Tunable broadband THz absorber using vanadium dioxide metamaterials

Tunable broadband terahertz absorber based on graphene metamaterials and VO2

With the fast development of terahertz (THz) techniques, THz absorbers have a variety of applications. However, current designs have a series of shortcomings, such as low absorption rate and a fixed and narrow absorption bandwidth. To solve such problems, in this paper, we design a tunable broadband THz absorber, which consists of two layers of graphene structures: the top layer is arrayed with the graphene concentric hexagonal rings and the bottom layer is arrayed with regular graphene hexagon rings; therefore, both layers support hybridization and stacking effects. The coexistence of both effects achieves the absorption rate of over 90% in a broad band from 0.93 THz to 1.80 THz. Moreover, the absorption spectrum can be tuned by adjusting the graphene chemical potential, which is insensitive to both incident angle and polarization. Considering advantages such as a tunable broad absorption band, a high absorption rate and insensitive incident angle and polarization, the proposed dual-layer graphene based tunable broadband THz absorber can be a useful reference for absorber design even in other electromagnetic wavebands.

Actively tunable ultrafast broadband terahertz (THz) polarimetry using a reconfigurable phase‐change material holds great potentials and prospects for the achievement of next‐generation versatile integrated THz components and systems in a variety of THz applications. Here, an optically tunable ultrafast broadband THz polarimetric device (THz‐PoD) composed of a phase‐change material Ge2Sb2Te5 (GST) and a thin mica substrate is demonstrated. This proposed novel THz‐PoD is verified for a frequency range of 0.1–2.5 THz, exhibiting broadband and ultrafast determination of polarization states for linearly polarized THz waves at polarization angles from −90° to 90°. It is shown that optical excitation with ns pulses allows easy and efficient control of the polarimetric properties of such THz‐PoD. The essential role of the GST film in switching the phase transition between the amorphous and crystalline phases is emphasized by the theoretical investigation of the optically tunable ultrafast polarimetric mechanism of the device. This phase transition allows optically changing the THz‐PoD's properties by ns‐pulsed laser in a controlled way to achieve THz polarimetry for linearly polarized THz waves. The combined advantages of this strategy can open up a new and promising way for realizing versatile reconfigurable and integrated THz devices, which may further promote the development of novel THz systems and applications.

A high-performance broadband terahertz absorber based on single layer cross-shaped graphene

A polarization-insensitive and broadband terahertz (THz) absorber based on metamaterial (MM) is presented. The absorber consists of two layers of metal and a single layer of medium. Each periodic cell of the upper metallic layer consists of five different sizes of metal patches which form a square array of 55. In the array, the size of each metal patch is different from that of its adjacent one, and each size of the metal patch generates a single resonance absorption peak. The broadband absorption is actually produced by the overlapping of five adjacent resonance absorption peaks. By studying the distribution of the surface current and the z-component of electric field, it is easy to know that the energy of the incident THz wave is absorbed by two factors: one is the electric dipole oscillation caused by the electric field in the y direction, and the other is the magnetic polariton caused by the magnetic field in the z direction. And the ohmic loss of metal layers plays a major role on the absorption of the absorber. Simulation results show that the bandwidth achieves 1.2 THz for the absorption beyond 80%, and the maximum absorption is up to 98.7%. It's full width at half maximum (FWHM) is 1.6 THz, and the thickness of the broadband absorber is only about one twentieth of the center wavelength. In addition, the absorber is insensitive to the polarization and has a wide-angle feature, and the potential applications of the absorber are electromagnetic stealth, THz thermal radiation detectors, and THz communication.

A tunable bifunctional terahertz (THz) metamaterial device based on Dirac semimetal films (DSFs) and VO2 is presented. The insulator-to-metal phase transition of VO2 enables bifunctional asymmetric transmission and dual-directional absorption to be switched in the THz range. When VO2 serves as a dielectric, tunable broadband asymmetric transmission of linearly polarized THz waves can be achieved. When VO2 is in a metallic state, the proposed device acts as a tunable dual-directional absorber with perfect absorption in both illumination directions. In each case, the response can be tuned by varying the Fermi energy of the DSFs. This offers a new pathway for the development of tunable multifunctional THz metamaterial devices.

Terahertz (THz) gas photonics uses gas as THz emitter and sensor for time-domain spectroscopy. Unique properties of the gas promise scalable, strong THz wave generation with broad spectral range covering the entire THz gas (0.3 THz to 35 THz). The systematic study of THz wave generation and detection in different gases shows that the generation efficiency is monotonically decreasing with the ionization potential of the gas molecules while the detection efficiency is linearly proportional to the third order nonlinear coefficient of the gas molecules. We also discuss the development of THz wave detection using laser-induced fluorescence and coherent control with THz gas photonics.

Multi-band terahertz (THz) absorbers have recently gained attention due to their favorable application prospects in communication, imaging, detection, and other fields. However, many multi-band THz absorbers are tuned by a single method, which limits their tuning effect. To address this issue, we propose a multi-band THz absorber that can be co-modulated by thermal and electrical methods. Our proposed absorber uses vanadium dioxide (VO2) to achieve this co-modulation. When VO2 is insulating, the frequency of the absorbing peaks originating from the lateral Fabry-Pérot resonance mode can be changed by adjusting the VO2 width. When VO2 is a conductor, the quality factor of the absorbing peak based on the inductor-capacitor resonance mode can be tuned by adjusting the width of VO2. By varying the top dielectric layer thickness, the frequency of the absorbing peaks can be tuned over a wide range. For devices with two or three layers of graphene nanoribbons-dielectric stacks, a modulation effect similar to that of varying dielectric layer thickness in a single-layer graphene device can be achieved simply by applying a 1 eV Fermi energy to graphene nanoribbons in different layers. By combining thermal and electrical modulation, the two or three-layer stacked device can be dynamically switched between four or six absorbing states, and a wider range of dynamic peak frequency modulation can be realized. Furthermore, the performance of the absorber does not deteriorate significantly at an incident angle of up to 70°. Our proposed thermal-electrical switchable wide-angle multi-band THz absorber provides a reference for the design, fabrication, and application of high-performance THz absorbers in different fields.

Despite significant advancements in terahertz (THz) generation and detection technologies, the practical deployment of portable THz systems remains constrained by existing modulator limitations, particularly stringent temperature requirements, inefficient thermal management, and high power consumption. Here, we demonstrate a breakthrough in magnetically controlled THz modulation using a CoFe2O4/MgO/F-Mica composite structure. The tunable optical response and dielectric properties of the composite under varying magnetic fields were systematically investigated via THz time-domain spectroscopy. Experimental results revealed a magnetic field-dependent THz transmission attenuation, with systematic suppression of transmission spectra proportional to the applied magnetic field intensity. A theoretical model accounting for external magnetic field variations was proposed, which agrees well with the experimental results regarding the imaginary component. Remarkably, the composite material realizes phase modulation simultaneously under magnetic field. This exceptional magnetic responsiveness significantly broadens the application potential of ferrites in the THz regime. These findings provide critical insights for designing tunable multifunctional THz magnetic devices in 6G communications, medical diagnostics, and nondestructive testing applications.

Many previous VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based perfect metamaterial absorbers can enhance light-matter interaction when VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is in metal phase but with weak light-matter interaction when VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is in insulator phase. In this work, an actively tunable terahertz (THz) absorber with the ability to switch between dual-band absorption and single ultra-narrowband absorption is theoretically proposed and numerically demonstrated. The electric dipole resonance, localized surface plasmon resonance, Fabry-Perot resonance and surface lattice resonance are utilized to enhance light-matter interaction. The simulation results show that an absorption peak has an absorptance of 98.6% and the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -factor reaches 351.5 when vanadium dioxide (VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) is in the insulating phase. Then, two absorption peaks with the absorptance of 96.2% and 99.5% are presented at 2.30 THz and 3.31 THz, respectively, when VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is in the metallic phase. To better understand the physical mechanisms of this absorber, the influence of parameters is further investigated. The proposed absorber with the dynamically tunable characteristic between such distinct states can pave numerous promising applications in sensing, imaging, active switch and modulator.

Terahertz (THz) technology is becoming a spotlight of scientific interest due to its promising myriad applications including imaging, spectroscopy, industry control and communication. However, one of the major bottlenecks for advancing this field is due to lack of well-developed solid-state sources and detectors operating at THz gap which serves to mark the boundary between electronics and photonics. Here, we demonstrate exceptionally wide tunable terahertz plasma-wave excitation can be realized in the channel of micrometer-level graphene field effect transistors (FET). Owing to the intrinsic high propagation velocity of plasma waves (>~108 cm/s) and Dirac band structure, the plasma-wave graphene-FETs yield promising prospects for fast sensing, THz detection, etc. The results indicate that the multiple guide-wave resonances in the graphene sheets can lead to the deep sub-wavelength confinement of terahertz wave and with Q-factor orders of magnitude higher than that of conventional 2DEG system at room temperature. Rooted in this understanding, the performance trade-off among signal attenuation, broadband operation, on-chip integrability can be avoided in future THz smart photonic network system by merging photonics and electronics. The unique properties presented can open up the exciting routes to compact solid state tunable THz detectors, filters, and wide band subwavelength imaging based on the graphene-FETs.

We report the design, fabrication, and measurements of reconfigurable terahertz (THz) filters using vanadium dioxide (VO2) phase-change material (PCM), which undergoes a transition between an insulator and metal phase at 68 °C (∼341 K). The filters are made of frequency selective surfaces (FSS) and heater are integrated for VO2 excitation. Two THz spatial filters are developed. A broadband on/off filter at 0.35 THz is demonstrated with 20 dB change in transmission between the on and off states. Then, a reconfigurable stopband FSS filter with tunable rejection from 0.75 to 0.55 THz is presented. The latter shows a peak with rejection of more than 90%. The two states of both filters are achieved at relatively low temperature of 68 °C, making them suitable for practical applications. Comparisons between simulation and measurement show excellent agreement for both 0.35 THz on/off filter and 0.75 to 0.55 THz tunable filters.

The ever-growing number of interconnected devices and the emergence of high-bandwidth technologies such as holographic communications in mixed reality are driving an exponential growth in the demand for terabits-per-second (Tbps) data rates with sub-milliseconds latency. For this reason, the sixth-generation (6G) wireless communications are moving towards the terahertz (THz) frequency band because of its wider spectral bandwidth availability to enable higher channel capacity and data rates according to the Shannon-Hartley theorem. However, the efficiency of wireless THz signal transmission is reduced by the increased path loss and atmospheric attenuation due to water vapor absorption at THz frequencies, which significantly decreases the signal-to-noise ratio and the spectral efficiency of THz wireless communications. The realization of THz band communications is further impeded by the limited power output of existing compact THz sources due to the extremely low conversion efficiencies. Therefore, it is crucial to develop broadband THz integrated circuits with waveguiding and beamforming functionalities for supporting high data-rate on-chip communications and to provide sufficient gains to counteract the path loss and limited power of THz wireless transmissions. This thesis aims to improve the spectral efficiency of 6G wireless communications via the spatiotemporal shaping of THz waves using silicon photonics. Spatiotemporal light shaping is the basis of nearly all THz photonic devices, ranging from THz wavefront manipulation using metasurfaces to THz planar waveguiding using photonic crystal waveguides. Silicon photonics is a promising material platform for THz wave shaping due to the low-loss propagation of THz waves in high-resistivity silicon and the prospect of a monolithic integrated circuit due to the complementary metal-oxide semiconductor (CMOS) fabrication process compatibility. Silicon metasurfaces allow the wavefront modulation of incident THz radiation by exploiting the generalized Snell’s law of refraction and reflection to beamform highly directional THz waves in a dynamic multi-user communication environment for enhancing the spectral efficiency. Likewise, broadband silicon photonic crystal waveguides allow the in-plane and lossless transport of THz waves to support on-chip high data-rate THz communications that can also seamlessly integrate multiple THz photonic devices in a THz integrated circuit. In addition, silicon electron emitters and silicon gratings potentially allow the on-chip generation of highly intense and broadband THz radiation to counteract the significant THz attenuation. In this thesis, spatiotemporal wave shaping techniques using silicon photonics for enhancing the spectral efficiency of THz band communications is presented. First, a self-adaptive and deep reinforcement learning model for THz beamforming is proposed, which can beamform multiple highly directional THz radiation beam in real-time to serve a dynamically changing multi-user communication environment and improve the signal-to-noise ratio of THz wireless transmission while potentially reducing the communication latency. Second, a broadband THz topological silicon waveguide designed based on valley photonic crystals is proposed for the robust and lossless transport of broadband THz waves for the construction of THz integrated circuits with broadband channel capacity for enabling higher data-rate on-chip communication. Last, the versatility of free-electron radiation from silicon gratings is investigated for the generation of highly intense spatiotemporal wave packets with user-designed group velocities and tunable intensity profile, with the potential to realize integrated high-power THz sources. The deep learning beamforming model, the broadband topological waveguide designs, and the free-electron radiation intensity shaping techniques presented in this thesis extends beyond the THz regime and are scalable across the entire electromagnetic spectrum. The CMOS process compatibility of silicon metasurfaces, silicon planar waveguides, along with silicon electron emitters and silicon gratings offers the opportunity to realize a broadband THz photonic integrated circuit with waveguiding, wave generation and beamforming functionalities for enhancing the spectral efficiency of THz communications in 6G wireless networks.

The combination of VO2 and metasurfaces has opened an attractive route to dynamically control terahertz (THz) waves based on the giant conductivity change. However, the high-precision control of microfabrication and single performance of conductivity change limit the multifunctional application of VO2-based metasurfaces. Here, we proposed a VO2-metallic hybrid metasurface by in situ depositing high-quality VO2 thin films onto a metasurface composed of asymmetric Fano resonance units. It exhibits agile frequency and amplitude modulation for THz transmission across tuning the dielectric constant and conductivity of VO2. The metallic metasurface is designed as a matrix to achieve high transmission at 0.61 and 0.78 THz due to the split-ring resonance. During the thermally triggered phase transition of VO2, we found that the resonance frequency and amplitude can be tuned dominantly by the change of dielectric constant and conductivity, respectively. In particular, the increased dielectric constant enables red shift of the frequency by around 0.48 THz and the conductivity increases lead to a giant THz amplitude modulation of 88%. These results provide a route for developing VO2-based THz smart devices combined with functional metasurfaces and hold great promise for applications in THz sensor and modulation.

Active broadband terahertz (THz) polarization manipulation devices are challenging to realize, but also of great demand in broadband terahertz systems. Vanadium dioxide (VO2) shows a promising phase transition for active control of THz waves and provides broadband polarization characteristics when integrated within grating-type structures. We creatively combine a VO2-based grating structure with a total internal reflection (TIR) geometry providing a novel interaction mechanism between the electromagnetic waves and the device, to realize a powerful active broadband THz polarization-controlling device. The device is based on a Si-substrate coated with a VO2 layer and a metal grating structure on top, attached to a prism for generating the TIR condition on the Si-VO2-grating interface. The grating is connected to electrodes for electrically switching the VO2 between its insulating and conducting phases. By properly selecting the incident angle of the THz waves, the grating direction, and the incident polarization state, we first achieved a broadband intensity modulator under a fused silica prism with an average modulation depth of 99.75% in the 0.2-1.1 THz region. Additionally, we realized an active ultra-broadband quarter-wave converter under a Si prism that can be switched between a 45° linear rotator and a quarter wave converter in the 0.8-1.5 THz region. This is the first demonstration of an active quarter-wave converter with ultra-broad bandwidth performance. Our work shows a highly flexible and multifunctional polarization-controlling device for broadband THz applications.