Gold Ground Plane Research Articles

A polarization insensitive terahertz tri-band graphene metasurface absorber

A graphene-based tri-band perfect metasurface absorber is presented for the far-infrared frequency range. A two-dimensional periodic pattern of graphene patches is created on a thin SiO2 dielectric layer, which is backed by a gold ground plane of sufficient thickness ( t ) ensuring zero transmission of the impinging electromagnetic (EM) wave. An array of graphene elliptical discs is placed around graphene square patches and thin graphene strips are used to connect these elliptical patches to the square patches. In terms of the lowest frequency of absorption, the proposed absorber seems to be a thin one as t∼λmax/13. It is also considered compact because the periodicity of the cells is on the order of λmax/22. The absorption spectrum gives three perfect absorption peaks with absorptivity greater than 99% at 5.76 and 9.06 THz. Further, the absorptivity is better than 96% at 7.86 THz. The performance of the proposed absorber remains invariant with change in polarization of the incident wave. It is revealed that this absorber performs satisfactorily throughout a broad angular range of incidence of about ± 60°.

Graphene-Enabled Tunable Phase Gradient Metasurface for Broadband Dispersion Manipulation of Terahertz Wave.

With the increasing demand for the miniaturization and flexibility of optical devices, graphene-based metasurfaces have emerged as a promising ideal design platform for realizing planar and tunable electromagnetic or optical devices. In this paper, we propose a tunable metasurface with low-dispersion phase gradient characteristics that is composed of an array of double-layer graphene ribbons sandwiched with a thin insulating layer and a polymer substrate layer with a gold ground plane. As two typical proof-of-concept examples, metasurfaces act as a planar prism and a planar lens, respectively, and the corresponding performances of tunable broadband dispersion are demonstrated through full-wave simulation experiments. By changing the Fermi level of each graphene ribbon individually to introduce abrupt phase shifts along the metasurface, the broadband continuous dispersion effect of abnormal reflection and beam focusing is achieved within a terahertz (THz) frequency region from 3.0 THz to 4.0 THz, and the dispersion results can be freely regulated by reconfiguring the sequence of Fermi levels via the bias voltage. The presented graphene metasurface provides an avenue for the dispersion manipulation of a broadband terahertz wave and may have great prospects in the fields of optics, imaging, and wireless communication.

Bicontrollable all dielectric metasurface absorber for chemical and biosensing applications

A bi-controllable all dielectric metasurface structure is designed at terahertz, which consists of an InAs micro-cylindrical array on the gold ground plane. The maximum absorptance frequency is tunable with the help of an external magnetic field and also by geometrical rotation of the metasurface as InAs is a gyrotropic material whose property depends on the magnitude and direction of an external magnetic field. Simulation results indicate that the proposed metasurface can achieve an absorptance of more than 99 % at 9.756 THz as well as it can provide a high tunability rate of around 2.55 THz T−1 in lower band and 3.33 THz T−1 in the upper band frequency spectrum with the application of the magnetic field. Additionally, the metasurface also acts as polarization insensitive when no external magnetic field is applied. Furthermore, the maximum absorptance frequency shows sensitivity towards the change in the refractive index value of the surrounding medium, thus the metasurface can be used for chemical and biosensing applications. It shows high chemical sensitivity of around 2727 GHz/RIU for the detection of different chemicals and bio sensitivity of 787 GHz/RIU for the detection of various viruses.

Highly sensitive tunable terahertz absorber for biosensing applications

A multicontrollable terahertz perfect absorber has been proposed and demonstrated numerically through an L-shaped metasurface tri-layer structure. The proposed tri-layer structure comprises a top L-shaped InAs layer and gold ground plane separated by a dielectric polyimide layer which shows a perfect absorption of more than 99.8% with the application of magnetic field from 0.5–0.8 T along with a higher tunability rate of 0.76 THz/T in the frequency spectrum. The multicontrollability and tunability in the frequency response are achieved by the application of a magnetic field as well as the geometrical rotation of meta-atom. Thus, the proposed structure act as a bicontrollable metasurface providing an efficient approach for control over the maximum absorptance frequency. Furthermore, the performance of the proposed absorber shows greater applicability for the bio-chemical sensors with a high value of sensitivity of around 1109 GHz/RIU for the detection of different chemicals, 1012 GHz/RIU for different pathogens, and 1066 GHz/RIU for different hemoglobin concentrations. Thus, the proposed metasurface provides a bicontrollable approach for efficient absorption and prevents transmission as well as acts as multifunctional metasurface which provides potential applications in designing bio-chemical sensors at terahertz frequency regimes.

A graphene based dual-band metamaterial absorber for TE polarized THz wave

A graphene based tunable metamaterial perfect absorber is designed, which is a classic and simple sandwich structure composed of a non-rotationally symmetric graphene pattern layer, a TOPAS dielectric layer and a thick enough gold ground plane. The dynamically adjustable absorption property of the proposed absorber is polarization sensitive, which is novel compared with the widely studied polarization insensitive metamaterial absorber. For TE polarized incident electromagnetic waves (EMW), the dual-band absorption can be generated at frequencies of 4.268 THz and 6.032 THz with peak absorption coefficients up to 99.99% and 99.79%, respectively. But for TM light, there is only one near unity absorption band located at about 4.148 THz. So it will have potential application value for the polarization state resolution of THz waves. The research mainly concentrates on the dual-band tunable absorption under TE polarized incident light. The factors affecting the absorption properties of the proposed absorber are studied by discussing the dependence of the geometric parameters as well as chemical potential and relaxation time of the graphene on the absorption. The simulation results are verified by using multiple reflection interference theory (MRIT) and impedance matching theory (IMT), and the results show that the theoretical results match well with the simulation results. The absorber can achieve good absorption performance in a wide range of incident angles for TE light. Furthermore, by selecting two different graphene chemical potential, good performance of THz switching can be realized. In addition, the application in the field of refractive sensing is also investigated in detail, and higher sensitivity and better figure of merit (FOM) can be obtained. Thus, the designed graphene-based dual-band THz metamaterial absorber has great potential in polarization resolution, EMW absorbing, THz switching, refractive index sensing and so on.

Narrowband-to-broadband switchable and polarization-insensitive terahertz metasurface absorber enabled by phase-change material

A terahertz absorber with controllable and switchable bandwidth that is insensitive to polarization is of great interest. Here, we propose and demonstrate a metasurface absorber with switchable bandwidth based on a phase-change material of vanadium dioxide (VO2) and verify its performance by finite element method simulations. The metasurface absorber is composed of a hybrid cross fractal as a resonator separated from a gold ground plane by a polyimide spacer. Switching from narrowband to broadband absorber is achieved via connecting VO2 patches to the gold first-order cross fractal converting the resonator to a third-order cross fractal. In the insulator phase of VO2, the main narrowband absorption occurs at the frequency of 6.05 THz with a 0.99 absorption and a full-width half-maximum (FWHM) of 0.35 THz. Upon insulator-to-metal transition of VO2, the metasurface achieves a broadband absorption with FWHM of 6.17 THz. The simulations indicate that by controlling the partial phase transition of VO2, we can tune the bandwidth and absorption level of the absorber. Moreover, the designed absorber is insensitive to polarization due to symmetry and works well for a very wide range of incident angles. In the metallic state of VO2, the absorber has an absorption exceeding 0.5 in the 3.57–8.45 THz frequency range with incident angles up to 65°.

Reflected metasurface carrying orbital angular momentum for a vortex beam in the terahertz region

Vortex beams carry orbital angular momentum (OAM), which can increase channel capacity in communication systems. In 2019, the International Telecommunication Union designated the terahertz band as the sixth generation wireless communication carrier, so it is very meaningful to study the OAM of terahertz waves. In this letter, we demonstrated a reflective metasurface terahertz vortex beam generator with a wide operating bandwidth (1.6 ∼ 2.4 THz), which consists of a gold metasurface array and a gold ground plane separated by a polyimide layer. Under illumination of circularly-polarized waves, the proposed metasurface produces vortex beams with topological charges of l = ±1, l =±2, and l = ±3 with the mode purity larger than 0.6. Simulation results are in good agreement with the theoretical predictions. Our approach provides a new idea for the design of terahertz vortex wave generators.

A Switchable Ultra-Wideband Metamaterial Absorber with Polarization-Insensitivity and Wide-incident Angle at THz Band

In this paper, we report a switchable ultra-wideband metamaterial absorber with polarization-insensitivity and wide-incident angle at THz band which is composed of VO2 disk, polyimide dielectric substrate, and gold ground plane. The results show that the absorption is greater than 90% from 3.5–8 THz for a temperature of 300 K and this absorption band disappears when the temperature rises to 350 K. The absorption property of our proposed metamaterial absorber is insensitive to polarization states and angles and it can withhold high absorption of more than 80% for wide-incident angles, up to 60° for TE mode and TM mode. The wideband absorption mechanism is elucidated using an effective medium and surface current analysis.

Switchable dual-band and ultra-wideband terahertz wave absorber

In this paper, we introduced a switchable dual-band and ultra-wideband terahertz wave absorber based on photoconductive silicon combining with vanadium dioxide (VO2). In the terahertz absorber, photoconductive silicon cross array, silicon dioxide layer, vanadium dioxide windmill type array, silicon dioxide dielectric layer, and gold ground plane are placed from the top layer to bottom layer in sequence. When VO2 is in a metallic state and the conductivity of photoconductive silicon is 2.5×10−4 S/m, the designed structure represents an ultra-wideband absorber with an absorption larger than 90% in the range of 3.14∼7.80 THz. As VO2 is in an insulation state and the conductivity of photoconductive silicon becomes 8.0×104 S/m, the designed device acts as two absorption bands, with a terahertz wave absorber with absorption more than 98% at 1.78∼2.90 THz and 7.35∼8.45 THz. The results show that the absorption band (dual-band or ultra-wideband) and absorption intensity (from 2% to 99%) can be switched by changing the phase transition of the VO2 and the conductivity of photoconductive silicon. Furthermore, the proposed device exhibits polarization insensitive and wide incident angles (lager than 70°) for TE- and TM- polarizations incidence.

Actively tunable dual-broadband graphene-based terahertz metamaterial absorber* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11504006 and 61805072) and the Key Scientific Research Project of Colleges and Universities in Henan Province, China (Grant No. 22A140001).

A tunable metamaterial absorber (MA) with dual-broadband and high absorption properties at terahertz (THz) frequencies is designed in this work. The MA consists of a periodic array of flower-like monolayer graphene patterns at top, a SiO2 dielectric spacer in middle, and a gold ground plane at the bottom. The simulation results demonstrate that the designed MA has two wide absorption bands with an absorption of over 90% in frequency ranges of 0.68 THz–1.63 THz and 3.34 THz–4.08 THz, and the corresponding relative bandwidths reach 82.3% and 20%, respectively. The peak absorptivity of the absorber can be dynamically controlled from less than 10% to nearly 100% by adjusting the graphene chemical potential from 0 eV to 0.9 eV. Furthermore, the designed absorber is polarization-insensitive and has good robustness to incident angles. Such a high-performance MA has broad application prospects in THz imaging, modulating, filtering, etc.

Broadband dynamically tunable terahertz absorber based on a Dirac semimetal.

In this paper, we propose a broadband tunable metamaterial absorber in the terahertz (THz) region. The absorber comprises a Dirac semimetal film, a dielectric layer, and a gold ground plane. Numerical results show that the absorptivity remains above 90% in the range from 5.7 THz to 8.4 THz when the Fermi level is 65 meV. By varying the Fermi energy of the Dirac semimetal film from 40 meV to 80 meV, the absorption bandwidth and absorption peaks can be dynamically tuned. To explain the mechanism of high absorption, the magnetic field, surface current, and power loss density distributions at different resonant frequencies were presented. Our work may have potential applications in various fields such as sensors, detectors, and photovoltaic devices in THz regions.

Independently tunable perfect absorber based on the plasmonic properties in double-layer graphene

Graphene can be utilized in designing tunable photonic devices due to its unique properties such as high optical transparency and tunable conductivity. In this paper, a tunable triple-band perfect absorber is designed and numerically demonstrated, which is composed of a stacked graphene nanodisk, L-shape graphene layer and a gold ground plane spaced by insulator layers. The results show that there are three higher than 99% absorption peaks in the infrared range with a wide incident angle range up to 50°, and the absorber is insensitive for TE and TM polarized incident light. Moreover, the perfect absorption peak wavelength can be tuned by changing Fermi levels or geometric parameters of graphene layers. The findings above indicate that the designed perfect absorber can be used in the field of photodetectors, thermal emitters and photovoltaics.

Umbrella-shaped graphene/Si for multi-band tunable terahertz absorber

We demonstrated a triple-band terahertz wave absorber, which consists of umbrella-shaped graphene metasurface array and a gold ground plane separated by a high-resistivity dielectric layer. Simulated results show that the proposed absorber has three distinctive absorption peaks at frequencies 0.506 THz, 1.638 THz, and 2.687 THz with the absorption rates of 0.998, 0.997, and 0.998, respectively. The terahertz absorber is valid to attain 0° ~ 60° range of incident angles for both transverse electric (TE) and transverse magnetic (TM) polarizations. For the graphene-based metasurface structures, the absorption curves can be tuned by controlling applied bias voltage. It is a promising candidate to design a high-performance absorber having the potential applications in bolometer, spectroscopic imaging, thermal detector, etc.

Flexible dual-band all-graphene-dielectric terahertz absorber

We report a polarization-insensitive dual-band tunable graphene absorber in the terahertz region, which is composed of a top patterned graphene and a gold ground plane, separated by a silicon dielectric layer. Numerical simulations verify the amplitude of the absorption peaks have reached 98.6% and 98.2% at 0.512THz and 1.467THz, respectively. It exhibits excellent performances with high absorptivity and wide incident angles for both transverse electric (TE) and transverse magnetic (TM) polarizations. The frequency and amplitude of the absorption peaks can be regulated by chemical potential of graphene via bias voltage. These characteristics provide great potential applications in imaging, detecting, and sensing in the terahertz regime.

Multifocal terahertz lenses realized by polarization-insensitive reflective metasurfaces

In this work, we report two multifocal terahertz (THz) lenses (MTLs) based on reflective metasurfaces with focusing efficiencies of 57.3% and 46.3% at a frequency of 3.11 THz, respectively. The reflective metasurfaces were constructed by patterning the polarization-insensitive cross resonator array atop a continuous gold ground plane spaced by a layer of parylene film, which provides a complete 2π phase coverage. The elaborated phase profiles of the two MTLs were arranged based on the Fresnel-Kirchhoff diffraction theory. The two MTLs were theoretically and experimentally verified to converge continuous THz waves into two and four beam spots at their focal planes, respectively. Meanwhile, the MTLs were also experimentally demonstrated with multiple imaging abilities.

An air-spaced terahertz metamaterial perfect absorber

Metamaterial absorbers are typically comprised of a layer of split-ring resonators and a ground plane with a dielectric spacer layer that provides structural support and in which absorbed energy is deposited. We address the question “What happens to the absorption if the spacer layer is removed?” through the design, fabrication, and characterization of a terahertz metamaterial absorber with air as the spacer layer. Reflection based terahertz time-domain spectroscopy was employed to measure the absorption and interference theory was used to interpret the results. The surface current in the gold ground plane and split-ring resonator layer is solely responsible for the absorption in the form of joule heating. In comparison to dielectric spacer layer absorbers, the quality factor is increased by a factor of ∼3. The electric field is highly concentrated in the volume between split-ring resonator layer and the ground plane offering the potential for novel sensing application if materials can be incorporated into this region (e.g. with microfluidics). In the spirit of this possibility, simulations of the absorption have been performed. The variation of the real part of the permittivity of the spacer material results in an absorption peak frequency shift, while a change in the imaginary part affects the quality factor and amplitude. Ultimately, the high quality factor and the absence of the spacer material provide the air-spacer metamaterial absorber with unique advantages for sensing applications.

A multi-functional plasmonic metasurface for anomalous reflection and optical rotation on the basis of anisotropic building blocks

We design and demonstrate a multi-functional plasmonic metasurface. Theoretical analyses and numerical simulations are performed and they agree well with each other. The metasurface contains gold bars aligned in different directions, a gold ground plane and a silica spacer. These anisotropic building blocks produce a phase gradient and their responses vary with the handedness of the circular polarization. The metasurface can achieve anomalous reflection and optical rotation simultaneously. The rotation angle is controllable and these optical features remain well in different incident conditions.

Terahertz saturable absorption in superconducting metamaterials

We present a superconducting metamaterial saturable absorber at terahertz frequencies. The absorber consists of an array of split ring resonators (SRRs) etched from a 100nm YBaCu3O7 (YBCO) film. A polyimide spacer layer and gold ground plane are deposited above the SRRs, creating a reflecting perfect absorber. Increasing either the temperature or incident electric field (E) decreases the superconducting condensate density and corresponding kinetic inductance of the SRRs. This alters the impedance matching in the metamaterial, reducing the peak absorption. At low electric fields, the absorption was optimized near 80% at T=10K and decreased to 20% at T=70K. For E=40kV/cm and T=10K, the peak absorption was 70% decreasing to 40% at 200kV/cm, corresponding to a modulation of 43%.

Frequency-Tunable Mid-Infrared Cross Polarization Converters Based on Graphene Metasurface

We have proposed two designs of graphene-enabled cross polarization converters, which are capable of high-efficiency polarization conversion rate and can work equally well for a wide range of incident wave angles. The first type is carefully constructed by an ellipse-shaped graphene sheet printed on a dielectric material backed up by a gold ground plane, while the second one comprises a graphene ring embedded an ellipse resonator. Numerical results demonstrate that the polarization conversion rate of the first polarizer reaches 99.38 % at 22.541 THz when the Fermi energy is fixed at 0.9 eV. The second one can simultaneously work at two frequencies with its polarization conversion rate being 96.74 and 95.88 %, respectively. Therefore, for two devices, the incident linearly polarized beams are almost completely rotated to its orthogonal counterpart after reflection in the mid-infrared spectral range. More importantly, the cross polarization amplitude and resonant frequencies can be dynamically tuned by shifting the Fermi energy without changing the nanostructure, which will exhibit enormous potential applications in photonics field.

Nonlinear terahertz metamaterial perfect absorbers using GaAs [Invited

We investigate the nonlinear response of terahertz (THz) metamaterial perfect absorbers consisting of electric split ring resonators on GaAs integrated with a polyimide spacer and gold ground plane. These perfect absorbers on bulk semi-insulating GaAs are characterized using high-field THz time-domain spectroscopy. The resonance frequency redshifts 20 GHz and the absorbance is reduced by 30% as the incident peak field is increased from 30 to 300 kV/cm. The nonlinear response arises from THz field driven interband transitions and intervalley scattering in the GaAs. To eliminate the Fresnel losses from the GaAs substrate, we design and fabricate a flexible metamaterial saturable perfect absorber. The ability to create nonlinear absorbers enables appealing applications such as optical limiting and self-focusing.