A tunable broadband terahertz MoS2 absorber using series-parallel hybrid network design.

A method for designing a broadband absorber using a series-parallel hybrid network is proposed. The performance of the broadband absorber is improved by using frequency-selective surface patterns based on a series-parallel hybrid equivalent circuit. The results indicate that the tunable single-layered terahertz MoS2 absorber has excellent broadband characteristics. Between 0.84 and 2.34 THz, the absorption and relative absorption bandwidth exceed 90% and 94.3%, respectively. Also, the absorption level can be adjusted from 90% to 10% by applying a bias voltage on the electrodes. The effects of different types of MoS2 layers and surface fluctuations in monolayered MoS2 on the properties of the absorber are demonstrated. In the 60° (TM) and 50° (TE) ranges, the polarization of the terahertz absorber is insensitive to the incidence angle. Overall, this method enables the single-layered absorber to exhibit excellent broadband characteristics comparable to those of multilayered structures, as well as simplifies the structure. Consequently, this method significantly broadens the usefulness of tunable single-layered absorbers for radar stealth, terahertz imaging, and electrically tunable modulation.

Electrically controllable broadband reflective linear cross-polarization conversion based on liquid crystals.

This paper presents an electrically controllable reflective broadband linear polarization (LP) converter based on liquid crystals (LCs) for cross-polarization conversion (CPC) in the terahertz frequency range. The proposed structure achieves a high polarization conversion ratio (PCR) exceeding 0.9 within the frequency range of 236.8 - 269.6 GHz. A vital feature of this design is the dynamic control of polarization conversion by re-orienting the nematic liquid crystal molecules through voltage bias switching between 'on' and 'off' states, allowing for precise manipulation of cross-polarized and co-polarized reflected waves. Experimental results validate the simulation outcomes, demonstrating excellent agreement. In contrast to conventional reflective polarization converters with fixed frequency responses, the proposed electrically controllable polarization conversion offers significant potential in imaging and optical communications.

Manipulation of sub-terahertz waves using digital coding metasurfaces based on liquid crystals.

This paper presents a novel sub-terahertz liquid crystal (LC) phase shifter based on digital coding metasurfaces. The proposed structure consists of metal gratings and resonant structures. They are both immersed in LC. The metal gratings function as reflective surfaces for electromagnetic waves and electrodes for controlling the LC layer. The proposed structure changes the state of the phase shifter by switching the voltage on every grating. It allows the deflection of LC molecules within a subregion of the metasurface structure. Four switchable coding states of the phase shifter are obtained experimentally. The phase of the reflected wave varies by 0°, 102°, 166°, and 233° at 120 GHz. Due to the presence of the transverse control electric field, modulation speed is approximately doubled compared to the free relaxation state. This work provides a novel idea for wavefront modulation of phase.

Wideband linear cross-polarization conversion with 3D metamaterial configuration.

In this Letter, a broadband and wide-angle linear polarization (LP) converter based on a 3D resonator for cross-polarization conversion (CPC) is presented. The designed structure performs a CPC with a polarization conversion ratio (PCR) of more than 90% in the frequency range 16.32 to 34.12 GHz, corresponding to a relative frequency bandwidth of 70.6%. Moreover, the presented structure possesses broad angle stability and miniaturized configuration. The efficiency of CPC remains over 90% in the operational frequency band with the incident angles up to 40°, and the cell size is 0.18λ for the lowest frequency of the CPC operational band. The proposed 3D structure is fabricated using the multi-material hybrid microdroplet jetting modeling (MHMJM) technique, and the experiments agree closely with the simulated results. Compared with traditional polarization converters based on a planar resonant structure, the proposed design shows excellent bandwidth, wide-angle performance, and miniaturization advantages.

High-Efficiency Multi-Channel Orbital Angular Momentum Multiplexing Enabled by the Angle-Dispersive Metasurface.

Orbital angular momentum (OAM) multiplexing of electromagnetic (EM) waves is of great significance for high-speed wireless communication and remote sensing. To achieve high-efficiency OAM multiplexing for multi-channel incident EM waves, this paper presents a novel angle-dispersive meta-atom structure, which can introduce the required anti-symmetric phase dispersion as well as high transmission efficiency for OAM multiplexing. These meta-atoms are then arranged delicately to form an angle-dispersive metasurface working at the X band, which enables three-channel OAM multiplexing by converting highly directional transverse-magnetic (TM) waves incident from 0 and ±45° to coaxial OAM beams with l = 0 and ±2 modes, respectively. The simulation and experimental results reveal that the proposed metasurface can convert a higher proportion of energy to the required OAM modes compared to the conventional OAM multiplexing metasurfaces, which can significantly improve the coaxial transmission efficiency of multi-channel OAM multiplexing.

Liquid crystal-based wide-angle metasurface absorber with large frequency tunability and low voltage.

A tunable metasurface absorber (MA) based on polymer network liquid crystal is introduced in this paper. Despite the well-designed unit cell patterns, the proposed MA can achieve both large frequency tunability and wide-angle stability. Compared with traditional liquid crystal-based metasurfaces, the measured results suggest that the recovery time of the proposed structure was reduced by half. By applying an external voltage on the top electrode of the liquid crystal layer from 0 to a saturation voltage of 10 V, the absorption peak of the MA can be tuned from 112.7 GHz to 102.2 GHz, with a maximum frequency tunability of 9.3%, which is significantly higher than other proposed liquid crystal-based metasurfaces. Moreover, the proposed tunable absorber can maintain absorption greater than 90% with incident angles reaching up to 60° for both transverse electric and transverse magnetic polarizations. This design provides an efficient way for developing low-power consumption terahertz devices with large frequency tunability and wide-angle stability.

Active continuous control of terahertz wave based on a reflectarray element-liquid crystal-grating electrode hybrid structure.

In this work, a new and efficient terahertz reflective phase shifter is proposed. The phase shifter is composed of a metal-dielectric-metal structure with a double dipole patch array, as well as copper grating electrodes immersed within the nematic liquid crystal. More specifically, the employed copper grating electrodes consist of two sets of cross-distributed comb grids, whereas at each set of comb grids can be applied an external bias voltage separately. On top of that, the electric field in the liquid crystal (LC) layer can be continuously changed by enforcing an innovative technique. Consequently, the orientation of the LC molecules was fully controlled by the applied electric field, since the dielectric constant of the LC is controlled by the biased voltage. The phase of the reflective electromagnetic wave can be continuously manipulated. Under this direction, the experimental results show that the phase shift exceeds the value of 180° in the range of 102.5 GHz-104.3 GHz, where the maximum phase shift is 249° at 103 GHz. The proposed work provides a new regulation concept for the implementation of LC-based terahertz devices and the respective applications in the terahertz reconfigurable antennas field.

Rapid terahertz wave manipulation in a liquid-crystal-integrated metasurface structure.

A terahertz phase shifter based on liquid-crystal-integrated metasurface is proposed, which contains a three-slotted array structure and comb grating. The orientation of the liquid crystal molecules can be completely controlled by the direction of the electric field. From the acquired experimental results, it was demonstrated that the phase shift exceeds 300° in the range of 378.6 - 390.8 GHz, whereas the maximum phase shift reaches 374.1° at 383.1 GHz. The molecular reorientation transient process induced by the external electric field in the liquid crystal was measured and analyzed. Based on the molecular reorientation mechanism, which can be divided into three processes, a rapid modulation mechanism was demonstrated. From the performance of the proposed device, an actively controllable phase delay and reflectance with a cycle switching time of approximately 0.3 s was achieved, which is remarkably faster than the usual cycle time that exceeds 8 s. Our work provides useful ideas for improving the response speed of LC-based terahertz devices, which is considered of great significance for several applications, in terms of terahertz reconfigurable devices.

Electrically active control of terahertz waves in a hybrid metasurface-grating structure with a liquid crystal.

A reflective terahertz phase shifter for wide-range dynamic and continuous phase modulation is proposed. By injecting a tunable liquid crystal between the slotted metasurface and the grating microstructure, the phases of reflected waves can be modulated with different electrically driven methods. Numerical simulations show that the proposed terahertz phase shifter has a phase difference of more than 360° between unbiased and biased states. Furthermore, an array of 35×35 patch elements was designed and fabricated. The performance of the phase shifter provides more than 360° between 379.6 and 391.8 GHz, where the maximum phase shift reaches 422.4° at 385.9 GHz. Moreover, the fully electrically controlled phase modulation of more than 180° is achieved between 382.0 and 394.1 GHz, with a maximum phase modulation of 248.4° at 383.3 GHz. This work may provide a reflective terahertz phase modulator for beam steering.

Highly efficient and tunable terahertz polarization converter based on double subwavelength metallic gratings infiltrated with liquid crystal.

In this work, a tunable cross-polarized transmission structure at the terahertz frequency was demonstrated, and the polarization state during modulation was investigated. The proposed structure can significantly enhance the polarization conversion performance of nematic liquid crystals by leveraging the formation of a Fabry-Perot-like resonant cavity that consists of two metal gratings. As a result, the incident waves are continuously reflected in the liquid crystal layer to complete the accumulation of polarization angle changes. From the experimental results acquired, it was concluded that the insertion loss of the cross-polarization transmission was less than 3 dB and the extinction ratio was larger than 28 dB in the frequency range of 388-426 GHz. Our work provides useful insights for improving the efficiency of cross-polarization conversion by enhancing the resonance process in a Fabry-Perot-like resonant cavity and, thus, significantly extending the equivalent optical path.

TM-polarized angle-dispersive metasurface for axisymmetric extension of beam steering angles.

Axisymmetric extension of beam steering angles by metasurfaces is of great interest due to its potential application in extending the angular scan range of existing phased arrays. This angle-multiplexed manipulation functionality requires anti-symmetrically angle-dispersive phase gradient, as well as anti-symmetric angular phase dispersion over continuous incident angles, which is difficult to be implemented by existing metasurfaces. In this work, a series of meta-atoms with asymmetric structures are developed to achieve the required phase response. Based on the asymmetric meta-atoms, an angle-dispersive metasurface for axisymmetric beam steering of transmitted transverse-magnetic (TM) wave is designed, fabricated and tested, whose simulation and experimental results demonstrate the axisymmetric extension capability of beam steering angles from -53° ≤ θi ≤ 53° to almost -90° ≤ θt ≤ 90°.

Enhanced broadband absorption with a twisted multilayer metal-dielectric stacking metamaterial.

This study proposes and experimentally demonstrates enhanced broadband absorption with twisted multilayer metal-dielectric stacking. Compared with the traditional metal-dielectric pyramid, the resonance frequencies of the third-order magnetic resonances in the twisted quadrangular frustum redshifted obviously. Hence, the proposed structure enables an ultra-broadband absorption by combining the third-order magnetic resonances with the fundamental mode. The broadband absorption is insensitive to the incident wave polarization, whereas the twisted angle of the stacking plays an important role in deciding the absorption bandwidth. The sample was fabricated via the multi-material hybrid micro-droplet jetting modeling (MHMJM) technology to verify the enhanced absorbing performance. The measured results suggest that the proposed strategy provides a potential path to realize broadband electromagnetic wave absorption. Moreover, it is possible to extend the twisted metamaterial to the terahertz and infrared frequencies using the advanced nano fabrication techniques.

An Ultrathin, Triple-Band Metamaterial Absorber with Wide-Incident-Angle Stability for Conformal Applications at X and Ku Frequency Band.

An ultrathin and flexible metamaterial absorber (MA) with triple absorption peaks is presented in this paper. The proposed absorber has been designed in such a way that three absorption peaks are located at 8.5, 13.5, and 17 GHz (X and Ku bands) with absorption of 99.9%, 99.5%, and 99.9%, respectively. The proposed structure is only 0.4 mm thick, which is approximately 1/88, 1/55, and 1/44 for the respective free space wavelengths of absorption frequency in various bands. The MA is also insensitive due to its symmetric geometry. In addition, the proposed structure exhibits minimum 86% absorption (TE incidence) within 60° angle of incidence. For TM incidence, the proposed absorber exhibits more than 99% absorptivity up to 60° incidence. Surface current and electric field distributions were investigated to analyze the mechanism governing absorption. Parameter analyses were performed for absorption optimization. Moreover, the performance of the MA was experimentally demonstrated in free space on a sample under test with 20 × 30 unit cells fabricated on a flexible dielectric. Under normal incidence, the fabricated MA exhibits near perfect absorption at each absorption peak for all polarization angles, and the experimental results were found to be consistent with simulation results. Due to its advantages of high-efficiency absorption over a broad range of incidence angles, the proposed absorber can be used in energy harvesting and electromagnetic shielding.

Comb-Shaped Graphene Nanoribbon Bandpass Filter.

In this study, a novel comb-shaped graphene nanoribbon wideband bandpass filter for use at midinfrared frequencies is proposed. In addition, numerical investigation was carried outwith finite difference time-domain (FDTD) numerical simulations. The filter includes one graphene nanoribbon (GNR) waveguide laterally coupled to six perpendicular GNRs on each side. With a simple geometric shape, the transmission bandwidth and efficiency of waves within the structure can be tuned by altering the width or length of the GNR teeth. Moreover, the transmission spectra can be easily tuned within a broad frequency region by tuning the chemical potential of the graphene teeth, thanks to the electronic tunability of monolayer graphene. This work offers a promising method for developing ultra-compact tunable graphene devices and for designing integrated all-optical architectures.

Graphene-based wavelength demultiplexing structure.

A wavelength demultiplexing (WDM) structure based on graphene nanoribbon resonators is proposed and simulated using the finite-difference time-domain (FDTD) method. Based on a simple structure, the demultiplexing wavelength and transmission characteristics of the WDM can be tuned by adjusting the length of the resonator, the nanoribbon width, or the chemical potential of graphene within a relative broadband frequency range. Moreover, the mechanism of the proposed WDM structure is analyzed in detail using the theory of Fabry-Perot (F-P) resonance and temporal coupled-mode theory. The proposed structure has promising potential in the field of ultracompact WDM systems in highly integrated optical circuits.

Electrically tunable liquid crystal terahertz device based on double-layer plasmonic metamaterial.

In this paper, a nematic liquid crystal (NLC)-based tunable terahertz (THz) plasmonic metamaterials (MMs) with large modulation depth (MD) and low insertion loss (IL) is designed and experimentally verified at THz frequencies. The proposed structure includes two-layered MM that is immersed in LC. The metal MM is used directly as electrode. The tunable device with a 46×46 array of sub-wavelength circular air loops was fabricated on a quartz glass substrate, with 2×2 cm2 area and 220 µm thickness. The obtained results show that the amplitude MD and IL for normally incident electromagnetic (EM) waves are about 96% and 1.19 dB at 421.2 GHz, respectively, when the bias voltage applied to the NLC layer varies from 0 to 16 V. Meanwhile, the transmission peak frequency gradually decreases from 421.2 to 381.8 GHz, and the frequency tunability (FT) of the proposed structure is greater than 9.35%. This study provides a potential solution for THz modulators, filters, and switches.

Tunable multi-wavelength absorption in mid-IR region based on a hybrid patterned graphene-hBN structure.

In this paper, we present a patterned graphene-hBN metamaterial structure and theoretically demonstrate the tunable multi-wavelength absorption within the hybrid structure. The simulation results show that the hybrid plasmon-phonon polariton modes originate from the coupling between plasmon polaritons in graphene and phonons in hBN, which are responsible for the triple-band absorption. By varying the Fermi level of graphene patterns, the absorption peaks can be tuned dynamically and continuously, and the surface plasmon-phonon polariton modes in the proposed structure enable high absorption and wideband tunability. In addition, how different structural parameters affect the absorption spectra is discussed. This work provides us a new method for the control and enhancement of plasmon-phonon polariton interactions.

Antireflection self-reference method based on ultrathin metallic nanofilms for improving terahertz reflection spectroscopy.

We present the potential of an antireflection self-reference method based on ultra-thin tantalum nitride (TaN) nanofilms for improving terahertz (THz) reflection spectroscopy. The antireflection self-reference method is proposed to eliminate mutual interference caused by unwanted reflections, which significantly interferes with the important reflection from the actual sample in THz reflection measurement. The antireflection self-reference model was investigated using a wave-impedance matching approach, and the theoretical model was verified in experimental studies. We experimentally demonstrated this antireflection self-reference method can completely eliminate the effect of mutual interference, accurately recover the actual sample's reflection and improve THz reflection spectroscopy. Our method paves the way to implement a straightforward, accurate and efficient approach to investigate THz properties of the liquids and biological samples.

Wideband Metamaterial Absorbers Based on Conductive Plastic with Additive Manufacturing Technology.

This paper proposes a wideband and polarization-insensitive metamaterial absorber (MA) based on tractable conductive plastic, which is compatible with an additive manufacturing technology. We provide the design, fabrication, and measurement result of the proposed absorber and investigate its absorption principle. The performance characteristics of the structure are demonstrated numerically and experimentally. The simulation results indicate that the absorption of this absorber is greater than 90% in the frequency range of 16.3-54.3 GHz, corresponding to the relative absorption bandwidth of 108%, where a high absorption rate is achieved. Most importantly, this additive manufactured structure provides a new way for the design and fabrication of wideband MAs.

Broadband terahertz metamaterial absorber based on tantalum nitride.

A broadband metamaterial absorber with a single layer of tantalum nitride (Ta2N3) frequency selective surface layer, printed on a foam substrate, is presented. The proposed design has been numerically examined at the terahertz region. The results have shown that a wideband absorption with absorptivity greater than 90% was achieved in the frequency range 1.17-2.99 THz, and the relative absorption bandwidth was up to 112.97%, which is significantly better than previously reported results. Moreover, the absorber was independent of wave polarization, and a high absorption for a wide range of oblique incidence was achieved. The surface current distribution, the electric field distributions, and the power loss analyses were used to explain the physical mechanism of a wideband absorption. However, the tantalum nitride layer has an important role in the energy absorption. According to the obtained results, the proposed absorber, which is compact and simple to design, has a potential application in evolving broadband terahertz absorbers and sensors.