Terahertz Communication Applications Research Articles

Easy-to-design wide stop-band multi-layer metamaterial filter based on symmetrical modified circular resonators for terahertz communication applications and electromagnetic interference shielding

Terahertz filters represent an essential element in modern communications system engineering. Designing a stopband filter with unique electrical qualities remains a relatively complicated task to achieve. In this context, the present paper suggests an easy-to-design bandstop filter to exploit it in terahertz communications and electromagnetic interference (EMI) shielding. The proposed filter is based on the multi-layer metamaterials stacking technique; it is constituted by a five-layer metal-insulator-metal-insulator-metal (MIMIM) microstructure. The two dielectric layers in polyimide with the three metallic layers in gold represent a total thickness of 43 μm. Each metallic layer is represented by a modified metamaterial resonator (MTMR) of the same symmetrical circular shape. The filter cell dimensions are optimized as 120 × 120 μm2 to have the desired stopband feature. The electrical qualities of the proposed filter have been analyzed by the finite element method (FEM) via the ANSYS high-frequency structure simulator software (HFSS). Through the simulation outcomes, salient filter features were highlighted. A wide −20 dB bandwidth of the order of 1.062 THz centered at 1.872 THz with a low transmission rate in the stopband which is less than 0.87 %, a significant square ratio of 0.86 and a relative bandwidth of around 57 %. Moreover, the absorption rate of the stopband has reached more than 97 % with excellent flat-top characteristics. To give more credibility to our results, we have presented a parametric analysis that included several components. In addition, the designed filter was insensitive to the polarization angle thanks to the symmetrical metamaterial resonators. Based on all these results and due to the simplicity of its design combined with all these unique features, our filter can be a prominent contender in the field of THz communications. It can be also required for terahertz imaging and EMI shielding.

Terahertz bandpass and bandstop filter based on the babinet complementary metamaterials

In this study, we experimentally investigate the terahertz (THz) bandpass and bandstop filters based on the elliptical Babinet complementary structure, including an elliptical hole structure (EHS) and an elliptical ring structure (ERS). The experimental results indicate that the monolayer structure exhibits an obvious polarization selectivity with consistent filtering curves under TE and TM mode excitation, but the spectral line has an apparent shift of 0.1 THz. Based on Babinet's principle, the filtering mode matching is experimentally verified between bandpass and bandstop under a 90° change in polarization. Furthermore, we propose a double-layer filter by integrating EHS and ERS on both sides of the silicon substrate, and realize the bandpass and bandstop filtering simultaneously. The simulation results show that the isolation degree of bandpass and bandstop filtering has a maximum of nearly 40 dB based on the angle-tunable filtering performance, and the Q-value of the filter is further improved than that of the single-layer structure. The filter designed based on the Babinet principle has the advantages of simple structure, easy processing, and sharper filtering effect, and shows great applications in THz communication, imaging, and sensing systems.

Polarization-multiplexing graphene-based coding metasurface for flexible terahertz wavefront control

In terahertz wireless communication systems, flexible wavefront control devices based on various structure metasurfaces have attracted enormous attention for next-generation communication. In general, tunable terahertz metasurfaces integrated with active materials or MEMS technologies are used for dynamic wavefront control. However, most existing metasurfaces suffer from various limitations, including intrinsic properties of active materials, low reliability of MEMS technologies, and single polarization mode of incident waves, which hinders their development and application. To address these challenges, herein, we design two types of reflective graphene-based coding metasurfaces for active wavefront control. The metasurface coding meta-atom is composed of a graphene split-ring resonator, a dielectric layer, and a metal ground plane. By simply rotating the coding meta-atom, independent 2π phase coverage for circularly polarized (CP) or linearly polarized (LP) illumination can be achieved, enabling polarization multiplexing. Thus, a metasurface (MS-1) is constructed based on the vortex phase profile to generate different wavefronts. Moreover, these wavefronts can be actively switched between a vortex beam, a multi-beam, and a specular reflection beam by altering the polarization mode of the incident waves and the Fermi level of the graphene coding regions Additionally, another metasurface (MS-2) is developed according to the parabolic phase profile to create a tunable metalens that allows active control over focal intensity and depth by adjusting the Fermi level of graphene. Such wavefront-controlled metasurfaces have high capacity and integration, making them very promising for potential applications in terahertz communication and imaging systems.

Terahertz angle sensor based on the asymmetry coupling of the square and L-shaped structure

This letter presents a terahertz angle sensor using a rectangular and double L-shaped structure. The structure unit of the sensor consists of a typical metal-medium structure, with the top metal pattern comprising a rectangle and a double L-shaped structure. At the same time, the bottom layer is made of polyimide, a dielectric material. By varying the position and number of L-shaped structures, a terahertz angle sensor based on a spring-shaped structure is created. The terahertz angle sensor achieves a Q value of 120.6 with a sensitivity of 3.45 GHz per degree. The terahertz angle sensor offers high-angle resolution and may find applications in terahertz communication, imaging, sensing, and other related fields.

All-Silicon Polarization-Insensitive Metamaterial Absorber in the Terahertz Range.

All-silicon terahertz absorbers have attracted considerable interest. We present a design and numerical study of an all-silicon polarization-insensitive terahertz metamaterial absorber. The meta-atoms of the metamaterial absorber are square silicon rings which can be viewed as gratings. By properly optimizing the structure of the meta-atom, we achieve a broadband absorptivity that is above 90% ranging from 0.77 THz to 2.53 THz, with a relative bandwidth of 106.7%. Impedance matching reduces the reflection of the terahertz waves and the (0, ±1)-order diffraction induce the strong absorption. The absorption of this absorber is insensitive to the polarization of the terahertz wave and has a large incident angle tolerance of up to 60 degrees. The all-silicon metamaterial absorber proposed here provides an effective way to obtain broadband absorption in the terahertz regime. Metamaterial absorbers have outstanding applications in terahertz communication and imaging.

Photonic time-crystalline behaviour mediated by phonon squeezing in Ta2NiSe5.

Photonic time crystals refer to materials whose dielectric properties are periodic in time, analogous to a photonic crystal whose dielectric properties is periodic in space. Here, we theoretically investigate photonic time-crystalline behaviour initiated by optical excitation above the electronic gap of the excitonic insulator candidate Ta2NiSe5. We show that after electron photoexcitation, electron-phonon coupling leads to an unconventional squeezed phonon state, characterised by periodic oscillations of phonon fluctuations. Squeezing oscillations lead to photonic time crystalline behaviour. The key signature of the photonic time crystalline behaviour is terahertz (THz) amplification of reflectivity in a narrow frequency band. The theory is supported by experimental results on Ta2NiSe5 where photoexcitation with short pulses leads to enhanced THz reflectivity with the predicted features. We explain the key mechanism leading to THz amplification in terms of a simplified electron-phonon Hamiltonian motivated by ab-initio DFT calculations. Our theory suggests that the pumped Ta2NiSe5 is a gain medium, demonstrating that squeezed phonon noise may be used to create THz amplifiers in THz communication applications.

Microstructure and dielectric properties of low-temperature sintered MgO-based ceramics at millimeter wave and terahertz frequencies

Low-temperature co-fired ceramics (LTCC) applied in millimeter/microwave and terahertz frequencies (5G/6G) have attracted a lot of attention recently. In this study, MgO-based dielectric ceramics were successfully sintered at 950∘C with the sintering aids: x wt.% of LiF fluoride ([Formula: see text], 4, 6, 8, 10) and 0.5[Formula: see text]wt.% of BBSZ (Bi2O3–B2O3–SiO2–ZnO) glass. BBSZ glass was introduced as another sintering aid to facilitate the sintering and densification. Crystalline structure and micro-morphology were investigated and analyzed. Dielectric properties ([Formula: see text], [Formula: see text], [Formula: see text]) at millimeter/microwave and terahertz wave frequencies were also studied. The ionic characteristics of Mg–O bond ([Formula: see text]), the lattice energy (U) and the bond energy (E) were calculated and analyzed. It is suggested that the optimal [Formula: see text], where [Formula: see text], [Formula: see text][Formula: see text]GHz (@12[Formula: see text]GHz) and [Formula: see text][Formula: see text]ppm/∘C at millimeter/microwave range. When the frequency was up to terahertz (1.0[Formula: see text]THz), the [Formula: see text] values were 8.8–9.35 and the tan[Formula: see text] were [Formula: see text]–[Formula: see text]. The experimental results indicated that the low-temperature sintered MgO-based ceramics have potential for millimeter/microwave and terahertz communication applications.

Dirac semimetal-enabled multi-bit coding metasurface for dynamic manipulation of terahertz beams

In this study, a switchable multi-bit coding metasurface that is applied under a terahertz (THz) frequency by adjusting the Fermi level (EF) of Dirac semimetals (DSMs) is proposed. At a EF of 0.2 eV, a 1-bit coding metasurface can be applied in the 2.58–2.62 THz. At 0.3 eV, a 3-bit coding metasurface is realized at 1.88 THz, and at 0.05 eV, the phase of the coding units coincides in the 1.5–3 THz. So, different functions of the metasurface can be realized. The proposed coding metasurfaces has promising applications in terahertz communication.

4096-QAM OFDM THz-Over-Fiber MIMO Transmission Using Delta-Sigma Modulation

A terahertz (THz)-over-fiber multiple-input multiple-output (MIMO) transmission of 4096-ary quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) supported by 46-GBaud quadrature phase shift keying (QPSK) signal at 320 GHz is experimentally demonstrated in this letter. By employing the polarization division multiplexing (PDM) technique and 1-bit delta-sigma modulation (DSM) quantization, we successfully implement 4.6-GHz 4096-QAM OFDM signal transmission over 20 km of standard single-mode fiber (SSMF) and a 3-m MIMO wireless link, and satisfy the soft-decision forward error correction (SD-FEC) threshold of 1.0 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> with 15% overhead, which corresponds to a net rate of 84.4 Gbps for THz-over-fiber transmission. To the best of our knowledge, fusing OFDM and PDM, this is the highest rate achievable for DSM applications in THz communication. This experimental demonstration can provide a promising solution for high-order QAM signals in a fiber-wireless seamless network in future 6G networks.

65,536-QAM OFDM signal transmission over a fiber-THz system at 320 GHz with delta-sigma modulation.

The transmission of a 65,536-ary quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) signal supported by a hybrid fiber-terahertz (THz) multiple-input multiple-output (MIMO) system at 320 GHz is experimentally demonstrated in this Letter. We adopt the polarization division multiplexing (PDM) technique to double the spectral efficiency. Based on a 23-GBaud 16-QAM link, 2-bit delta-sigma modulation (DSM) quantization enables 65,536-QAM OFDM signal transmission over a 20-km standard single-mode fiber (SSMF) and a 3-m 2 × 2 MIMO wireless delivery, and satisfies the hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10-3, corresponding to a net rate of 60.5 Gbit/s for THz-over-fiber transport. Meanwhile, below the fronthaul error vector magnitude (EVM) threshold of 0.34%, a maximum signal-to-noise ratio (SNR) of 52.6 dB is achieved. To the best of our knowledge, this is the highest modulation order achievable for DSM applications in THz communication.

Reconstructing Polarization Multiplexing Terahertz Holographic Images with Transmissive Metasurface

There is a growing trend towards the development of high resolution and multiplexing metasurface holograms. In this paper, we propose the reconstruction of polarization multiplexing terahertz (THz) holographic images based on transmissive metasurface. The metasurface composed of all-dielectric meta-atoms is designed as a multi-foci metalens and the focal points of the metalens are utilized as the pixels of a reconstructed image. We analyze the effects of focal length and phase pixel number of the metalens on focal point to achieve high-resolution holographic images. In addition, by switching the polarization of incident lights, holographic images with different patterns are reconstructed on its focal plane. Such high-resolution and polarization multiplexing metasurface holograms is promising for applications in THz communications, information engineering, and encryption.

Liquid Crystal-Tuned Planar Optics in Terahertz Range

Recently, terahertz waves of higher frequencies compared to microwave and radio frequency have shown great potential in radar detection and high-speed wireless communication. To spatially control the wavefront of terahertz beams, various novel components, such as terahertz filters, polarization converters and lenses, have been investigated. Metamaterials and metasurfaces have become the most promising technique for the free manipulation of terahertz waves. Metadevices integrated with liquid crystals have been widely used in active terahertz devices. In this review, the birefringence of liquid crystals in the terahertz band and terahertz devices based on liquid crystals are summarized. By integrating liquid crystals with plasmonic metamaterials, the functions become dynamically adjustable and are reconstructed. Utilizing liquid crystals to change the resonance of metamaterials, tunable filters, absorbers, and programmable metasurfaces are realized. To solve the problem of low efficiency, terahertz wavefront shaping devices based on dielectric metasurfaces and liquid crystals, such as a variable deflection angle grating and zoom metalenses, are presented. Finally, we discuss and anticipate the future developments of liquid-crystal-integrated meta-devices, which will inspire broad applications in terahertz communication and imaging.

Multifunctional transmission polarization conversion metasurface based on dislocation-induced anisotropy at the terahertz frequency.

Manipulating the polarization state of terahertz waves is critical for terahertz communication systems. This study proposes a terahertz band polarization conversion metasurface based on dislocation-induced anisotropy. Numerical simulation results revealed that the polarization conversion of orthogonal linearly polarized light, orthogonal circularly polarized light, linearly polarized light to circularly polarized light, and circularly polarized light to linearly polarized light can be realized. Furthermore, the simulation revealed that multifunctional polarization conversion could be achieved by various structures of the bilayer metasurface. Thus, the proposed design can be generalized. The proposed metasurface exhibits considerable potential for applications in terahertz communications.

A Circular Split Ring Resonator Absorber with Graphene Material for Terahertz Communication Applications

A Circular Split Ring Resonator Absorber with Graphene Material for Terahertz Communication Applications

All-Dielectric Tunable Terahertz Metagrating for Diffraction Control.

Recently, tunable metagratings have attracted substantial attention in manipulating the diffraction of electromagnetic waves with considerable flexibility, but they are usually limited to inherent ohmic loss due to the metal layers. The all-dielectric schemes can address this issue, but its design and optimization remain challenging in the terahertz regime, especially in the 6G communication window. In this work, an all-dielectric tunable terahertz metagrating is demonstrated in theoretical and experimental investigations. The metagrating operating in the 6G communication window bends the electromagnetic waves beam into the T-1 diffraction order by optimizing the unit cell. In the experiments, more than 72.46% of the transmitted energy is concentrated in the desired diffraction order for p-polarized light and more than 66.60% for s-polarized light, which agrees well with the theoretical design. The tunability by angular deflection is reported in this all-dielectric metagrating. Then, based on the all-dielectric metagrating arrays, a metalens with numerical aperture of NA = 0.39 at 0.14 THz is demonstrated. The subwavelength scale focal spot is obtained as 2.0 mm × 2.0 mm with the focusing distance of 117.8 mm. Imaging capability of the metalens is performed utilizing the transmission imaging manner. The measured and anticipated results are satisfactorily congruous with one another, which could validate our design. This work paves the way toward designing highly efficient and tunable devices with potential applications in terahertz communications, sensors, and super-resolution imaging.

Electrically addressable integrated intelligent terahertz metasurface.

Reconfigurable intelligent surfaces (RISs) play an essential role in various applications, such as next-generation communication, uncrewed vehicles, and vital sign recognizers. However, in the terahertz (THz) region, the development of RISs is limited because of lacking tunable phase shifters and low-cost sensors. Here, we developed an integrated self-adaptive metasurface (SAM) with THz wave detection and modulation capabilities based on the phase change material. By applying various coding sequences, the metasurface could deflect THz beams over an angle range of 42.8°. We established a software-defined sensing reaction system for intelligent THz wave manipulation. In the system, the SAM self-adaptively adjusted the THz beam deflection angle and stabilized the reflected power in response to the detected signal without human intervention, showing vast potential in eliminating coverage dead zones and other applications in THz communication. Our programmable controlled SAM creates a platform for intelligent electromagnetic information processing in the THz regime.

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.

High-efficiency reflective metasurfaces for terahertz vortex wave generation based on completely independent geometric phase modulations at three frequencies

One of the considerable disadvantages of a metasurface (MS) is frequency-dependent behavior, which largely restricts its practical applications. To overcome this challenge, a MS is designed to generate vortex beams at three distinct terahertz (THz) frequencies. Specifically, a unit-cell structure comprising three resonators is proposed to operate at three distinct THz frequencies. The cross-polarized reflection coefficients of the unit-cell structure are greater than 70% under circularly polarized (CP) incidence. Based on the geometric phase principle, the full 2 π phase shift can be obtained at each frequency by rotating the corresponding orientation angles of the three resonators. By carefully arranging the unit-cell structure, orbital angular momenta (OAMs) with topological charges of l = ± 1 , − 2 , and, − 3 at 0.706 THz, 1.143 THz, and 1.82 THz, respectively, can be generated for a normally incident right CP wave, and OAMs with topological charges of l = − 3 , − 2 , and − 1 can be generated for a normally incident left CP wave. The generated reflective vortex beams through the designed MS have good mode purity up to 85% at 0.706 THz, 84% at 1.143 THz, and 74.7% at 1.82 THz, respectively. Moreover, the designed reflective MS reveals a convenient and low-cost way to generate vortex beams with different/same OAM modes at three different resonance frequencies and is beneficial for potential applications in THz communication.

Terahertz Metagrating Accordion for Dynamic Beam Steering

AbstractMetasurfaces can be used to manipulate the beam direction beyond the Snell's law by the constitutive gradient elements. Independent control of the phase response of each element is usually required for real‐time beam steering, which is very challenging at terahertz (THz) frequencies. Here a type of accordion‐shaped single‐element metagratings for THz beam scanning and angle‐adaptive retroreflection is proposed. The metagrating is carefully designed to enhance the +1st order diffraction while suppressing other orders for efficient beam steering. Benefiting from the periodic arrangement, dynamic beam steering is realized by imparting desired momentum via mechanical compression and rotation, instead of the complicated element‐by‐element modulation. Beam scanning in a plane and over the 3D space is experimentally validated with elevation angles from 45° to 75° and azimuthal angles over 360°. An angle‐adaptive retroreflector by adding a metallic plate to the metagrating is studied with near perfect theoretical diffraction efficiency for incident angles within 40°–85°. The physical mechanism of wide‐angle high efficiency is detailed by the mode analysis, and can be generalized into other frequencies. The study here may find applications in THz communications and radar detections, and open a new avenue to facilitate reconfigurable metadevices for wavefront modulation.

Reconstructing subwavelength resolution terahertz holographic images.

Computer-generated holography typically generates terahertz (THz) holographic images with a pixel size larger than wavelength. We propose a multi-foci metalens model to reconstruct THz holographic images with subwavelength resolution. The designed devices are realized based on dielectric metasurfaces consisting of silicon micropillars with spatially variant orientations. By exploiting quasi-continuous profile of focal points as the pixels of a holographic image, a metalens can reconstruct a high-resolution target image on its focal plane. The effects of size and pitch of each sub-diffraction focal point on imaging quality and pixel resolution are discussed. The intensity distribution at each focal point indicates that the reconstructed images have subwavelength resolution. In comparison with conventional hologram designs, this design method can be used to reconstruct THz holographic images with subwavelength resolution, which have potential applications in THz communication, information security and anti-counterfeiting.