Tunable bandwidth of band-stop filter by metamaterial cell coupling in optical frequency

Adjustable strong circular dichroism based on a tricircular arc metasurface

Circular dichroism has promising applications in biology, molecular chemistry, and other fields. The key to obtaining strong circular dichroism is to introduce symmetry breaking into the structure, which leads to a great difference in the response to different circularly polarized waves. Here, we propose a metasurface structure based on three circular arcs, which produces strong circular dichroism. The metasurface structure combines the split ring with the three circular arcs and increases the structural asymmetry by changing the relative torsional angle. The causes of the strong circular dichroism are analyzed in this paper, and the influence of metasurface parameters on it is discussed. According to the simulation data, the response of the proposed metasurface to different circularly polarized waves varies greatly, with absorption of up to 0.99 at 5.095 THz for a left-handed circularly polarized wave and a maximum circular dichroism of over 0.93. In addition, the incorporation of the phase change material vanadium dioxide on the structure allows flexible modulation of circular dichroism and modulation depths of up to 98.6%. The change of angle within a certain range has little effect on the structural performance. We believe that this flexible and angle robust chiral metasurface structure is suitable for complex reality, and large modulation depth is more practical.

Design and Characterization of Wideband Terahertz Metamaterial Stop-Band Filter

We propose and design a metamaterial broadband stop-band filter with a steep cut-off in the terahertz region. The filter is based on the flexible structure of metal-dielectric-metal-dielectric-metal (MDMDM). Simulation results show that the filter has a center frequency of 1.08 THz, the square ratio reaches 0.95, and the −20 dB bandwidth reaches 1.07 THz. In addition, it has excellent flat-top characteristics with an average transmission rate in the resistive band of no more than 5%. The relative bandwidth has been up to 99%, and stopband absorption rate has reached more than 98%. The effects of the main structural parameters on the transmission characteristics are discussed. The role of each layer of metal in the filter is explored by studying the effect of the variation of the number of metal layers on the filter. The symmetry of the structure ensures the polarization insensitivity of the filter at normal incidence. The correctness of the simulation results was verified by analyzing the effective permittivity and magnetic permeability. To investigate the transmission characteristics of the metamaterial filter in-depth, we analyzed the electric field strength and surface current distribution at the center frequency of the filter. The designed terahertz filter may have potential applications in terahertz communications, sensors, and emerging terahertz technologies.

Switchable Terahertz Absorber from Single Broadband to Dual Broadband Based on Graphene and Vanadium Dioxide

A multifunctional switchable terahertz (THz) absorber based on graphene and vanadium dioxide (VO₂) is presented. The properties of the absorber are studied theoretically by the finite-difference time-domain (FDTD) method. The results illustrate that the structure switches between the single-broadband or double-broadband absorption depending on the temperature of VO₂. Moreover, the amplitude of the absorptivity can be adjusted by changing the Fermi energy level (E_F) of graphene or the conductivity of VO₂ separately. Via impedance matching theory, the physical mechanism of the absorber is researched. Furthermore, the effects of incidence angle on absorption have also been studied. It is found that the absorber is insensitive to the polarization of electromagnetic waves.

Tunable Dual-Broadband Terahertz Absorber with Vanadium Dioxide Metamaterial

A dual broadband terahertz bifunction absorber that can be actively tuned is proposed. The optical properties of the absorber were simulated and numerically calculated using the finite-difference time-domain (FDTD) method. The results show that when the conductivity of vanadium dioxide is less than σ0=8.5×103 S/m, the absorptance can be continuously adjusted between 2% and 100%. At vanadium dioxide conductivity greater than σ0=8.5×103 S/m, the absorption bandwidth of the absorber can be switched from 3.4 THz and 3.06 THz to 2.83 THz and none, respectively, and the absorptance remains above 90%. This achieves perfect modulation of the absorptance and absorption bandwidth. The physical mechanism of dual-broadband absorptions and perfect absorption is elucidated by impedance matching theory and electric field distribution. In addition, it also has the advantage of being polarization insensitive and maintaining stable absorption at wide angles of oblique incidence. The absorber may have applications in emerging fields such as modulators, stealth and light-guided optical switches.

Ultra-wideband tunable metamaterial perfect absorber based on vanadium dioxide

A dynamically adjustable ultra-wideband metamaterial perfect absorber (MPA) is proposed which consists of three resonance rings based on vanadium dioxide (VO₂) and a metal ground layer separated by a dielectric spacer. The simulation results show that the terahertz (THz) absorption bandwidth of more than 90% absorptance reaches 3.30 THz, which covers from 2.34 to 5.64 THz, under different incident polarization angles. The range is better than that of previous VO₂-based reports. Moreover, when the conductivity of VO₂ changes from 200 S/m to 2×10⁵ S/m, the absorption peak intensity can be adjusted continuously from 4% to 100%. The key is to optimize the geometric structure through interference cancellation and impedance matching theory, to achieve better absorption bandwidth and efficiency. Besides, the terahertz absorber has a wide-angle absorption effect both in TE and TM waves. Thus, the designed absorber may have many potential applications in modulating, sensing and imaging technology.

Plasmonic hybridization generation in self-aligned disk/hole nanocavities for multi-resonance sensing

Plasmonic nanostructures have proven an extensive practical prospect in ultra-sensitive label-free biomolecule sensing due to their nanoscale localization and large near-field enhancement. Here, we demonstrate a photonic plasmonic hybridization in the self-aligned disk/hole nanocavity array under two specific cases of nanogap and nanooverlap achieved by adjusting pillar height embedded into hole. The proposed disk/hole arrays in above two cases exhibit three hybridized modes with extremely high absorption, mainly arising from the in-phase (bonding) and out-of-phase (antibonding) coupling of dipolar modes of their parent disk and hole. Surprisingly, when the nanogap feature of the disk/hole array is transformed to the nanooverlap, crossing the quantum effect region, the bonding mode in the disk/hole array has an enormous transition in the resonant frequency. In comparison with the counterpart in the nanogap structure, the bonding mode in the nanooverlap structure supports strongest near-field localization (i.e., the decay length down to merely 3.8 nm), although charge transfer channel provided by the geometry connect between disk and hole quenches partial field enhancement. Furthermore, we systematically investigate the sensing performances of multiple hybridized modes in above two cases by considering two crucial evaluating parameters, bulk refractive index sensitivity and surface sensitivity. It is demonstrated that, in the nanogap structure, the bonding mode possesses both high bulk refractive index sensitivity and surface sensitivity. Dissimilarly, for the nanooverlap structure, the bonding and antibonding modes show different surface sensitivities in different regions away from the surface, which can be used to monitoring different bio-molecular sizes and achieve the most optimum sensitivity. Due to its unique sensing features, this disk/hole array mechanism is very valuable and promising for developing of high sensitivity sensing platform.

Broadband terahertz absorber with a flexible, reconfigurable performance based on hybrid-patterned vanadium dioxide metasurfaces

An actively tunable broadband terahertz absorber is numerically demonstrated, which consists of four identical vanadium dioxide (VO₂) square loops and a metal ground plane separated by a dielectric spacer. Simulation results show that an excellent absorption bandwidth of 90% terahertz absorptance reaches as wide as 2.45 THz from 1.85 to 4.3 THz under normal incidence. By changing the conductivity of VO₂, an approximately perfect amplitude modulation is realized with the absorptance dynamically tuned from 4% to 100%. This absorption performance is greatly improved compared with previously reported VO₂-based absorbers. The physical mechanisms of a single absorption band and the perfect absorption are elucidated by the wave-interference theory and the impedance matching theory, respectively. Field distributions are further discussed to explore the physical origin of this absorber. In addition, it also has the advantages of polarization insensitivity and wide-angle absorption. The proposed absorber may have many promising applications in the terahertz range such as modulator, sensor, cloaking and optic-electro switches.

Design of Broad Stopband Filters Based on Multilayer Electromagnetically Induced Transparency Metamaterial Structures

Broad stopband filters are proposed, based on multilayer electromagnetically induced transparency (EIT) metamaterial structures. The single EIT metamaterial consists of a U-shaped resonator and a strip on a polyimide substrate. The EIT-like spectral feature is firstly utilized to achieve stopband filters by properly coupling two layers of EIT structure. Influences of different rotation angles on the transmission properties of the two-layer EIT structure are investigated. It is found the wider low-transmission band can be obtained for the Transverse Magnetic (TM) polarization when the two EIT metal structures are vertical to each other. Furthermore, the bandwidth of the stopband can be controlled by increasing layers of the EIT structures with the proper architectural design. The design using a coupling effect of multi EIT-like resonances in the metamaterial would provide a new method for broad stopband filters in highly integrated optical circuits.

Comparative investigation of sensing behaviors between gap and lattice plasmon modes in a metallic nanoring array

Plasmonic nanostructures have become the most promising candidates for biosensing applications because of their miniature sizes, ease of integration, and high-throughput detection. Both propagating and localized surface plasmon modes in the nanostructures have been used for sensing and biomolecular detection. However, to maximize the biosensing potential of nanostructures, the choice of an optimized sensing detection strategy among two plasmon modes depends on the relationship between the biomolecule sizes and field decay length of plasmon modes. Here, we propose and investigate plasmonic coupling on a single-crystalline gold film, wherein there are two distinct optical modes, a gap mode (localized surface plasmon) originating from the parallel coupling of a nanoring and a surface lattice mode (propagating surface plasmon) originating from the anti-parallel coupling of a nanoring in an array. The sensing performances of the above two modes are thoroughly investigated and compared by considering two aspects, i.e., bulk and surface sensitivities. It is demonstrated that there is a reciprocal relationship between bulk and surface sensitivities for two modes, which also illustrates that the surface sensitivity is indispensable to fully describe the sensing performance of nanostructures. Furthermore, due to their different decay lengths, the gap and surface lattice modes on a single optical substrate can achieve simultaneous detection of target analytes with various sizes. Therefore, we can provide a high performance sensing platform based on a metallic nanoring array for a broad range of biomolecules with various sizes.

Study of THz-Plasmon hybridization of a loop Yagi-Uda absorber

In this article we present a three-dimensional loop Yagi-Uda array for efficient, polarization independent and directional absorption of THz radiation over a narrow frequency range (f0 = 0.657 THz & Q factor = 7.5). Unit cell of the array consists of three vertically stacked gold micro rings separated from each other by 30 µm thick SU-8 layers. The proposed array also exhibits a filtering response in its transmittance spectrum. The characteristics are explained by plasmon hybridization method. The transmission, reflection and absorption spectra of the structure are measured and they show a good agreement with corresponding simulated results.

Wavelength-tunable perfect absorber based on guided-mode resonances

We numerically investigate the triple-band perfect absorption in a metal-insulator-metal structure. The absorption peak from the TE-polarized guided-mode resonance (GMR) is highly sensitive to the incident angle. Thus a wavelength-tunable perfect absorber (PA) based on the TE GMR is proposed for the first time. By the precise control of the incident angle, the ∼5  nm narrowband perfect absorber can be modulated linearly about 3 nm/° in our structure. For single frequency light, the intensity tunability of the absorption between 6.2%-99.27% is realized only by changing the incident angle of 5°. The further study focused on TM polarization confirms the possibility to realize a polarization-independent wavelength-tunable PA. Such a PA possesses potential for applications in absorption filter, thermal emitter, surface-enhanced Raman scattering, biosensing, and nonlinear optics.

Three-dimensional movable metamaterial using electric split-ring resonators

We have demonstrated a three-dimensional movable metamaterial (MM) with reconfigurable electric split-ring resonators (eSRR) at terahertz frequencies. This is accomplished by making planar arrays of eSRR with movable stress curved beams (eSRR-MSCBs) which are actuated out-of-plane by electrostatic force. Our results have demonstrated that the eSRR-MSCBs possess blueshifting capabilities and polarization dependent state at terahertz frequencies, while such adaptive MMs offer significant potential in realizing electromagnetic functionality with diversified applications, such as sensors, switches, and filters.

Near- and far-field spectroscopic imaging investigation of resonant square-loop infrared metasurfaces

Optical metamaterials have unique properties which result from geometric confinement of the optical conductivity. We developed a series of infrared metasurfaces based on an array of metallic square loop antennas. The far-field absorption spectrum can be designed with resonances across the infrared by scaling the geometric dimensions. We measure the amplitude and phase of the resonant mode as standing wave patterns within the square loops using scattering-scanning near-field optical microscopy (s-SNOM). Further, using a broad-band synchrotron-based FTIR microscope and s-SNOM at the Advanced Light Source, we are able to correlate far-field spectra to near-field modes of the metasurface as the resonance is tuned between samples. The results highlight the importance of multi-modal imaging for the design and characterization of optical metamaterials.

Tunable broad-band perfect absorber by exciting of multiple plasmon resonances at optical frequency

A broad-band perfect absorber composing a two-dimensional periodic metal-dielectric-metal sandwiches array on dielectric/metal substrate is designed and numerically investigated. It is shown that the nearly-perfect absorption with a bandwidth of about 50 nm in visible region can be achieved by overlapping of two plasmon resonances: one originating from the coupling of electric dipoles between adjacent unit cells and another arising from magnetic dipole plasmon resonances. A capacitor-inductor circuit description is introduced to explain the dependence of resonance frequencies and band-width on geometrical parameters.