All-dielectric metamaterial analogue of electromagnetically induced transparency and its sensing application in terahertz range

Dynamically tunable multi-band plasmon-induced absorption based on multi-layer borophene ribbon gratings

In this paper, we propose a borophene-based grating structure (BBGS) to realize multi-band plasmon-induced absorption. The coupling of two resonance modes excited by upper borophene grating (UBG) and lower borophene grating (LBG) leads to plasmon-induced absorption. The coupled-mode theory (CMT) is utilized to fit the absorption spectrum. The simulated spectrum fits well with the calculated result. We found the absorption peaks exhibit a blue shift with an increase in the carrier density of borophene grating. Further, as the coupling distance D increases, the first absorption peak shows a blue shift, while the second absorption peak exhibits a red shift, leading to a smaller reflection window. Moreover, the enhancement absorption effect caused by the bottom PEC layer is also analyzed. On this basis, using a three-layer borophene grating structure, we designed a three-band perfect absorber with intensities of 99.83%, 99.45%, and 99.96% in the near-infrared region. The effect of polarization angle and relaxation time on the absorption spectra is studied in detail. Although several plasmon-induced absorption based on two-dimensional (2D) materials, such as graphene, black phosphorus, and transition metal dichalcogenides (TMDs), have been previously reported, this paper proposes a borophene-based metamaterial to achieve plasmon-induced perfect absorption since borophene has some advantages such as high surface-to-volume ratios, mechanical compliance, high carrier mobility, excellent flexibility, and long-term stability. Therefore, the proposed borophene-based metamaterial will be beneficial in the fields of multi-band perfect absorber in the near future.

Quasi-BIC based all-dielectric metasurfaces for ultra-sensitive refractive index and temperature sensing

In this paper, an all-dielectric metasurface that measures refractive index and temperature using silicon disks is presented. It can simultaneously produce three resonances excited by a magnetic toroidal dipole, magnetic toroidal quadrupole, and electric toroidal dipole, in the THz region. Asymmetric structures are used to generate two quasi-bound states in the continuum (BIC) resonances with ultra-high-quality factors driven by magnetic and electric toroidal dipoles. We numerically study and show the dominant electromagnetic excitations in the three resonances through near-field analysis and cartesian multipole decomposition. The effects of geometric parameters, scaling properties, polarization angles, incident angles, and silicon losses are also investigated. The proposed metasurface is an excellent candidate for sensing due to the extremely high-quality factor of the quasi-BICs. The results demonstrate that the sensitivities for liquid and gas detection are Sl = 569.1 GHz/RIU and Sg = 529 GHz/RIU for magnetic toroidal dipole, and Sl = 532 GHz/RIU and Sg = 498.3 GHz/RIU for electric toroidal dipole, respectively. Furthermore, the sensitivity for temperature monitoring can reach up to 20.24 nm/°C. This work presents a valuable reference for developing applications in the THz region such as optical modulators, multi-channel biochemical sensing, and optical switches.

Polarization-Tuned Fano Resonances in All-Dielectric Short-Wave Infrared Metasurface

The short-wave infrared (SWIR) is an underexploited portion of the electromagnetic spectrum in metasurface-based nanophotonics despite its strategic importance in sensing and imaging applications. This is mainly attributed to the lack of material systems to tailor light-matter interactions in this range. Herein, this limitation is addressed and an all-dielectric silicon-integrated metasurface enabling polarization-induced Fano resonance control at SWIR frequencies is demonstrated. The platform consists of a 2D Si/Ge_0.9 Sn_0.1 core/shell nanowire array on a silicon wafer. By tuning the light polarization, it is shown that the metasurface reflectance can be efficiently engineered due to Fano resonances emerging from the electric and magnetic dipoles competition. The interference of optically induced dipoles in high-index nanowire arrays offers additional degrees of freedom to tailor the directional scattering and the flow of light while enabling sharp polarization-modulated resonances. This tunablity is harnessed in nanosensors yielding an efficient detection of 10^-2 changes in the refractive index of the surrounding medium.

Dirac semimetal and an all dielectric based tunable ultrasensitive terahertz sensor with multiple bound states in the continuum

Herein, we propose an all dielectric, open-slit, U-shaped resonant arm-based metasurface that achieves three bound states in the continuum (BIC) in the high-frequency terahertz band whose quasi-BIC (QBIC) states can achieve Q factor of the magnitude order of 10⁴. The maximum sensitivities of accidental QBIC and the figure-of-merit were respectively equal to 1717 GHz/RIU and 16670. Furthermore, the dependencies of the three QBICs on the THz incidence angle were different. Finally, the tunable sensor based on the Dirac semimetal metasurface achieves active tuning of the sensitivity and sensing range. This overcomes the shortcomings of the fixed structure without tuning capabilities, and provides a broad applicability platform for THz sensors.

High-Q and tunable analog of electromagnetically induced transparency in terahertz all-dielectric metamaterial

We propose a novel all-dielectric metamaterial (ADMM), to the best of our knowledge, with a simple structure to achieve the analog of electromagnetically induced transparency (EIT) in the terahertz range. The ADMM is constructed by unit cells with two same silicon bar resonators on a quartz substrate. By breaking the symmetrical array of silicon resonators, the guided-mode resonance can be excited in the substrate, and the destructive interference between a broadband electric-dipole resonance and a narrowband guided-mode resonance gives rise to an EIT-like response. The EIT window can reach a high quality factor (Q-factor) over 1500 by carefully adjusting the asymmetry degree within the unit cell. A dynamically tunable ADMM was further developed by employing photoactive doped silicon. By varying the carrier density of the doped silicon through optical pump, the strength of the EIT-like resonance can be actively modulated, enabling an on-to-off switch of the slow-light effect. The designed ADMM can achieve a high-Q EIT-like response and dynamic modulation, which may give potential applications in bio/chemical sensing, optical switching, and slow-light devices.

Polarization and incidence insensitive analogue of electromagnetically induced reflection metamaterial with high group delay

In this work, we demonstrate an analogue of electromagnetically induced reflection (EIR) effect with hybrid structure consisting of a silica (SiO₂) square array layer embedded in graphene-dielectric-Au film constructed F-P cavity. It is shown that the SiO₂ square array and F-P cavity create transverse waveguide with high quality factor (Q-factor) and longitudinal F-P modes, and their destructive interference effectively forms the EIR-like effect, which benefits for obtaining high group delay. In addition, the C4 symmetric structure ensures the polarization-independent for this EIR-like effect. With high Q-factor at the reflection window, the ultra-high group delay as high as 245 ps can be obtained. This structure will be useful to develop the EIT-like devices with excellent performance such as high group delay, polarization and incident insensitivity, and environmental stability.

Dynamical manipulation of a dual-polarization plasmon-induced transparency employing an anisotropic graphene-black phosphorus heterostructure

Dynamical tunable plasmon-induced transparency (PIT) possesses the unique characteristics of controlling light propagation states, which promises numerous potential applications in efficient optical signal processing chips and nonlinear optical devices. However, previously reported configurations are sensitive to polarization and can merely operate under specific single polarization. In this work we propose an anisotropic PIT metamaterial device based on a graphene-black phosphorus (G-BP) heterostructure to realize a dual-polarization tunable PIT effect. The destructive interference coupling between the bright mode and dark modes under the orthogonal polarization state pronounced anisotropic PIT phenomenon. The coupling strength of the PIT system can be modulated by dynamically manipulating the Fermi energy of the graphene via the external electric field voltage. Moreover, the three-level plasmonic system and the coupled oscillator model are employed to explain the underlying mechanism of the PIT effect, and the analytical results show good consistency with the numerical calculations. Compared to the single-polarization PIT devices, the proposed device offers additional degrees of freedom in realizing universal tunable functionalities, which could significantly promote the development of next-generation integrated optical processing chips, optical modulation and slow light devices.

Analogue of Electromagnetically Induced Transparency in an All-Dielectric Double-Layer Metasurface Based on Bound States in the Continuum

Bound states in the continuum (BICs) have attracted much attention due to their infinite Q factor. However, the realization of the analogue of electromagnetically induced transparency (EIT) by near-field coupling with a dark BIC in metasurfaces remains challenging. Here, we propose and numerically demonstrate the realization of a high-quality factor EIT by the coupling of a bright electric dipole resonance and a dark toroidal dipole BIC in an all-dielectric double-layer metasurface. Thanks to the designed unique one-dimensional (D)-two-dimensional (2D) combination of the double-layer metasurface, the sensitivity of the EIT to the relative displacement between the two layer-structures is greatly reduced. Moreover, several designs for widely tunable EIT are proposed and discussed. We believe the proposed double-layer metasurface opens a new avenue for implementing BIC-based EIT with potential applications in filtering, sensing and other photonic devices.

Analogue of electromagnetically induced transparency in a metal-dielectric bilayer terahertz metamaterial

We realize and numerically demonstrate the analogue of electromagnetically induced transparency (EIT) with a high-Q factor in a metal-dielectric bilayer terahertz metamaterial (MM) via bright-bright mode coupling and bright-dark mode coupling. The dielectric MM with silicon dimer rectangular-ring-resonator (Si-DRR) supports either a bright high-Q toroidal dipole resonance (TD) or a dark TD with infinite Q value, while plasmonic MM with metallic rectangular-ring-resonator (M-RR) supports a low-Q electric dipole resonance (ED). The results show that the near-field coupling between the dark TD and bright ED behaves just as that between the two bright modes, which is dependent on the Q factor of the TD resonance. Further, due to the greatly enhanced near-field coupling between the bright ED and dark TD, the coupling distance is significantly extended to about 1.9 times of the wavelength (in media), and robust EIT with large peak value over 0.9 and high Q-factor is achieved. The proposed bilayer MM provides a new EIT platform for design and applications in high-Q cavities, sensing, and slow-light based devices.

Bidirectional Electromagnetically Induced Transparency Based on Coupling of Magnetic Dipole Modes in Amorphous Silicon Metasurface

A bidirectional electromagnetically induced transparency (EIT) arising from coupling of magnetic dipole modes is demonstrated numerically and experimentally based on nanoscale a-Si cuboid-bar metasurface. Analyzed by the finite-difference time-domain (FDTD) Solutions, both the bright and dark magnetic dipole mode is excited in the cuboid, while only the dark magnetic dipole mode is excited in the bar. By breaking the symmetry of the cuboid-bar structure, the destructive interference between bright and dark magnetic dipole modes is induced, resulting in the bidirectional EIT phenomenon. The position and amplitude of simulated EIT peak is adjusted by the vertical spacing and horizontal spacing. The EIT metasurface was fabricated by Electron-Beam Lithography and deep silicon etching technique on the a-Si film deposited by Plasma-Enhanced Chemical Vapor Deposition. Measured by a convergent spectrometer, the fabricated sample achieved a bidirectional EIT peak with transmission up to 65% and 63% under forward and backward incidence, respectively. Due to the enhanced magnetic field induced by the magnetic dipole resonance, the fabricated bidirectional EIT metasurface provides a potential way for magnetic sensing and magnetic nonlinearity.

Theoretical design of a reconfigurable broadband integrated metamaterial terahertz device

An actively reconfigurable broadband terahertz (THz) metamaterial functional device based on the phase-change material vanadium dioxide (VO₂) and two-dimensional graphene material is theoretically proposed and demonstrated. The device has excellent tolerance under oblique incidence. When the VO₂ is in the metallic state, and the Fermi energy of graphene is fixed at 0.1 eV, the designed device acts as a broadband THz absorber in the transverse magnetic (TM) polarization mode. The absorptance bandwidth exceeds 0.55 THz with a complete absorption intensity of more than 90%. In this state, the absorber operates as a broadband modulator with the total modulation depth exceeding 91.5% as the continually decreased conductivity of VO₂ from 200000 S/m to 10 S/m. In the transverse electric (TE) polarization process, the structure behaves as a dual-band absorber with two perfect absorption peaks. When the conductivity of VO₂ is changed, the tunable absorber can also be regarded as an absorptance modulator, with a maximum modulation intensity of 92.1%. Alternatively, when VO₂ behaves as an insulator at room temperature in the TE polarization mode, a strong broadband electromagnetically induced transparency (EIT) window is obtained, with a bandwidth exceeding 0.42 THz in the transmittance spectrum. By varying the Fermi energy of graphene from 0 to 0.9 eV, the EIT-like window or broadband transmission spectrum (in TM mode) can be switched. The results indicate that the device can also be operated as a modulator in the transmission mode. The impedance matching theory is used, and electric field distributions are analyzed to quantify the physical mechanism. An advantage of the manipulation of the polarization angle is that the modulation performance of the proposed multi-functional THz device can be regulated after fabricated.

Broadband transparency with all-dielectric metasurfaces engraved on silicon waveguide facets: effect of inverted and extruded features based on Babinet's principle

Building blocks of photonic integrated circuitry (PIC), optical waveguides, have long been considered transparent. However, the inevitable Fresnel reflection from waveguide facets limits their transparency. This limitation becomes more severe in high-index waveguides in which the transparency may drop to 65%. We overcome this inherent optical property of high-index waveguides by engineering an appropriate facet landscape made of sub-wavelength artificial features unit cells. For this, we develop a semi-analytical formalism for predicting the metasurface parameters made of high-index dielectric materials, to be engraved on the facets of optical waveguides, based on Babinet's principle: either extruded from the waveguide facet or etched into it. Our semi-analytical model predicts the shape of anti-reflective metasurface unit cells to achieve transmission as high as 98.5% in near-infrared from 1 μm to 2 μm. This new class of metasurfaces may be used for the improvement of PIC devices for communication and sensing, where device transparency is crucial for high signal-to-noise ratios.

Analog of multiple electromagnetically induced transparency using double-layered metasurfaces

We reported an analog of electromagnetically induced transparency (A-EIT) featured by double transparent peaks in the spectrum. The A-EIT is realized by double-layered metasurface which consists of spoof localized surface plasmons (S-LSP) and cut-wire (CW)-square rings (SR) hybrid. Electric and magnetic S-LSP are excited as bright and dark modes respectively then couple with resonant modes of CW and SR simultaneously to achieve multiple A-EIT. Two bright modes of the electric S-LSP and SR are excited by external electric field directly that produce a bright-bright mode A-EIT. Moreover, the magnetic S-LSP, which cannot be excited by external field directly, is excited through near field coupling from CW, inducing another bright-dark mode A-EIT. Theoretical analysis with corresponding experiment in microwave band are introduced for better insights into physical essence of the double-peaks A-EIT.

Ultrasensitive specific sensor based on all-dielectric metasurfaces in the terahertz range

An ultrasensitive specific sensor based on all-dielectric metasurfaces in the terahertz range was proposed. The designed metasurfaces consist of multi pairs of tilted silicon bars on a SiO₂ substrate with a high-Q Fano resonance feature. The peak of this high Q Fano resonance can form a wide reflection spectrum band by scanning the angle of the incident THz waves. Utilizing this angle-scanning strategy, we designed a metasurface sensor and its reflection spectrum band can cover the absorption peak of tyrosine and santonin. By depositing different thicknesses of tyrosine and santonin on the sensor, we have successfully identified them with a detection limit of 6.7 μg cm⁻² and 59.35 μg cm⁻², respectively. The performance of the sensor with high sensitivity has been analyzed in detail, showing an exciting prospect for identification of ‘fingerprint’ spectra in the terahertz region.

An ultrasensitive specific sensor based on all-dielectric metasurfaces in the terahertz range was proposed.