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Generation and manipulation of terahertz (THz) waves are of vital importance to advance THz technology. Nonlinear metasurfaces allow effective integration of both processes into a single compact device. However, such existing THz devices commonly rely on utilizing a single meta-atom, which has fixed THz generation properties and thus limits the range of achievable functionalities. Here, for the first time to our knowledge, we introduce a coupling effect between different meta-atoms into the unit-cell design, and experimentally validate this new degree of freedom in controlling the nonlinear THz generation, where achiral coupling provides full control over the amplitude of the generated THz field, while chiral coupling makes the THz generation sensitive to the handedness of the pump polarization. In particular, the chiral coupling case further allows a multiplexing-based handedness-selective nonlinear metasurface, which generates THz beams with different orbital angular momenta (OAMs). This approach is promising in developing various integrated nonlinear THz devices.

Actively controlling the performance of surface-enhanced Raman scattering (SERS) holds great promise in enabling novel applications of this technique. As a popular two-dimensional material, graphene shows intriguing tunable properties and thus has become a promising material in many potential active device applications. Here we proposed and experimentally demonstrated low-voltage electrically tunable SERS by using a plasmonic gold nanorod-graphene/ion-gel hybrid structure. The SERS signals of molecules adsorbed on such a structure were found to be continuously tunable through an external small bias voltage. As such a tuning process is highly related with the charge transfer between the adsorbed molecules and graphene, the proposed design is also helpful in understanding the charge transfer-based enhancement mechanism of SERS.

In terahertz (THz) technologies, generation and manipulation of THz waves are two key processes usually implemented by different device modules. Integrating THz generation and manipulation into a single compact device will advance the applications of THz technologies in various fields. Here, we demonstrate a hybrid nonlinear plasmonic metasurface incorporating an epsilon-near-zero (ENZ) indium tin oxide (ITO) layer to seamlessly combine efficient generation and manipulation of THz waves across a wide frequency band. The coupling between the plasmonic resonance of the metasurface and the ENZ mode of the ITO thin film enhances the THz conversion efficiency by more than 4 orders of magnitude. Meanwhile, such a hybrid device is capable of shaping the polarization and wavefront of the emitted THz beam via the engineered nonlinear Pancharatnam-Berry (PB) phases of the plasmonic meta-atoms. The presented hybrid nonlinear metasurface opens a new avenue toward miniaturized integrated THz devices and systems with advanced functionalities.

Recent demonstrations of metalensing have realized the selective coupling from free-space light into surface plasmon focusing beams, providing great possibilities for on-chip integration of division multiplexing and light routing. However, the surface plasmon wavefront control strategies of these metalenses can only work for a limited number of polarizations and wavelengths, resulting in four or less multiplexing channels. In order to break through the limitations of current SP metalens designs, we propose a holographic method with which we demonstrate, both theoretically and experimentally, that several surface plasmon metalenses can couple and steer the incident circular polarizations of four wavelengths and two spin directions to high-quality surface plasmon focusing beams, yielding a novel eight-channel multiplexing. The focusing directions for different free-space incidences can be flexibly designated, either different or the same. The proposed design strategy is straightforward and versatile, which may open up new opportunities in the realms of integrated photonics, on-chip spectroscopy, or plasmonic demultiplexers.

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Actively controlling the polarization states of terahertz (THz) waves is essential for polarization-sensitive spectroscopy, which has various applications in anisotropy imaging, noncontact Hall measurement, and vibrational circular dichroism. In the THz regime, the lack of a polarization modulator hinders the development of this spectroscopy. We theoretically and experimentally demonstrate that conjugated bilayer chiral metamaterials (CMMs) integrated with Ge2Sb2Te5 (GST225) active components can achieve nonvolatile and continuously tunable optical activity in the THz region. A THz time-domain spectroscopic system was used to characterize the device, showing a tunable ellipticity (from ‒36° to 0°) and rotation of the plane polarization (from 32° to 0°) at approximately 0.73 THz by varying the GST225 state from amorphous (AM) to crystalline (CR). Moreover, a continuously tunable chiroptical response was experimentally observed by partially crystallizing the GST225, which can create intermediate states, having regions of both AM and CR states. Note that the GST225 has an advantage of nonvolatility over the other active elements and does not require any energy to retain its structural state. Our work allows the development of THz metadevices capable of actively manipulating the polarization of THz waves and may find applications for dynamically tunable THz circular polarizers and polarization modulators for THz emissions.

Miniaturized nonvolatile reconfigurable optical components with a subwavelength thickness, extremely compact size, high-speed response, and low power consumption will be the core of next-generation all-optical integrated devices and photonic computing to replace traditional bulky optical devices and integrated circuits, which are reaching physical limitations of Moore’s law. Metasurfaces, as ultrathin planar surfaces, have played a major role in controlling the amplitude, phase, and polarization of electromagnetic waves and can be combined with various active modulation methods to realize a variety of functional devices. However, most existing reconfigurable devices are bounded in volatile nature with constant power to maintain and single functionality, which restricts their further extensive applications. Chalcogenide phase change materials (PCM) have attracted considerable attention due to their unique optical properties in the visible and infrared domains, whereas in the terahertz (THz) regime, research on the reversible phase transition in large-scale areas and applications of Ge2Sb2Te5 (GST) are still under exploration. Here, we achieved reversible, repeated, and large-area switching of GST with the help of optical and thermal stimuli. Large-area amorphization with a 1 cm diameter of GST is realized by using a single laser pulse. Then, we incorporate GST into metasurface designs to realize nonvolatile, reconfigurable, multilevel, and broadband terahertz modulators, including the anomalous deflector, metalens, and focusing optical vortex (FOV) generator. Experimental results verify the feasibility of multilevel modulation of THz waves in a broadband frequency range. Moreover, the modulators are reusable and nonvolatile. The proposed approach presents novel avenues of nonvolatile and reconfigurable metasurface designs and can enable wide potential applications in imaging, sensing, and high-speed communications.

Fast and efficient information processing and encryption, including writing, reading, and encryption memory, is essential for upcoming terahertz (THz) communications and information encryption. Here, we demonstrate a THz multi-level, nonvolatile, optically rewritable memory and encryption memory based on chalcogenide phase-change materials, Ge

High-performance terahertz (THz) devices with reconfigurable features are highly desirable in many promising THz applications. However, most of the existing reconfigurable THz elements are still limited to volatile responses, single functionality, and time-consuming multistep manufacturing procedures. In this paper, we report a lithography-free approach to create reconfigurable and nonvolatile THz components by exploring the reversible, nonvolatile, and continuous THz modulation capability of the phase change material Ge2Sb2Te5 . As a proof of concept, THz gratings with significant Rayleigh anomalies and diffraction as well as ultrathin THz flat lenses with subwavelength and ultra-broadband focusing capabilities are designed and fabricated on ultrathin Ge2Sb2Te5 films using the presented photo-imprint strategy. Moreover, such a method can also be adopted to create more complex THz devices, such as Pancharatnam–Berry phase metasurfaces and grayscale holographic plates. With these findings, the proposed method will provide a promising solution to realize reconfigurable and nonvolatile THz elements.

The achievable functionalities of metasurfaces are mainly determined by the capability of their unit cells to control the local electromagnetic responses. To realize full control over light, it is highly desired to find a kind of unit cell that can achieve complete control of the phase, amplitude, and polarization, which remains a big challenge and cannot be fully replaced with iteration algorithms. Here, such control is achieved by exploiting interference in a dielectric unit cell composed of multiple meta-atoms, namely, a meta-molecule. This strategy provides sufficient effective geometric degrees of freedom within the meta-molecule to enable full control over its optical properties, as is demonstrated here with a complete meta-molecule database. As a proof-of-concept demonstration, polarization-independent, polarization-dependent, and vectorial meta-holograms are proposed and experimentally characterized. High imaging quality is achieved in both the measured intensity and phase distributions. This is allowed by the ability of the meta-molecules to fully record the information of the target images. This strategy opens a direct way toward optical devices with various desirable functionalities.

Polarization conversion dichroism is of particular interest in manipulating the polarization state of light, whereas high-performance asymmetric transmission (AT) of linearly polarized waves is still inaccessible in the terahertz range. Here, a bilayer chiral metamaterial consisting of orthogonally chained S-shaped patterns with broken symmetry along the light propagation direction is proposed and demonstrated experimentally to realize a dual-band dichroic AT effect for linearly polarized terahertz waves. The AT effects are robust across a wide range of incident angles. The observed strong AT can be theoretically explained by a multiple reflection and transmission interference model and the transfer matrix method. The proposed bilayer chiral metamaterial may have broad applications in polarization manipulation, chiral biosensing and direction-dependent information processing.

The introduction of single-pixel imaging and compressed sensing (CS) techniques into terahertz (THz) imaging has sped up image acquisition and avoided raster scanning. At present, most single-pixel terahertz imaging developments are based on simple metal samples, researchers rarely study the reconstruction of complex structural samples with large attenuation in the terahertz domain, such as metasurface holographic images. Here, we present an implementation of the single-pixel compressed sensing approach into THz metasurface holography reconstruction. By laser projecting a set of binary patterns on a 500-μm thick silicon wafer using a digital micromirror device (DMD), THz wavefront of the holographic metasurface is spatially encoded. Single-element detector is used to measure electric field amplitude of the transmitted THz radiation for each pattern, and then the hologram is reconstructed by Total variation Augmented Lagrangian and Alternating Direction Algorithm (TVAL3). Besides, the reconstruction effects are also analyzed by reducing measurement number, it can maintain more than 95% of the image information under 20% compression. The demonstrated combination of terahertz holography and single-pixel compressed sensing imaging provides new possibilities for metasurface imaging, verifies the stability of terahertz single-pixel imaging, and the scheme may lead to advances in fast terahertz imaging.

Hysteretic behavior in ion gel-graphene hybrid terahertz (THz) modulator is presented. The ion gel gated graphene modulator was designed and fabricated by conventional wet-based graphene transfer method. The modulation performance and hysteretic behavior of the device was characterized by THz time-domain spectroscopy. The dependence of hysteresis on the sweeping voltage rate and sweeping range was explored in a continuous wave terahertz system. The temporal response of the sample was also measured and fitted by a double-exponential expression, which indicated that there were two mechanisms that might cause the hysteresis in the THz modulator. This hysteretic behavior is promising in developing nonvolatile memory devices.

Chiral materials can exhibit different levels of transmission for opposite propagation directions of the same electromagnetic wave. Here we demonstrate thermal switching of asymmetric transmission of linearly polarized terahertz waves. The effect is observed in a terahertz metamaterial containing 3D-chiral metallic inclusions and achiral vanadium dioxide inclusions. The chiral structure exhibits pronounced asymmetric transmission at room temperature when vanadium dioxide is in its insulator phase. As the metamaterial is heated, the insulator-to-metal phase transition of vanadium dioxide effectively renders the structure achiral and the transmission asymmetry vanishes. We demonstrate the effect numerically and experimentally, describe it analytically and explain the underlying physical mechanism based on simulated surface current distributions. Potential applications include directionally asymmetric active devices as well as intensity and polarization modulators for electromagnetic waves.

Extending the optical control capabilities of metasurfaces for broader functionalities has recently attracted extensive attention. Simultaneously achieving amplitude and phase control is an effective route as it allows rebuilding the full information of the field. However, related previous studies mostly rely on anisotropic meta-atoms, which restrict the available incident polarizations. Here, a polarization-insensitive amplitude and phase control method is proposed and experimentally demonstrated in the terahertz regime, which is actualized by introducing interference effect in reflective-type meta-molecules composed of isotropic meta-atoms. Two kinds of functional meta-mirror devices, i.e., multi-order meta-gratings and multi-focal meta-lenses, are designed and characterized, where the results verify this method very well. This proposed method further enriches the routes to control amplitude and phase and may also find broad applications in realizing flexible wavefront control devices with complex functionalities.

Light is a complex vectorial field characterized by its amplitude, phase, and polarization properties, which can be further represented by four basic parameters, that is, amplitudes and phases of two orthogonally polarized components. Controlling these parameters simultaneously and independently at will using metasurfaces are essential in arbitrarily manipulating the light propagation. However, most of the studies so far commonly require a great number of different meta-atoms or rely on diffraction under oblique incidence, which lack convenience and flexibility in design and implementation. Here, a new metasurface paradigm is proposed that can completely manipulate the amplitudes and phases of two spin components based on the interference effect, where only two different meta-atoms are applied. For proof-of-concept demonstration, two five-channel meta-holograms for imaging and information encryption are designed and experimentally characterized. The interference method provides a simple route toward compact complex and multifunctional meta-devices.

Surface plasmons (SPs) is a vital form of electromagnetic wave confined to the metal-dielectric interface. Since the capability of guiding light within subwavelength scale, a great deal of plasmonic devices emerged successively. Over the past few years, in-plane SPs controlled by artificially structured surfaces are of great promise for information transmission in ultracompact integrated optical circuit, especially in terahertz (THz) near-field photonics. Owing to the exploitation of holographic principles in two-dimensional surface, we theoretically and experimentally demonstrate a generic design strategy to steer the SP waves to several focus spots depending on the wavelength and chirality.

Radiation at terahertz frequencies can be used to analyze the structural dynamics of water and biomolecules, but applying the technique to aqueous solutions and tissues remains challenging because such radiation is strongly absorbed by water. Here we develop terahertz optoacoustics as a method for analyzing water, tissues and aqueous solutions even in thick samples. We demonstrate that adjusting the temperature to alter the terahertz optoacoustic signal of water improves the sensitivity with which it can be analyzed and, conversely, can reduce or even "silence" its signal, allowing detection of solutes such as glucose and ions, potentially with greater sensitivity than terahertz time-domain spectroscopy. Terahertz optoacoustics may be a powerful tool for spectroscopy and potentially imaging of aqueous solutions and tissues to study molecular interactions and biochemical processes.

Radiation at terahertz frequencies can be used to analyze the structural dynamics of water and biomolecules, but applying the technique to aqueous solutions and tissues remains challenging since terahertz radiation is strongly absorbed by water. While this absorption enables certain analyses, such as the structure of water and its interactions with biological solutes, it limits the thickness of samples that can be analyzed, and it drowns out weaker signals from biomolecules of interest. We present a method for analyzing water-rich samples via time-domain terahertz optoacoustics over a 104-fold thickness ranging from microns to centimeters. We demonstrate that adjusting the temperature to alter the terahertz optoacoustic (THz-OA) signal of water improves the sensitivity with which it can be analyzed and, conversely, can reduce or even “silence” its signal. Temperature-manipulated THz-OA signals of aqueous solutions allow detection of solutes such as ions with an order of magnitude greater sensitivity than terahertz time-domain spectroscopy, and potentially provide more characteristic parameters related to both terahertz absorption and ultrasonic propagation. Terahertz optoacoustics may be a powerful tool for spectroscopy and potential imaging of aqueous solutions and tissues to explore molecular interactions and biochemical processes.

Bound state in the continuum (BIC) refers to the trapped state in the radiation continuum of a system. In the terahertz band, BIC provides a unique and feasible method to design devices with ultra-high quality factor (Q factor) and to achieve intense terahertz-matter interaction, which is of great value to terahertz science and technology. Here, multiple BICs protected by the resonance symmetry in the terahertz metasurface consisting of metallic split ring resonators (SRR) is demonstrated. The evolution from the BIC to the quasi-BIC (QBIC) is induced by changing the gap width of the SRRs. The proposed BICs are experimentally demonstrated and analyzed by the coupled mode theory along with the numerical simulation. It is found that the leakage behavior of these QBICs is strongly affected by the intrinsic Ohmic loss in the SRRs while it is quite robust to the tilted incidence.

Metasurfaces, especially tunable ones, have played a major role in controlling the amplitude, phase, and polarization of electromagnetic waves and attracted growing interest, with a view toward a new generation of miniaturized devices. However, to date, most existing reconfigurable devices are bounded in volatile nature with sustained external energy to maintain and single functionality, which restrict their further applications. Here, we demonstrate for the first time, to our knowledge, nonvolatile, reconfigurable, and dynamic Janus metasurfaces by incorporating phase-change material Ge 2 Se 2 Te 5 (GST) in the terahertz (THz) regime. First, we experimentally show the reversible switching characteristic of GST on large areas by applying a single nanosecond laser pulse, which exhibits excellent contrast of THz properties in both states. Then, we present a multiplex metasurface scheme. In each metasurface, three sets of structures are adopted, in which two sets integrate GST. The effective structures can be reversely modulated by the amorphization and crystallization of GST. As a proof of concept, the dynamic beam splitter, bifocal metalens, dual-mode focusing optical vortex generators, and switchable metalens/focusing optical vortex generators are designed, fabricated, and experimentally characterized, and can be switched reversibly and repeatedly with the help of optical and thermal stimuli. Our scheme will pave the way toward the development of multifunctional and compact THz devices and may find use for applications in THz imaging, sensing, and communications.

Efficient and flexible manipulation of electromagnetic waves using metasurfaces has attracted continuous attention in recent years. However, previous studies mainly apply sole resonance effect to accomplish the task. Here, we show that introducing a meta-coupling effect would reveal further physical insights in the electromagnetic wave control. To demonstrate this, a reflection-type coupling system composed by two identical linear resonances in a metal-insulator-metal configuration is theoretically proposed using the coupled-mode theory, whose phase diagram can be well controlled upon the coupling changes. Such intriguing optical property is verified by a double C-shaped resonator in the terahertz regime, where the coupling effect can be tuned by changing their either relative distance or rotation. More importantly, the reflection phase shift around the working frequency can be efficiently engineered without having to change the dimensions of the resonators. Two efficient anomalous metasurface deflectors are designed and experimentally characterized, whose maximum measured efficiency is more than 70%. The proposed controlling strategy further enriches the designing freedoms of metasurfaces and may find broad applications in realizing efficient and tunable functional devices.

Extreme terahertz (THz) science and technologies, the next disruptive frontier in nonlinear optics, provide multifaceted capabilities for exploring strong light-matter interactions in a variety of physical systems. However, current techniques involve the need for an extremely high-field free space THz source that is difficult to generate and has limited investigations to a rather weak and linear regime of light-matter interactions. Therefore, new approaches are being sought for the tight confinement of THz waves that can induce nonlinear effects. Here, a nonlinear “tera-nano” metasurface is demonstrated exhibiting extremely large THz nonlinearity and sensitive self-modulation of resonances at moderate incident THz field strengths. A record deep-subwavelength (≈λ/33 000) confinement of strongly enhanced (≈3200) THz field in a nano-gap (15 nm) exhibits remarkable THz field-tailored nonlinearity. Further, ultrafast injection of photocarriers reveals a competition between nonlinear THz field-induced intervalley scattering and optically driven interband excitations. The results on “tera-nano” metasurfaces enable a novel platform to realize enhanced nonlinear nano/micro composites for field-sensitive extreme THz nonlinear applications without the need for intense THz light sources. Ministry of Education (MOE) This work was supported by the National Natural Science Foundation of China (61905007, 11827807), the National Key R&D Program of China (2019YFB2203102), and the Open Project Program of Wuhan National Laboratory for Optoelectronics No. 2018WNLOKF001, and the Open Fund of Guangdong Provincial Key Laboratory of Information Photonics Technology (Guangdong University of Technology, No. GKPT20). M.M. and R.S. acknowledge research funding support from the Ministry of Education, Singapore (AcRF Tier 1, Grant RG191/17, MOE2017-T2-1-110, and MOE2016-T3-1-006(S)).

Chiral metamaterials have attracted wide interest because strong optical activity at designed frequencies could be achieved beyond that in natural materials. Here we propose an all-dielectric metamaterial with strong extrinsic circular dichroism and circular birefringence by periodically arranging symmetry-broken dielectric Mie resonators at terahertz frequencies. The strong interaction between the electric and magnetic resonances from circularly polarized incident waves dominates the performance of the all-dielectric metamaterial, which exhibits a 60% circular dichroism in transmission and a polarization rotation angle of 60° at maximum, respectively. Additionally, the spectral range of the circular dichroism with preserved amplitude can be adjusted continuously in the frequency range from 0.67–0.79 THz by tuning the tilt angle of the incident wave. Our findings will be of great potential in polarization control applications such as asymmetric transmission, optical isolation, and on-chip chiral manipulation.

Nondestructive testing of packaged chips is essential for ensuring product performance, yet existing methods have serious drawbacks. Here, we apply photoacoustic remote sensing microscopy using a high-frequency pulse laser with a pulse width of 1.2 ps and a wavelength of 1030 nm to inspect silicon-based semiconductor chips for internal defects. Joint optical-mechanical scanning allowed high-resolution imaging of a large field of view. The basis for photoacoustic imaging was explained using a solid-state physics model, which was confirmed experimentally by measuring photoacoustic amplitudes at different doping concentrations. Our method appears capable of rapidly imaging chips over a large field of view with depth-to-resolution ratios of around 200 without the need for a couplant, which could support nondestructive inspection in industrial applications.

Two-dimensional black phosphorus (BP) recently emerged as an outstanding material for optoelectronics and nanophotonics applications. In contrast to graphene, BP has a sufficiently large electronic bandgap and its high carrier mobility allows for efficient free-carrier absorption in the infrared and terahertz regimes. Here, we present a reflective structure to enhance the response of nanostructured monolayer BP at terahertz frequencies and investigate localized surface plasmon resonances in BP nanostrip arrays. Anisotropic absorption is observed in the proposed BP metamaterials due to the puckered crystal structure of the monolayer BP, and further investigations show that the plasmonic resonances are strongly depending on the geometric parameters of the nanostrips and the coupling between the adjacent nanostrips. We expect that the monolayer BP is an outstanding candidate of highly anisotropic plasmonic material for ultrascaled optoelectronic integration.

A switchable chiral metasurface based on a phase change material Ge2Sb2Te5, which can switch between a right-handed circularly polarized mirror and a left-handed circularly polarized mirror, is theoretically discussed. When the conductivity of Ge2Sb2Te5 σ is 0 S/m, the metasurface will reflect incident right-handed circularly polarized light and absorb incident left-handed circularly polarized light at 0.76 THz. As σ is set to 3 × 105 S/m, the response of the metasurface to circularly polarized light will be reversed. That is, it reflects the incident left-handed circularly polarized light and absorbs the incident right-handed circularly polarized light at 0.66 THz. The circular dichroism is from 76% to −64%. Then, we also study the performance of the mirror structure of the initial metasurface. By simulating the reflected spectra with different conductivities and the surface current distribution, the reason for the switchable function is clear. Moreover, the switchable chiral metasurface can be applied in spin-selective beam deflectors, which is proven by simulation. This work provides a new strategy for the development of tunable chiral devices.

As one kind of special beams, a terahertz (THz) cylindrical vector Bessel beam processes centrosymmetric polarization distribution and nondiffractive propagation properties. Such a vector Bessel beam is potential in various practical applications ranging from THz communication and electron acceleration to sensing and imaging. Here, we propose and experimentally demonstrate a method to directly emit a broadband THz cylindrical vector Bessel (CVB) beam using nonlinear effects. By photo-exciting an indium tin oxide film coated axicon with a circularly polarized femtosecond laser pulse, a high-quality CVB beam is obtained in a broadband THz frequency range. The proposed method is universal and low-cost, opening a simple avenue for the emission of broadband THz specialty beams.

Metamaterial-based Fano resonances are very promising for the design of sensors, lasers, and slow light devices, which are particularly important for the terahertz band where functional devices have long been lacking. However, most of the reported Fano metamaterials are designed to manipulate free-space terahertz waves. Here, we present a Fano resonance metamaterial design to manipulate the waveguide mode in a parallel plate waveguide. Different from the free-space cases, the interaction length between the waveguide mode and the Fano metamaterials can be adjusted flexibly, allowing a manipulation of the Fano resonance intensity. Moreover, we also demonstrate thin-film sensing of a monolayer graphene film with the proposed Fano resonance metamaterials. The proposed design scheme may motivate the design and fabrication of future practical devices in waveguide systems, and promote a wider application of Fano resonances.

In this study, a series of uniaxial and multiaxial ratcheting tests were conducted at room temperature on zirconium alloy tubes. The experimental results showed that for uniaxial symmetrical cyclic test, the axial ratcheting strain ɛ x did not accumulate obviously in initial stage, but gradually increased up to 1% with increasing stress amplitude σ xa . For multiaxial ratcheting tests, the zirconium alloy tube was highly sensitive to both the axial stress amplitude σ xa and the internal pressure p i . The hoop ratcheting strain ɛ θ increased continuously with the increase of axial stress amplitude, whereas the evolution of axial ratcheting strain ɛ x was related to the axial stress amplitude. The internal pressure restricted the ratcheting accumulation in the axial direction, but promoted the hoop ratcheting strain on the contrary. The prior loading history greatly restrained the ratcheting behavior of subsequent cycling with a small internal pressure.

The absorption region of a water-based absorber was expanded by introducing tetramethylurea (TMU) into the inclusion, whose dielectric properties are tunable through the concentration of TMU. The dielectric spectroscopy of a TMU/water mixture was deconstructed using a Debye model. We designed a four-layer ultra-broadband microwave absorber with a supernatant micro-structure. Simulation and experiment results indicate that the absorber can achieve 90% perfect absorption, covering a broad frequency range of 4–40 GHz. The concentration dependence of the absorber was also studied experimentally and numerically. The concentration control provides a more practical and large frequency-region modulation of perfect absorption.

Weyl points are discrete locations in the three-dimensional momentum space where two bands cross linearly with each other. They serve as the monopoles of Berry curvature in the momentum space, and their existence requires breaking of either time-reversal or inversion symmetry1–16. Although various non-centrosymmetric Weyl systems have been reported15, demonstration of Weyl degeneracies due to breaking of the time-reversal symmetry remains scarce and is limited to electronic systems17,18. Here, we report the experimental observation of photonic Weyl degeneracies in a magnetized semiconductor—InSb, which behaves as a magnetized plasma19 for electromagnetic waves at the terahertz band. By varying the magnetic field strength, Weyl points and the corresponding photonic Fermi arcs have been demonstrated. Our observation establishes magnetized semiconductors as a reconfigurable20 terahertz Weyl system, which may prompt research on novel magnetic topological phenomena such as chiral Majorana-type edge states and zero modes in classic systems21,22. Photonic Weyl points—topologically chiral singularity points in three-dimensional momentum space—have been realized in a homogeneous non-reciprocal material without a crystal lattice structure.

Active control of metasurfaces has attracted widespread attention because of the adjustable electromagnetic properties obtained. Here we designed and experimentally studied a dynamically controllable polarization converter in the terahertz band. By designing the structural parameters and utilizing the insulator-to-metal phase transition of vanadium dioxide and principle of current resonance, dynamic tunability of the polarization conversion function from dual-broadband (0.45∼0.77 THz and 0.97∼1.2 THz) to ultra-broadband (0.38∼1.20 THz) can be realized with a high polarization conversion ratio. The scheme proposed here can find potential applications in integrated terahertz systems, sensing, imaging and communications areas.

Terahertz (THz) metamaterials are widely used in biosensor devices due to their unique superiority, and the demand for new high sensitivity biosensors based on THz metamaterials is increasing. This paper presents a polarization-insensitive terahertz metamaterial sensor used for BSA detection. Simulation reveals that the peak of transmission spectrum shifts obviously when the sensor is covered with analytes of different refractive index and thickness. After the sensor is covered with 10 μm thick non-destructive analytes, its sensitivity is as high as 135 GHz/RIU. Experiments show that the lowest detectable concentration of BSA solutions by this sensor is 0.1 mg/mL, the peak red shift of the transmission spectrum reaches 137 GHz when the concentration is 17.6 mg/mL, and the frequency shift percentage is 16.4%. This study provides a highly sensitive solution for biosensor detection in the pharmaceutical and food fields.

Originating from the investigation of condensed matter states, we have discovered a new material with phase transition no-trivial properties——topological insulators. With the progress in artificial metamaterials, the photonic crystals which can give rise to topological edge states that propagate around sharp corners immune to scattering have captured much attention. In this paper, according to the spatial and time reversal symmetry and quantum spin Hall effect theory, we presented a topological photonic crystal made of metallic cylinders at terahertz frequencies. By slightly expanding and shrinking the unit cell, the spatial reversal symmetry was broken and therefore, the band degeneracy was broken, realizing the topological band gap. Unidirectional topological edge states were proven to exist on a domain wall connecting two photonic crystals with opposite topological properties without back-scattering. Then we investigated the reflection spectra of the designed topological photonic crystals in the terahertz regime, which provided a new means to verify topological photonic crystals.

Metasurfaces are 2D engineered structures with subwavelength granularity, offering a wide range of opportunities to tailor the impinging wavefront. However, fundamental limitations on their efficiency in wave transformation, associated with their deeply subwavelength thickness, challenge their implementation in practical application scenarios. Here, it is shown how the coherent control of metagratings through multiple wave excitations can provide new opportunities to achieve highly reconfigurable broadband metasurfaces with large diffraction efficiency, beyond the limitations of conventional approaches. Remarkably, energy distribution between the 0th and higher diffraction orders can be continuously tuned by changing the relative phase difference between two excitation waves, enabling coherent control, with added benefits of enhanced efficiency and bandwidth. This concept is demonstrated for a thin electric metagrating operating at terahertz frequencies, showing that coherent control can overcome several of the limitations of single-layer ultrathin metastructures, and extend their feasibility in various practical scenarios.

Graphene has been recognized as a promising candidate in developing tunable terahertz (THz) functional devices due to its excellent optical and electronic properties, such as high carrier mobility and tunable conductivity. Here, we review graphene-based THz modulators we have recently developed. First, the optical properties of graphene are discussed. Then, graphene THz modulators realized by different methods, such as gate voltage, optical pump, and nonlinear response of graphene are presented. Finally, challenges and prospective of graphene THz modulators are also discussed.