Recently, quasi-bound states in the continuum (quasi-BICs) of localized spoof surface plasmons (LSSPs) have been developed on several metasurfaces with broken symmetry, which have high quality factors ($Q$-factors) because of low radiation loss. They are transverse electric (TE) modes. In this paper, the quasi-BICs of LSSPs transverse magnetic (TM) modes are realized on a nonparallel circular grating metasurface with a linearly polarized plane wave. When the circular gratings are placed in parallel, the LSSPs TM modes on the metasurface cannot be excited by the plane wave, because they are true-BICs protected by rotational symmetry; when the circular gratings are mechanically reconfigured to be nonparallel, owing to the asymmetry of the structure, the LSSPs TM modes turn into quasi-BICs from the true-BICs and can be excited. Owing to the strong coupling between the circular gratings in the vertical coupling configuration, the quasi-BICs of the LSSPs TM modes have higher $Q$-factors compared with the quasi-BICs of the LSSPs TE modes. An experiment was conducted and the $Q$-factor of $\text{TM}_{2.1}$ was measured to be 305.0. This study provides a new idea for developing LSSPs with low radiation loss. It also extends the LSSPs from two-dimensional to three-dimensional, and brings new potential of transformation.

Harmonic multiplying provides a key solution to addressing the shortage of high-frequency driving sources and the strong magnetic field requirements for gyro-TWTs in terahertz band. In this article, broadband harmonic multiplying gyro-TWTs are proposed and theoretically investigated, which enable the use of commercially available G-band solid-state sources as the driver and extends the gyro-TWT operation into unprecedented terahertz regime. The composite interaction circuit with different radii separated by a long drift section is proposed, and the associated nonlinear beam dynamics are analyzed in detail to guide the optimization. Distributed losses and the axis-encircling electron beam are employed to ensure the stable harmonic operation. A magnetic cusp gun with a velocity spread below 1.2% is developed and the cathode thermal profile is characterized, demonstrating the feasibility of the proposed interaction scheme. Finally, numerical simulations demonstrate that stable third and fourth harmonic multiplying are achieved at 80 kV voltage and 1 A current, with 3-dB bandwidths of 10 GHz and 11 GHz around 660 GHz and 865 GHz, corresponding to high efficiencies of 7.68% and 2.61%, respectively. This article demonstrates a practical and realizable route toward broadband, high-power terahertz amplifiers, paving the way for significant advances in terahertz radar and spectroscopic applications.

Broadband achromatic metalenses are key components for millimeter-wave (MMW) integrated sensing and communication (ISAC). This work introduces a group-delay-dispersion (GDD) matching strategy to streamline achromatic metalens design. An MMW device is fabricated and tested. The measured results show a near-unity numerical aperture (NA) of 0.98, a relative bandwidth of 27.1% (7.4 GHz), subwavelength focusing with a minimum full-width at half-maximum (FWHM) of 0.41λ, and a 3-dB beamwidth of 6.12°. Such metalenses are promising for future broadband ISAC applications, including low-altitude platforms, vehicular systems, immersive communications, and intelligent mobility sensing.

In this paper, a novel scheme is introduced that combines a traveling wave tube (TWT) with a metasurface to generate high-power E-band vortex electromagnetic waves. The TE10 mode electromagnetic wave emitted by the TWT is initially converted into a plane wave via a horn antenna and subsequently transformed into a vortex electromagnetic wave by the metasurface. The metasurface is designed and simulated, and the results show that this approach can convert the TE10 mode from the TWT into vortex electromagnetic waves with a specific topological charge of l=+1 within the 71-76 GHz frequency range, achieving a remarkable mode purity of up to 97%. The experiment at 73.5 GHz was successfully carried out, generating vortex electromagnetic waves with the designated topological charge of l=+1 using this method. Although the experimentally measured mode purity was limited to 30.6%, this outcome confirms the effectiveness of the proposed method.

The gyrotron equipped with pulsed magnets is one of the most promising candidates to generate high-power electromagnetic wave in the terahertz band. However, the narrow available radiation duration of several to tens of microseconds in conventional pulsed gyrotrons limits their performance. In this paper, a terahertz gyrotron based on the 20-T high-field pulsed magnet is developed and experimentally investigated. Using the natural time-varying pulsed field strength, millisecond-level radiation can be achieved. With a voltage of 18 kV and a current of 1.5 A, cumulative energy near 1 Joule is obtained. The time-varying dynamic process is recorded and exhibits a unique asymmetric waveform, which is in good agreement with the theoretical predictions. This work will promote the development of high-power terahertz sources and high-resolution spectrometers.

In this study, we propose a polarization-converted phase gradient metasurface (PCPGM). It achieves highly efficient retroreflection at large oblique incidence angles in the broadband frequency range. This technology is based on the integration of the generalised Snell's law, the polarization conversion characteristics of elements, and a phase gradient array. Simulation results reveal that the proposed polarization-converted metasurface composed of dual-resonant ring patches provides a phase shift greater than 303.7 degrees, while maintaining a polarization conversion efficiency of >80 %. The designed phase gradient array produces a maximum radar cross-section of 15.3 dBm(2) during backscattering enhancement at an optimized frequency of 12 GHz and oblique incidence angle of 60 degrees. Furthermore, efficient retroreflection is maintained in a broadband frequency range of 10-20 GHz. The 3 dBm(2) effective bandwidth for accurate retroreflection is 4.57 %. A prototype metasurface with a total area of 300 x 300 mm(2) was fabricated and measured, confirming that the PCPGM achieved a cross-polarization isolation of 35.7 dBm(2) at 12 GHz and a large incident angle of 55.8 degrees. This cross-polarization isolation value at a large incident angle is larger than that reported for multilayer bias-control network structures. The single-layer PCPGM is easy to fabricate and exhibits stable performance at large incidence angles, excellent polarization isolation, high resistance to interference, and low transmission loss. This study contributes to the realization of efficient and stable radar echo signals for practical applications. The proposed metasurface improves resistance to interference under different polarization states and enhances scattering performance in low-profile decoy systems.

This article investigates a passive microwave pulse compression (PMPC) technique that achieves a peak power gain exceeding 100 in X-band experiments. Rigorous mathematical derivations demonstrate that for a given dispersive system with a fixed frequency response, a theoretical maximum peak power gain exists. This maximum gain is attained if and only if the input signal equals the sign function of the time-reversed time-domain response. A compact microwave pulse compression system, fabricated with aluminum, consisting of two cascaded overmoded short-circuit arms, is designed. Each arm can propagate nine modes, and they can be coupled with each other, leading to substantial group delay differences among various frequencies. In low-power experiments, the S-parameters of the system are first measured using a vector network analyzer (VNA) to theoretically determine the optimal input signal, and input and output signals are then captured by a high-speed oscilloscope—yielding a measured peak power gain of 108 times with a 1500-ns input pulse—with good agreement between the experimental waveform and theoretical calculation. This validates the feasibility of the proposed theory. As a PMPC technology, the system offers long-term operational stability and holds promise for generating ultrahigh-peak-power high-power microwaves (HPMs) in future applications.

Stable operation of a gyrotron necessitates a precise axial magnetic field distribution within its room-temperature bore. The magnetic field must satisfy the distinct requirements for field strength and uniformity across the electron gun, interaction, and collector regions. This paper presents the electromagnetic design and optimization of a 0.7 T MgB2 superconductingmagnet specifically tailored for K-band gyrotron applications. Based on the practical application scenario of solid nitrogen cooling, the operating temperature is fixed within the 10–20 K range. To achieve an economical and manufacturable design, a hybrid Linear Programming-Nonlinear Programming (LP-NLP) optimization framework is proposed. In the LP stage, the total coil volume is minimized under multi-point axial field constraints to determine a preliminary optimal coil distribution. To bridge the gap between the irregular LP results and practical fabrication, a rectangularization strategy is implemented to reconstruct the preliminary coils into regular solenoidal coils. The rectangularized coil parameters then serve as robust initial values for the subsequent NLP stage, where refined optimization is conducted by integrating comprehensive engineering constraints. Finally, electromagneticsimulations validate the 0.7 T MgB2 magnet design and confirm that the synthesized magnetic field strictly adheres to the performance specifications of each functional region. The proposed magnet design provides a technical reference for the design and application of MgB2 superconducting magnets for gyrotrons.

Advances in high-stability flat-top pulsed magnets and programmable power supplies have enabled the precisely controllable radiation of gyrotrons exceeding the 1-THz band. However, high-efficiency operation under the “hard-excitation” condition and mode shadowing is still difficult for terahertz gyrotrons. In this article, a novel temporal profile of the pulsed field is introduced to achieve efficiency enhancement and operating-domain extension. An overshoot at the pulse front realizes smooth conversion between the soft and hard excitation regions within the pulsewidth scale. As an advanced demonstration, a 1-THz gyrotron equipped with a 40-T flat-top magnet is developed and investigated. Steady oscillation with power above 8 kW at 1.001 THz is predicted in this drastic time-varying system. The efficiency is more than doubled compared to the value that can be directly excited. The feasibility of this waveform based on the model predictive control (MPC) is also discussed. This scheme is generalized as it imposes no specific requirements on the gyrotron itself. This work will promote the research on the high-power terahertz sources and their applications such as high-resolution spectrometers.

Meta‐lenses can break the performance limits of traditional bulky lenses and are essential for the development of advanced meta‐devices and systems. However, chromatic dispersion represents a significant challenge in the metalenses design for their widespread applications. Previous studies on broadband achromatic metalenses are mostly based on all‐dielectric metasurfaces, and very little efforts are devoted to the design of achromatic plasmonic metalenses. Herein, an ultra‐thin broadband hybrid achromatic plasmonic metalens with only 0.08 λ 0 thickness (λ 0 is the central wavelength) is designed, presented, and characterized in low spectrums. Compared to previous broadband plasmonic metalenses with single positive or negative chromatic dispersion, the aberration of hybrid plasmonic metalens is significantly reduced by an order of magnitude. High average numerical aperture exceeding 0.8 is achieved in the simulations across the considered broad bandwidth. Besides, a high average focusing efficiency of 60% (the maximum is nearly 80%) is also measured in the experiments. These excellent focusing performances can break the tradeoffs of current achromatic metalenses. More importantly, the presented hybrid plasmonic metalens provides a novel and general concept to develop broadband achromatic metalenses with a fully planar and low‐profile device footprint.

Circular polarization is a key electromagnetic property for applications like wireless communication, sensing technologies, radar systems, and detection platforms. Dynamic control over polarization states and radiation patterns is crucial for enhancing performance. Spin-momentum locking (SML), a phenomenon in evanescent waves, plays a vital role in achieving circular polarization. SML involves the strong coupling between a wave’s propagation direction, spatial decay, and the spin of the electric field, forming a right-handed triplet. This relationship allows precise manipulation of circularly polarized radiation, minimizing the impact of material losses or complex designs. This study demonstrates that leveraging SML enables highly efficient circularly polarized radiation over a broad frequency range with a simple structural design. Simulations show high polarization purity and dynamic control over polarization and radiation direction, making the design adaptable and cost-effective for various applications in communication and sensing systems.

The conventional generalized Snell’s law (GSL), derived from classical laws of optical reflection and refraction, governs wavefront manipulation via phase gradients but neglects higher-order spatial harmonics inherently excited by the mutual coupling among meta-atoms on a metasurface. Here, we introduce a spatial harmonic-expanded GSL (SH-GSL) framework by unifying phase-gradient control with Floquet periodicity, establishing spatial harmonics as independent degrees of freedom rather than conventional parasitic disturbances. The SH-GSL framework rigorously identifies the intrinsic harmonic dynamics inherent to metasurfaces, which is a critical feature absent in GSL. Furthermore, this framework further reveals that all gradient-phase metasurfaces inherently function as multichannel platforms due to full spatial harmonics, with this multifunctionality rooted in nonlocal Floquet-Bloch modal interactions. Experimental validation demonstrates: abnormal spatial-harmonic reflection with angular precision ( < 5° deviation), multi-beam splitting (dual/quad configurations) via the relationship between specific harmonics and compensation wave vectors, and a perfect three-channel retroreflector achieving up to 99

Metasurface-based retroreflectors hold significant potential for applications in communication systems; however, existing designs still face challenges such as narrow bandwidth, low reflection efficiency, and insufficient multi-channel performance at high frequencies. While three-channel designs have been extensively studied, research on five-channel designs remains limited, despite the critical importance of multi-channel expansion for advanced beam steering and signal processing. To address these issues, this paper proposes a 20GHz five-channel metasurface-based retroreflector. Through theoretical analysis and electromagnetic simulations, its efficiency is evaluated. The results demonstrate that three incident angles achieve efficient retroreflection, while the remaining two angles exhibit anomalous reflections due to channel crosstalk. This study lays the foundation for further theoretical analysis, experimental validation, and practical implementation, offering new insights for applications in next-generation wireless communication, radar, and satellite networks.

Spoof surface plasmons (SSPs) on the micro-scale corrugated metallic grating can find many novel applications in Terahertz (THz) band due to its many distinct electromagnetic properties. Specifically, free electron beam driven enhanced SSPs radiation sources have received great attentions in recent years. The significant near-field enhanced effect of SSPs mode provides unique opportunity to develop this kind of THz electronic radiation sources. In this work, we numerically demonstrates SSPs mode can be efficiently excited on the uniform metal grating by using low-energy electron beam with a pulse current as small as 50mA (the total electron energy is below 1kWatts). Numerical simulations indicates that the maximum generated SSPs power can reach 0.46Watts at around 1THz frequency within a 6mm length waveguide. Furthermore, the generated SSPs power can be enhanced significantly to above 1.5Watts by extending the waveguide length to a long-distance of 11.4mm. Compared to other existed Smith-Purcell or Cherenkov radiations using low-energy electron beam on the meta-materials or meta-structure, this work provide novel avenues to develop on-chip high-efficiency THz radiation sources by employing the bounded SSPs modes.

Smith-Purcell radiation (SPR) occurs when an electron skims above a spatial grating, but the fixed momentum compensation from the static grating imposes limitations on the emission wavelength. It has been discovered that a temporally periodic system can provide energy compensation to generate light emissions in free space. Here, we introduce temporal SPR (t-SPR) emerging from a time grating and propose a generalized t-SPR dispersion equation to predict the relationship between radiation frequency, direction, electron velocity, modulation period, and harmonic orders. Compared to conventional SPR, t-SPR can: 1) Provide a versatile platform for manipulating SPR emission through temporal modulation (e.g., period, amplitude, wave shape). 2) Exhibit strong robustness to the electron-grating separation, alleviating the constraints associated with extreme electron near-field excitation. 3) Introduce additional energy channels through temporal modulation, enhancing and amplifying emission.

Circularly polarized (CP) radiation is critical in modern wireless communications and radar systems, yet conventional CP antenna designs are often constrained by limited tunability, structural complexity, and high sensitivity to minor deviations. A generalized methodology for realizing high-purity CP radiation is presented, leveraging the spin-momentum locking (SML) phenomenon inherent to evanescent waves. By engineering an efficient coupling mechanism facilitating radiation from spoof surface plasmons (SSPs), this approach circumvents the structural complexities and potential sensitivities inherent in many conventional CP antenna designs. To validate this concept, a low-profile CP antenna is proposed to validate the method, and a systematic exploration of structural configurations provides insights into optimizing gain enhancement or polarization purity. Simulated results demonstrate high-purity CP radiation across 13.4-14.0 GHz, with an average axial ratio (AR) of 1.48 dB and a minimum AR of 0.25 dB. Experimental results confirm these findings, yielding an average AR of 0.99 dB, significantly outperforming existing designs. Furthermore, the antenna architecture demonstrates robust polarization fidelity during beam scanning. This work establishes a paradigm for developing simplified yet high-performance CP antennas, grounded in fundamental wave physics, with significant implications for advanced wireless communication and radar systems.

This paper presents a novel 220 GHz photonic crystal gyrotron design capable of generating a maximum power of 10.8 MW with a corresponding working efficiency of 3.09%, addressing the critical challenge of mode competition in overmoded high-power terahertz sources. By integrating a partially loaded metal photonic bandgap (PBG) resonator with synthetic dimension optimization, the proposed architecture achieves robust single-mode operation while minimizing diffraction losses. Through precise control of the PBG lattice position and thickness, the design demonstrates a significant reduction in competing mode excitation compared to conventional cylindrical cavities. Particle-in-cell (PIC) simulations validate stable operation at megawatt power levels, representing an improvement in output power over existing PBG gyrotron simulations. This breakthrough enables transformative applications in directed energy, plasma diagnostics, and terahertzdriven material processing.