Optical computing, renowned for its light-speed processing and low power consumption, typically relies on the coherent control of two light sources. However, there are challenges in stabilizing and maintaining high optical spatiotemporal coherence, especially for large-scale computing systems. The coherence requires rigorous feedback circuits and numerous phase shifters, introducing system instability and complexity. Here we propose an innovative logic gate using a single light source, with frequency and polarization serving as two virtual inputs. Our design leverages frequency-polarization multiplexed metasurfaces to achieve all basic logic operations by selectively routing surface plasmon polaritons. This single-channel logic gate maintains inherent coherence between frequency and polarization, thereby considerably eliminating stringent light-source specifications and numerous rigid phase controls and resulting in higher stability. Our device showcases unique application potentials in on-chip readout of encryption information by using random sequences as a one-time pad, unlocking fresh prospects for information protection and optical computing with other simple light sources.

Electromagnetic waves carrying orbital angular momentum (OAM), namely OAM beams, are important in various fields including optics, communications, and quantum information. However, most current schemes can only generate single or several simple OAM modes. Multi-mode OAM beams are rarely seen. This paper proposes a scheme to design metasurfaces that can generate multiple polarization-multiplexed OAM modes with equal intervals and intensities (i.e., OAM combs) working in the terahertz (THz) range. As a proof of concept, we first design a metasurface to generate a pair of polarization-multiplexed OAM combs with arbitrary mode numbers. Furthermore, another metasurface is proposed to realize a pair of polarization-multiplexed OAM combs with arbitrary locations and intervals in the OAM spectrum. Experimental results agree well with full-wave simulations, verifying a great performance of OAM combs generation. Our method may provide a new solution to designing high-capacity THz devices used in multi-mode communication systems.

Investigation of absolute instability (AI) and convective instability (CI) of the electron cyclotron maser has independently led to the birth of gyrotron oscillators and amplifiers. Here, it is demonstrated that these instabilities can form a cooperative relationship owing to the nonlinear behavior of the stimulated electron beam. The CI can be induced in a zero‐drive system with the assistance of AI, and the ohmic losses of all the excited waves inside the system are greatly reduced, which is called “instability entanglement” here. According to the theoretical and experimental study of a 167/330 GHz gyrotron, when instability entanglement occurs, the ohmic dissipation decreases to one‐ninth of the AI‐only condition, and the output power is enhanced by 20%. This discovery is promising for surpassing the ceiling of output power and frequency of gyrodevices placed by ohmic losses.

Recent advances in vortex Smith–Purcell radiation (V‐SPR) have spurred many breakthroughs in the generation of vortex beams for their potential applications in chiral detection, particle acceleration, communication, and imaging. However, advanced multi‐mode applications of the V‐SPR suffer from the incoherent spectrum and poor purity in producing high‐order topological charges. Here, by exploiting the rotational symmetry and Brillouin‐folding phenomenon, it is proved that the coherent V‐SPR with independently steerable high‐order topological charge is directly emitted from chiral spoof surface plasmons. Moreover, both topological charges and operation frequencies can be readily tuned, thus enabling flexible adaptability over a broad bandwidth. A proof‐of‐concept prototype is designed and fabricated in the microwave regime, in which the method is demonstrated from the perspective of symmetry and tunability. The proposed methodology can be exploited as a platform to investigate the interaction between chiral plasmons and swift electrons, promising a feasible scheme for generating coherent and tunable vortex beams from the microwave to the terahertz band.

Plasmonic metasurfaces have shown great potential to manipulate the electromagnetic fields within the subwavelength scale. Nanoslits are one type of the commonly used building blocks of metasurfaces. In this paper, through controlling the orientation angles of two separated slits under 90° linear polarization incidence, the wavefront of the excited surface plasmon polaritons (SPPs) can be freely manipulated. More importantly, the SP profiles on the two sides keep inverse in a broadband regime. For verification of the proposed design rule, this paper demonstrates two terahertz structures by CST simulation and theoretical calculation. The profiles on the two sides are focusing (deflecting) and diverging (opposite deflecting) respectively. Our work explores one new plasmonic function, providing ideas for the further implementation of arbitrary field generation.

As an active microwave coherent imaging technology, synthetic aperture radar (SAR) images suffer from severe speckle noise and low-resolution problems due to the limitations of the imaging system, which cause difficulties in image interpretation and target detection. However, the existing SAR super-resolution (SR) methods usually reconstruct the images by a determined degradation model and hardly consider multiplicative speckle noise, meanwhile, most SR models are trained with synthetic datasets in which the low-resolution (LR) images are down-sampled from their high-resolution (HR) counterparts. These constraints cause a serious domain gap between the synthetic and real SAR images. To solve the above problems, this paper proposes an unsupervised blind SR method for SAR images by introducing SAR priors in a cycle-GAN framework. First, a learnable probabilistic degradation model combined with SAR noise priors was presented to satisfy various SAR images produced from different platforms. Then, a degradation model and a SR model in a unified cycle-GAN framework were trained simultaneously to learn the intrinsic relationship between HR–LR domains. The model was trained with real LR and HR SAR images instead of synthetic paired images to conquer the domain gap. Finally, experimental results on both synthetic and real SAR images demonstrated the high performance of the proposed method in terms of image quality and visual perception. Additionally, we found the proposed SR method demonstrates the tremendous potential for target detection tasks by reducing missed detection and false alarms significantly.

With the rapid development of nanophotonics for enhancing free-electron radiation, bound states in the continuum (BICs) have emerged as a promising approach for emitting intense Smith–Purcell radiation (SPR) with enhanced intensity. However, current BIC-based methods are limited to single-frequency operation, thereby restricting their applications requiring spectral and angular tunability, such as particle detectors and light sources. To overcome this limitation, this work proposes an approach for constructing plasmonic BICs over a broad spectral range in symmetry-broken systems. By leveraging the high- Q resonances near the BICs, we achieve intense SPR with broadband tunability, potentially improving the radiation intensity by six orders compared to traditional methods. Experimentally, we validate the construction of BIC using plasmonic antennas and achieve broadband demonstration. Our proposed concept can be extended to other plasmonic or guided-wave systems, paving the way toward compact and efficient free-electron sources in hard-to-reach frequency regimes.

Free-electron radiations prompt numerous advanced applications such as particle detectors, biological imaging, and light sources from microwave to X-ray. While their polarization, directionality, and phase could be shaped with prosperous metamaterials, their natural broad spectra have not been tailored to yield frequency combs, which are state-of-the-art technology in metrology, spectroscopy, and precision frequency synthesis. Here, the frequency comb directly emitted from the free-electron radiation is demonstrated by simultaneously exciting a series of modes with equidistant spectral lines. Both the offset and repetition frequencies are readily adapted by structural parameters, and relative spectrum intensities between the comb teeth are customizable based on selective coupling, thus permitting flexible tunability over broadband. Moreover, the repetition rate is experimentally verified at the microwave regime. The proposed methodology implies that swift electrons present a natural and versatile platform for generating frequency combs, facilitating the development of metrology and spectroscopy in the terahertz band.

Metasurface is a kind of two-dimensional surface artificial material with periodic subwavelength metal or dielectric structure. It can control the amplitude, phase and polarization of electromagnetic wave by using the resonant coupling of surface elements. In this paper, the principle of phase gradient metasurface manipulation of electromagnetic wave is introduced. A reflective metasurface with linear phase gradient is designed and demonstrated by simulation. With appropriate phase gradient arrangement, the abnormal reflection function is realized, and then the metasurface retroreflectors with different phase gradient are constructed to reflect the incident wave in the same direction, which have double-channel and single-channel functions respectively.

Metasurfaces have recently experienced revolutionary progress in sensing and super-resolution imaging fields, mainly due to their manipulation of electromagnetic waves on subwavelength scales. However, on the one hand, the addition of metasurfaces can multiply the complexity of retrieving target information from detected electromagnetic fields. On the other hand, many existing studies utilize deep learning methods to provide compelling tools for electromagnetic problems but mainly concentrate on resolving one single function, limiting their versatilities. In this work, a multifunctional deep learning network is demonstrated to reconstruct diverse target information in a metasurface-target interactive system. First, a preliminary experiment verifies that the metasurface-involved scenario can tolerate the system noises. Then, the captured electric field distributions are fed into the multifunctional network, which can not only accurately sense the quantity and relative permittivity of targets but also generate super-resolution images precisely. The deep learning network, thus, paves an alternative way to recover the targets' information in metasurface-target interactive systems, accelerating the progression of target sensing and superimaging areas. Besides, another new network that allows forward electromagnetic prediction is also proposed and demonstrated. To sum up, the deep learning methodology may hold promise for inverse reconstructions or forward predictions in many electromagnetic scenarios.

Surface waves (SWs) are of great importance in terahertz (THz) photonics applications due to their subwavelength properties. Hence, it is crucial to develop surface wavefront shaping techniques, which is urgent in modern information technologies. In this paper, a new scheme is proposed to realize SW excitation and spin-decoupled wavefront shaping with an ultracompact planar meta-device working in the THz range. The meta-device is composed of two parts: meta-atoms (in the center) and plasmonic metals (on the left and right sides). By carefully setting the geometry size and rotation angle of each meta-atom, the encoded spin-decoupled phase distributions for both left circularly polarized (LCP) and right circularly polarized (RCP) incident THz waves are determined. In this way, circularly polarized (CP) incident THz waves can be converted to SWs propagating along plasmonic metals with unique wavefront profiles, i.e., Bessel and focusing profiles. Full-wave simulations and THz near-field scanning experiments were performed to verify the functionalities of the meta-device, both of which are in great agreement with theoretical predictions. Our findings may provide more solutions to design THz integrated photonic devices and systems.

Recent advances in graphene plasmonics offer numerous opportunities for enabling the design and manufacture of a variety of nanoscale optical devices. Here, a method of designing metagratings and hyperbolic metamaterials based on the geometrical transformation of the proposed equivalent graphene is reported. The physical mechanism underlying this method is the strongly enhanced light-matter interaction of equivalent graphene plasmonics, which can be characterized by effective conductance and remains constant in geometrical transformation. As proof of the method, we design and demonstrate a compact retroreflector that can anomalously reflect the incident wave along its original path utilizing the roll-up plasmonic structure. In addition, an air-medium hyperbolic metamaterial and hyperlens consisting of periodic plasmonic structures are also theoretically predicted and experimentally validated by anomalous diffraction. Compared with the existing approaches of designing these metadevices, the proposed method significantly lowers the requirement on their compositional materials. The concept of equivalent graphene effectively links optical and microwave plasmonics on the basis of the effective conductance model. In this case, the methodology of geometrical transformation can function in both the microwave and nanofields and serves as a platform for designing nanoscale and microwave focusing lenses and diverse on-chip optical wave control devices.

Based on the principle of electron cyclotron maser(ECM),gyrotrons are among the most promising devices to gener-ate powerful coherent terahertz(THz)radiation and play a vital role in numerous advanced THz applications.Unfortunately,THz ECM systems using a conventional high-Q cavity were theoretically and experimentally demonstrated to suffer from strong ohmic losses,and,accordingly,the wave output efficiency was significantly reduced.A scheme to alleviate such a challenging problem is systematically investigated in this paper.The traveling-wave operation concept is employed in a 1-THz third harmonic gyrotron oscillator,which strengthens electron-wave interaction efficiency and reduces the ohmic dissipation,simultaneously.A lossy belt is added in the interaction circuit to stably constitute the traveling-wave interac-tion,and a down-tapered magnetic field is employed to further amplify the forward-wave(FW)component.The results demonstrate that the proportion of ohmic losses is nearly halved,and output efficiency is nearly doubled,which is promising for further advancement of high-power continuous-wave operation of the ECM-based devices.

Cherenkov radiation based on effective surface plasmon polaritons is introduced in this work. The effective surface plasmon polaritons excited by a moving electron bunch in a parallel plate waveguide can be transformed into Cherenkov radiation. The operating frequency can be tuned by adjusting the relative permittivity of the filling medium and the geometry parameters. Moreover, this system has strong robustness. This mechanism defies the limitations of materials and can be achieved at arbitrary frequency ranges by tuning corresponding parameters. This work offers a platform for the study of Cherenkov radiation in waveguide structures and promotes related applications.

The design of a 330-GHz second harmonic gyrotron with broadband tunability is presented in this article. Loaded with a prebunched cavity, the output power is almost doubled (~216 W, corresponding to the output efficiency of 3.6%) compared with a conventional open-cavity scheme, and the tuning range is ~1.85 GHz. To further reveal the feasibility of the prebunching section for the harmonic operations, it is theoretically demonstrated that the phase shift inside the prebunching Section plays an important role in improving the interaction performance. The dependence of the phase shift on the cavity dimension and propagation constant is explored, which instructs the circuit optimization. Moreover, the engineering designs of the magnetron injection gun, the output window, and the collector are presented under a 10-T super-conducting magnet. The overall configuration of the tube is exhibited at the end of the article.

A free-electron-driven terahertz (THz) vortex generator based on Smith-Purcell radiation and Archimedean spiral grating (ASG) is proposed in this paper. The structural parameters are optimized to operate in the THz band. When the electron beam rotates around the singularity of the ASG constrained by the magnetic field, the vortex beam with the value of the topological charge of 1 can be generated above the grating. The vorticity of the vortex beam is determined by the rotation direction of the structure. For instance, if the vorticity of the ASG is anticlockwise, the topological charge of the vortex beam should be +1. The proposed scheme indicates its promising applications in developing compact on-chip THz vortex generators and the next-generation communication technology based on orbital angular momentum multiplexing.

Orbital angular momentum (OAM) is a kind of momentum which is generally carried by electromagnetic waves. Since OAM has the potential to satisfy the demand of channel capacity, it has attracted more and more attention recently. The demand of generating the OAM mode is growing rapidly. In this work, we study the OAM generated by the localized spoof surface plasmons in curved meanderline with a special excitation. As a result, an orbital angular momentum mode with the topological charge being 2 is generated, which may be further extend to higher modes and may provide a potential way for the multiplex communication and object feature detection.

In this paper, two bandpass filters based on localized spoof surface plasmons (LSSPs) are proposed. Magnetic coupling is selected as the scheme to connect the input and output ports with the LSSPs resonator. Simulation results show that the 3-dB bandwidth is 1.44 GHz (24.7%) from 5.10 GHz to 6.54 GHz, centered at 5.82 GHz. Compared with the counterparts coupled by microstrip lines, this work has the advantage of wide bandwidth. Compared with the conventional filters, this work has the advantages of lightness and compactness.

As a kind of artificial electromagnetic device, metasurface is applicable in many areas for its distinctive characteristics. Plasmon-Induced Transparency (PIT) is a typical application of metasurface, with special electromagnetic characteristics such as high Q-factor and slow light effect. However, most of the current work has paid little attention to the ability of nonlinear manipulation of PIT metasurface. Using the excellent photoconductivity effect of MAPbI3 Perovskite, it is possible to design optically controlled nonlinear modulation devices based on a hybrid PIT metasurface and achieve an effective manipulation of terahertz waves. In this paper, a MAPbI3-Gold-Si hybrid metasurface is designed and fabricated to achieve a high Q-factor PIT effect. The experiment implies that the PIT metasurface can be actively and nonlinearly modulated with high efficiency after spin-coating the MAPbI3 film. In addition, the hybrid PIT metasurface is compatible with integrated circuit (IC) technology, hence it may provide a potential way in the region of terahertz communication, active filters, highly sensitive sensors and slow-light devices.

A free-electron-based vortex Smith-Purcell radiation with the higher-order topological charge is proposed in this paper based on uniform helical grating. Due to the spiral constraint of azimuth, the evanescent field of the electron beam will be diffracted and possess the orbital angular momentum. When the wave vectors of the diffraction wave match those of the free space, the vortex beam will be generated with different topological charges. Moreover, the higher-order topological charge can be affirmatively generated by utilizing the higher spatial harmonics according to Bloch's theorem of the helical periodic system. Moreover, the vortex beam with mixed orbital angular momentum can also be achieved. Furthermore, and the mode purities of these vortex beams are analyzed. The proposed scheme may facilitate the journey of the vortex beam from optical into terahertz band.