreement with theoretical predictions. The impact of asymmetric parameter on transparency window is also investigated. Simulation results show the spectral properties and the group index of the proposed 2D metamaterial can be tunned by adjusting asymmetric structure parameter, temperature and also the metal used in the metamaterial. Furthermore, the electromagnetic field distributions, excited surface currents, induced electric dipole and quadruples, and slow light properties of the metamaterial are investigated in details as well as transmission spectral responses. The outstanding result is that, the 2D-metamaterial is in a high decrease of the group velocity and therefore slow light applications, because in the best state, the group velocity in our structure decreases by a factor of 221 at T=100 K using copper as metal in optimization asymmetric case.     

Broadband Mid-infrared Dual-Band Double-Negative Metamaterial: Realized Using a Simple Geometry

We design and numerically demonstrate a novel metamaterial structure consisting of a dielectric layer sandwiched between two silver films and is perforated with two kinds of square-shaped holes at different angles, which is a dual-band double-negative (each band possesses simultaneously negative permittivity and permeability) metamaterial with broad NRI bands in mid-infrared region(3–30 μm). The broadband of NRI contributed to the strong magnetic resonance caused by the excitation of surface plasmon polaritons. The influence of the number of square-shaped holes on the properties of the designed structures are also investigated by analyzing and comparing the transmission, permeability, permittivity, refractive index, and figure of merit. Then, by optimizing the structural parameters, the proposed structure exhibits a negative band with a figure of merit of 3.3, which is to our knowledge larger than previously reported plasmonic metamaterial in mid-infrared region(M-IR). The value of negative refractive index(NRI) reaches −6 and the bandwidth of NRI can reach up to 4.2 THz in the low-frequency band of M-IR region, which is the widest NRI band in M-IR spectrum at present as far as we know. Moreover, the metamaterial structure is simple and easy to be manufactured with standard fabrication techniques. This work will be very meaningful in designing dual-band negative-index material with broad NRI band and low loss. Finally, the proposed metamaterial has huge potential applications in multiband or broadband devices.

     

Dual-Band Infrared Perfect Absorber Based on a Ag-Dielectric-Ag Multilayer Films with Nanoring Grooves Arrays

In this paper, we demonstrate a dual-band metamaterial perfect absorber based on a Ag-dielectric-Ag multilayer nanostructure. The structure of top metal film covers nanoring grooves array. A dielectric layer has a function of confining electromagnetic fields. Theoretical analysis shows that two absorption peaks (1059 nm and 1304 nm) with the absorption of 99.2% and 99.9% have been achieved, respectively. The physical origin of perfect absorption peaks are related to the Fabry-Perot resonance effect and localized surface plasmon resonance (LSPR) of the nanoring grooves. Its perfect absorption and resonance wavelength can be well regulated by adjusting the relevant structural parameters. Additionally, the absorber demonstrates good operation angle-polarization-tolerance at wide incident angles (0–60°). We believe that our design has a promising application in plasmon-enhanced photovoltaic, optical absorption switching, and modulator optical communications in the infrared regime.

     

Ultra-Broadband Linear Polarization Conversion via Diode-Like Asymmetric Transmission with Composite Metamaterial for Terahertz Waves

In this paper, a tri-layer metamaterial composed of a split-disk structure array sandwiched with two layers of twisted sub-wavelength metal grating is proposed and investigated numerically in terahertz region. The numerical results exhibit that linear polarization conversion via diode-like asymmetric transmission for terahertz waves within ultra-broadband frequency range is achieved due to Fabry-Perot-like resonance. In our design, the conversion polarization transmission coefficient for normal incidence is greater than 90 % in the range of 0.23–1.17 THz, equivalent to 134.3 % relative bandwidth. The physical mechanism of the broadband linear polarization conversion effect is further illustrated by simulated electrical field distributions.

     

Tunable Triple-Band Plasmonically Induced Transparency Effects Based on Double π-Shaped Metamaterial Resonators

Tunable triple-peaks with the transmission intensity of more than 90% plasmonically induced transparency metamaterial resonator based on nested double π-shaped metallic structure is proposed at the terahertz frequency region, which is consisted of three sets of gold nanorods with different sizes placed on a dielectric substrate of SiO₂. The coupling effect of localized electric field between different parts of the proposed structure can be used to explain the physical mechanism of three transparent windows. The finite-difference time-domain (FDTD) is used to study the spectral properties of the proposed structure, and the influence of the size of the nanorods and the relative distance between them on the spectral characteristics are also discussed. It can be seen that some obvious shift phenomena occur in the spectra with the change of these nanorods. These results indicate that the proposed structure opens up new avenues in many related applications, especially for multi-channel filters, optical switches, and sensors.

     

A Design for Band Enhanced Dielectric Absorber Based on Fractal-Like Structure

A dielectric metamaterial absorber has been proposed, which consists of fractal-like structure and conductive sheet. The fractal-like structure is made by the high permittivity dielectric and also is covered by the conductive sheet. Absorptivity of such a dielectric metamaterial absorber is 99.1%, which can be found at 10.196 GHz; meanwhile, the absorber is polarization insensitive. To enhance the bandwidth of absorber, a novel absorber also is proposed, whose bandwidth is 0.566 GHz, which ranges from 9.752 to 10.318 GHz, and relative bandwidth is 5.64%. The maximum absorptivity can reach to 99.8%, and the proposed absorber also is polarization insensitive. In the meantime, the absorber shows excellent performance which is incident angle insensitive; when the incident angle is increased to 70°, the absorptivity is larger than 75%.

     

Resonance Bandwidth Controllable Adjustment of Electromagnetically Induced Transparency-like Using Terahertz Metamaterial

In the fields of communication and sensing, resonance bandwidth is a very critical index. It is very meaningful to implement a broadband resonance device with a simple metamaterial structure in the terahertz band. In this paper, we propose a simple metamaterial structure which consists of one horizontal metal strip and two vertical metal strips. This structure can achieve an electromagnetically induced transparency-like (EIT-like) effect in the frequency range of 0.1~3.0 THz to obtain a transparent window with a resonance bandwidth as high as 1.212 THz. When the relative distance between two vertical metal strips is changed, the bandwidth can be effectively controlled. Furthermore, we found that the EIT-like effect can be actively adjusted by replacing vertical metal strips with photosensitive silicon.

     

Ultrahigh-Q and Polarization-Independent Terahertz Metamaterial Perfect Absorber

Ensuring a good trade-off between high-quality factor (Q-factor) and polarization independency is a key challenge for designing practicable terahertz metamaterial devices. We propose a symmetric composite aluminum-structured metamaterial absorber to achieve high Q-factor beyond 80 and near-unity absorbance of arbitrary polarization waves in the terahertz regime. Ultrahigh Q-factor reaches 84, and polarization-independent absorption is as high as 99% for resonant frequency tuning from 7.58 to 8.97 THz, covering 14% of the standard THz gap. The geometric effect of the symmetric sublattice on resonant frequency tuning is analyzed. The large Q-factor and strong absorption by oblique incidence is discussed. Designed high-Q metamaterial perfect absorber has various applications, including terahertz hyperspectral imaging, filtering, and sensing.

     

Analogue Electromagnetically Induced Transparency Based on Low-loss Metamaterial and its Application in Nanosensor and Slow-light Device

In this paper, we demonstrated a low-loss and high-transmission analogy of electromagnetically induced transparency based on all-dieletric metasurface. The metamaterial unit cell structure is composed of two mutually perpendicular silicon nanoscale bars. Under the joint effects of the neighboring meta-atoms’ coherent interaction and significant low absorption loss, the transmission and the Q-factor can reach up to 93 % and 139, respectively. Moreover, we use the coupled harmonic oscillator model to analyze the near field interaction between the two elements in the electromagnetically induced transparency (EIT) metamaterial unit cell qualitatively and the effects of parameters on EIT. The figure-of-merit of 42 and the group delay of 0.65 ps are obtained. These characteristics make the metamaterial structure with potential to apply for ultrafast switches, sensor, and slow-light devices.     

A Terahertz Tunable Metamaterial Reflective Polarization Converter Based On Vanadium Oxide Film

In this paper, on the basis of metamaterial, a simply single-layer and tunable reflective polarization converter has been numerically investigated, which is composed of vanadium dioxide film (VO₂) component combined with two-corner-cut square patch cut by a slit and reflective ground layer. Calculated results obtained by the CST Microwave Studio show that in the frequency of 2.22–5.42 THz, high polarization conversion efficiency (polarization conversion ratio (PCR) above 90%) can be normally achieved at the temperature about 25 °C for both the linearly and circularly polarized wave incidence. At the same time, the cross-polarization converter can be analyzed and obtained from the view on qualitative variable of polarization azimuth angle (θ) and ellipticity (η). Moreover, a tunable polarization conversion property can be realized by the designed device with vanadium dioxide utilizing changing different conductivities. Even so, to be demonstrated, the physical mechanism of the merits of controllability and uniqueness has been discussed by the distributions of current densities and E-field map, respectively. According to the prior results, the designed metamaterial could be applied in the area of temperature-controlled sensing, THz wireless communication, tunable polarized devices.

     

Design of Tunable Multi-Band Metamaterial Perfect Absorbers Based on Magnetic Polaritons

A tunable multi-band metamaterial perfect absorber is designed in this paper. The absorber made of a composite array of gold elliptical and circular disks on a thick metallic substrate, separated by a thin dielectric spacer. The absorptivity and the field enhancement of proposed structures are numerically investigated by the finite difference time domain method. Three absorption peaks (1.15, 1.55, and 2.05 μm) with the maximal absorption of 99.2, 99.7, and 97.3% have been achieved, respectively. By altering the dimensions of associated geometric parameters in the structure, three resonance wavelengths can be tuned individually. Physical mechanism of the multi-band absorption is construed as the resonance of magnetic polaritons. And the absorber exhibits the characteristics that are insensitive to the polarization angle due to its symmetry. The research results can have access to selective control of thermal radiation and the design of multi-band photodetectors.

     

Dynamic-Shift Single- and Double-Negative Refractive Index in a Novel Three-Dimensional Metamaterial

A novel three-dimensional terahertz metamaterial by integrating planar and standup resonators on a sapphire substrate is proposed. This structure shows the capability of dynamic-shifting single- and double-negative refractive index bands under temperature control. When the temperature rises from 160 K to 290 K, the single- and double-negative bands of the metamaterials exhibit blue-shift, which is attributed by the simultaneously tuning of the electromagnetic resonance frequencies of the resonators. If the temperature rises to 300 K, the bianisotropy of the metamaterials increases rapidly, while it performs stable between 160 K and 290 K.

     

All-Optical Switching and Routing with a Nonlinear Metamaterial

We report the dynamic control characteristics of electromagnetic wave propagation in a nonlinear metamaterial by an applied electric field, which is constructed by an array of metallic nanowires embedded into a nonlinear dielectric. Numerical results show that the composite structure can appear three kinds of interesting interconversion characteristics among positive refraction, negative refraction, and cut-off states by adjusting the intensity of the applied electric field. Consequently, we can switch all-optically light states between the total reflection state (OFF state) and the total transmission state (ON state), as well as control light propagation route dynamically. Moreover, we also elaborate on the dependency of the refraction angles of energy flow and wave vector, and Brewster angle on the applied electric field and the orientation angle φ. These properties open up an avenue for potential applications of nonlinear metamaterials in nanophotonic devices such as all-optical switches, routers, and wave cut-off devices.     

Structure Metallic Surface for Terahertz Plasmonics

Plasmonics is the field of study of the interaction between incident light and electrons in metals. It is used widely for developing nanophotonic devices. The structured metallic surface such as metamaterials can be used to produce spoof surface plasmons at any frequencies with the dimensions of unit cell less than the incident wavelength. Terahertz plasmonics is attracted to the field of research since it is used for sensing biological components even in a weak environment. The issue with planar metamaterials is a lower quality factor value. Several methods have been adopted for obtaining high Q-value in metamaterials. Among them, Fano- and Toroidal-based metamaterials offer high Q-factor and string localized field enhancement. This article discusses the importance and developments in the field of high-Q terahertz metamaterial for plasmonics applications. The nonlinear responses of terahertz metamaterial under high-intense THz pulses are also discussed.

     

Multi-layered Graphene Silica-Based Tunable Absorber for Infrared Wavelength Based on Circuit Theory Approach

Graphene can be utilized as a tunable material for a wide range of infrared wavelength regions due to its tunable conductivity property. In this paper, we use Y-shaped silver material resonator placed over the top of multiple graphene silica-layered structures to realize the perfect absorption over the infrared wavelength region. We propose four different designs by placing the graphene sheet over silica. The absorption and reflectance performance of the structures have been explored for 1500- to 1600-nm wavelength range. The proposed design also explores the absorption tunability of the structure for the different values of graphene chemical potential. We have reported the negative impedance for the perfect absorption for proposed metamaterial absorber structures. All the metamaterial absorbers have reported 99% of its absorption peaks in the infrared wavelength region. These designs can be used as a tunable absorber for narrowband and wideband applications. The proposed designs will become the basic building block of large photonics design which will be applicable for polariser, sensor, and solar applications.

     

Miniaturized Five-Band Perfect Metamaterial THz Absorber with Small Frequency Ratio

A five-band polarization-insensitive perfect metamaterial absorber (PMA) is reported in this paper for THz detection and sensing applications. The proposed absorber is constructed using interconnected circular ring elements enclosed by a square loop. The ring elements are interconnected using short strip lines which increases the electrical length to offer resonance at the lower frequencies of the THz regime without increasing the electrical length. The proposed absorber has a footprint of 0.12 λ_eff?×?0.12 λ_eff where λ_eff is the effective wavelength calculated at the lowest operating frequency. The absorber provides 92%, 84%, 90%, 100%, and 100% absorption at 0.24, 0.56, 0.65, 0.82, and 0.95 THz, respectively. The proposed structure offers structural symmetry, and hence, it is polarization-insensitive. The proposed five-band absorber has good angular stability consistent with many research works reported in the literature and has a small frequency ratio of 1:2.3:2.7:3.4:3.9. The proposed absorber can be used as a permittivity sensor and its sensitivity is estimated to vary from 5.8 GHz/permittivity unit (PU) to 23.56 GHz/PU.

     

A Tunable Ultra-Broadband Metamaterial Absorber with Multilayered Structure

In this article, we demonstrate a tunable ultra-broadband metamaterial absorber (TUMA) in terahertz (THz) band which is based on the multilayered structure composed of an Au reflective layer, polyimide dielectric layers, and vanadium dioxide (VO₂) periodic structures, respectively. We gain the tunable absorption spectra because of the room temperature phased-changed character of VO2. The relative bandwidth reaches to 81.2% and the absorption rate is over 90% at the frequency range of 1.63–3.86 THz when the temperature (t₁) is 350 K, but when t₁ = 300 K, the presented absorber is acted as a reflector whose absorption is small besides the frequency points of 9.75 THz and 9.81 THz. For the sake of comprehending the physical mechanism in-depth, the electric field (E-field) diagrams, the surface current distributions and the power loss density (PLD) of the TUMA are investigated. The influences of structural arguments and incident angle (θ) on the absorption are also analyzed. The emulated consequences show that the absorption spectrum can be regulated by changing structural parameters and incident angle and the tunable absorption regions can be obtained by altering the external temperature.

     

Manipulate the Transmissions Using Index-Near-Zero or Epsilon-Near-Zero Metamaterials with Coated Defects

We investigate the wave transmissions through an index-near-zero (INZ) or epsilon-near-zero (ENZ) metamaterial containing various kinds of coated cylindrical defects. We find that thin coatings of the defects can dramatically change the transmission behaviors. For example, perfect magnetic conductor (PMC) defects embedded in an INZ or ENZ metamaterial yield total reflections for transverse magnetic polarized waves (Hao et al., Appl Phys Lett 96:101109, 2010). However, if the PMC defects are coated with dielectric shells, total transmissions could be achieved by tuning their permittivity values or geometric sizes. The permittivity differences of dielectric shells for total reflections and transmissions in the INZ or ENZ metamaterial could be very small, implying potential applications, such as ultrasensitive sensors and switches.     

Light Humidity Sensor of Surface Plasmon Resonance by Symmetric Metal Film

The field of plasmonics has experienced a renaissance in recent years by providing a large variety of new physical effects and applications. Here, we demonstrate a light humidity sensor of surface plasmon resonance (SPR) by a symmetric metal film, which uses P polarised light of emergent He–Ne laser to stimulate SPR. Resonance angle change received by the spectrum detector can determine humidity via the relationship between humidity and effective refractive index. When the relatively short wavelengths are shown in the model, the evanescent wave penetration depth is shallow, resonance state is weak and energy loss is low. The opposite results are obtained, when the wavelength is long. Also, with increasing thickness, the resonance peak becomes acute, thereby indicating the improvement in detection accuracy. When the metal thickness of our model is 50 nm, the resonance peak of the reflection spectrum is narrower, accuracy is high and reflectivity is close to 0. By analysing the experimental results, the SPR resonance phenomenon is shown. The electromagnetic field energy is highly concentrated near the interface between the metal and SiO₂, which appears highly localized. The resolution of the structure can reach 0.37% RH (relative humidity), which is significantly more than the resolution of capacitive humidity sensor, i.e., resolution is usually 1% RH to 2% RH. The optical sensor of our development can provide a key device for long-distance transmission sensing, in special conditions such as low temperature.

     

Tunable Dielectric Properties of Ferrite-Dielectric Based Metamaterial

A ferrite-dielectric metamaterial composed of dielectric and ferrite cuboids has been investigated by experiments and simulations. By interacting with the electromagnetic wave, the Mie resonance can take place in the dielectric cuboids and the ferromagnetic precession will appear in the ferrite cuboids. The magnetic field distributions show the electric Mie resonance of the dielectric cuboids can be influenced by the ferromagnetic precession of ferrite cuboids when a certain magnetic field is applied. The effective permittivity of the metamaterial can be tuned by modifying the applied magnetic field. A good agreement between experimental and simulated results is demonstrated, which confirms that these metamaterials can be used for tunable microwave devices.