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A simple synthetic method of poly(hexamethylene guanidine hydrochloride) grafted hollow nano-silica (HSN-PHMG), a novel antibacterial material, was proposed. The water dispersibility and antibacterial property of HSN grafting of PHMG were improved. The hollow structure of nano-silica was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis were used to characterize the successful grafting of PHMG and the mass fraction of PHMG is about 9.5% in HSN-PHMG. Antibacterial test using Escherichia coil (E.coil) and Staphylococcus aureus (S.aureus) as test strains and the results show that the minimum inhibitory concentration (MIC) of HSN-PHMG on E.coil and S.aureus are all 32 mg/L; E.coil can be completely killed within 2 h when the concentration of HSN-PHMG is 64 mg/L.

A simple synthetic method of poly(hexamethylene guanidine hydrochloride) grafted hollow nano-silica (HSN-PHMG), a novel antibacterial material, was proposed. The water dispersibility and antibacterial property of HSN grafting of PHMG were improved. The hollow structure of nano-silica was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis were used to characterize the successful grafting of PHMG and the mass fraction of PHMG is about 9.5% in HSN-PHMG. Antibacterial test using Escherichia coil (E.coil) and Staphylococcus aureus (S.aureus) as test strains and the results show that the minimum inhibitory concentration (MIC) of HSN-PHMG on E.coil and S.aureus are all 32 mg/L; E.coil can be completely killed within 2 h when the concentration of HSN-PHMG is 64 mg/L.

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The rapid rise of two-dimensional (2D) materials has aroused increasing interest in the fields of microelectronics and optoelectronics; various types of 2D van der Waals heterostructures (vdWHs), especially those based on MoS2, have been widely investigated in theory and experiment. However, the interfacial properties of MoS2 and the uncommon crystal surface of traditional three-dimensional (3D) metals are yet to be explored. In this paper, we studied heterostructures composed of MoS2 and metal(001) slabs, based on the first-principles calculations, and we uncovered that MoS2/Au(001) and MoS2/Ag(001) vdWHs reveal Schottky contacts, and MoS2/Cu(001) belongs to Ohmic contact and possesses ultrahigh electron tunneling probability at the equilibrium distance. Thus, the MoS2/Cu(001) heterostructure exhibits the best contact performance. Further investigations demonstrate that external longitudinal strain can modulate interfacial contact to engineer the Schottky–Ohmic contact transition and regulate interfacial charge transport. We believe that it is a general strategy to exploit longitudinal strain to improve interfacial contact performance to design and fabricate a multifunctional MoS2-based electronic device. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this article, the design of a fourth-harmonic gyrotron with 1-kW-level output power at 1 terahertz (THz) is presented. A unique advantage of this design is that, with a well-optimized magnetic-cusp large-orbit electron gun, the designed gyrotron can operate from first-harmonic to fourth-harmonic in multiple discrete bands by varying the confining magnetic field strength. By carefully balancing the competing modes, the gyrotron can be tuned to operate at six candidate modes, including TE1,2 for fundamental harmonic, TE2,3 and TE2,4 for second-harmonic, TE3,5 and TE3,6 for third-harmonic, and TE4,8 for fourth-harmonic interactions. As the main operating mode, the TE4,8 can generate a peak output power of 1.68 kW, with an output efficiency of about 2.1%, and a magnetically controlled frequency tuning range of about 1.5 GHz around 1 THz. The impact of the longitudinal nonuniformity of the cavity at high-harmonic interaction was also studied. It shows that a radial tolerance of several micrometers will significantly elevate the start-oscillation current and deteriorate output performance. This design is to produce a THz source with ultra-wideband tuning capability ranging from the submillimeter-wave to 1-THz bands. [ABSTRACT FROM AUTHOR]

In this study, high‐speed ballistic impact tests were conducted on GH4169 alloy samples with the aeroengine compressor blade leading‐edge feature to simulate the notch‐type foreign object damages (FOD). Macroscopic and microscopic characterization of FOD and high cycle fatigue tests were performed to investigate the effect of FOD depth on GH4169 alloy fatigue strength along with numerical analysis using the Kitagawa–Takahashi diagram. Results show that the incident side of notch‐type FODs is relatively smooth, whereas the exit side is rugged. The FOD depth ranges from 0.18 to 1.33 mm, and the fatigue strength of damaged samples is 37.93% – 97.04% of the undamaged samples. As the FOD depth increases, damage length, material losses, and the stress concentration coefficient of the FOD increase significantly and accompanied by the increasing of adiabatic shear bands, micro voids, and cracks, resulting in fatigue strength reduction. Numerical analysis indicates that the Kitagawa–Takahashi diagram can provide a basic model for the design of FOD tolerance. Highlights: The macro and micro features of FOD notches with varying damage depths were characterized.Grooves at FOD notch bottom may lead to premature initiation of high cycle fatigue cracks.Fatigue strength of FOD samples decreases with the increase of the damage depth.The K‐T diagram can provide a basis for the design of foreign object damage tolerance. [ABSTRACT FROM AUTHOR]

An orbital angular momentum (OAM) antenna based on chiral harmonic diffraction of spoof surface plasmon polaritons (SSPP) is demonstrated in this letter. Due to the diffraction of the helical wire, SSPP is transformed into the high-purity OAM mode. The operation frequency is determined by SSPP, and the topological charge is manipulated by modulating the harmonic order. Additionally, this system achieves a frequency-controlled beam-scanning characteristic in a wide bandwidth from 7.8 to 12.6 GHz, and the measurement data agrees well with the theoretical analysis and the simulation results. Furthermore, the OAM antenna validated in this letter is compact, easy-fabricated, and robust.The underlying principle opens up a new train of thought for generating OAM mode and paves the versatile applications involving wireless communication. [ABSTRACT FROM AUTHOR]

High-quality axis-encircling electron beam is essential to drive a high-harmonic gyrotron. A magnetic cusp gun that can generate a low-spread axis-encircling electron beam at an extremely high magnetic-compression ratio was developed for a 1-THz fourth-harmonic gyrotron toward continuous-wave operation. The optimized gun could generate an axis-encircling electron beam with 80-kV voltage, 1-A current, and a nearly constant pitch factor of about 1.5 in a wide range of magnetic field strength. The transverse velocity spread was less than 2.1% and the guiding center radius was $26.6 ~\mu \text{m}$. The design was verified by using MAGIC, EGUN, and CST simulations and good agreements were achieved. [ABSTRACT FROM AUTHOR]

Terahertz (THz) vortex beams carrying orbital angular momentum (OAM) with high purity and tunable topological charge (TC) will undoubtedly bring extraordinary capacities for advanced imaging or communication systems. We propose a convenient and efficient method to generate a tunable and broadband vortex beam that is excited by superradiant Smith-Purcell radiation (SSPR) on a helical grating. This scheme fully explores the advantages of natural broadband evanescent wave carried by the electron beam and the Bloch's theorem regulated helical periodic systems. An explicit relation is established between the index of the spatial harmonic wave on the grating and the topological charge of the vortex beam. The electron energy in the SSPR can be customized to ignite the specific spatial harmonics and manipulate the OAM beam, accordingly. The separated radiation region also promises the high purity of the OAM spectrum. The harmonic excitation reduces the communality between the wavelength and the device size and alleviates the difficulty of device fabrication in the THz band. The proposed vortex scheme can not only be handily scaled to microwave and mid-infrared regions, but also bring possibilities to applications based on compact tunable vortex beam sources. [ABSTRACT FROM AUTHOR]

A three-port plasmonic array is proposed to achieve a dual-field superfocusing using the forward-propagation method based on spatial Fourier transform. The algorithm of fast Fourier transform is used to facilitate the design process. Both the electric and magnetic fields can be superfocused on the focal plane of z = 0.1λ (λ is the free-space wavelength) based on an effect of radiationless electromagnetic interference. The full widths of the half-maximum of both two fields are 0.060λ and 0.072λ, respectively. Besides, the metrics of the dual-field superfocusing are 1.67 and 1.39. The proposed theory can simplify the design process of superfocusing, and the plasmonic array with brief three ports can sufficiently facilitate the construction of the desired source. This letter can be beneficial for the development of beam steering, sophisticated holography, super-resolution imaging, and other applications of near- or far-field devices. [ABSTRACT FROM AUTHOR]

A wideband terahertz (THz) output network consisting of a pyramidal horn and a diamond Brewster window has been developed for 0.7–1.0-THz traveling-wave tube (TWT) amplifier and is measured at 0.325–0.5 THz owing to the limitation of the measurement facility. A pyramidal horn is designed to convert the waveguide mode into a high-purity quasi-plane polarized mode to meet the demand of the Brewster window. A diamond Brewster window plate with low THz transmission loss is fabricated via chemical vapor deposition (CVD). The plate is directly placed on the pyramidal horn with a Brewster angle. A circular window plate supporter is intended to balance the stress distribution during diamond-metal brazing. The measurement shows transmission coefficient is better than 0.85 within a wide bandwidth of 0.325–0.5 THz. This letter provides a practical way for a single diamond window structure for THz TWT amplifiers. [ABSTRACT FROM AUTHOR]

A 1 bit single-polarization broadband space-time-coding digital metasurface element with a zero cross polarization is proposed. This element consists of a modified patch, an electronic switch, a dc-bias network, a ground plane, and two-layer dielectrics. The upper patch with edge loading and centered square hole is fully optimized to achieve broadband and stable phase difference between two states. By electronically controlling the diode, the element achieves a 1 bit phase shift with a relative bandwidth of 20% within 180° ± 30° from 7.71 to 9.43 GHz. Moreover, a negligible cross polarization is obtained due to the uniaxial symmetry of the structure, and the amplitude loss is less than 0.8 dB for two states. The experimental results measured in a rectangular waveguide are well matched with the simulations, further demonstrating validity of the proposed element. [ABSTRACT FROM AUTHOR]

The dispersion characteristics of electromagnetic waves have led to a wide range of applications, such as pulse compression, superluminal propagation, wave tunneling effect, and electron–wave interaction. The near‐cutoff wave inside a guiding system is strong dispersive. When the wave frequency reaches the terahertz band, this strong dispersive property is extremely sensitive to the losses induced by the limited conductivity of the component. This phenomenon is traditionally investigated through perturbation of boundary conditions in the frequency domain, which does not deal well with broad‐spectrum signals. Two simple and efficient time‐domain schemes to study the strong dispersive propagation of terahertz waves are proposed. One is based on relatively concise principles, called the constant frequency method. The second is the recursive convolution method, which is suitable for dealing with a wave with wider spectrum range. The reliability of both approaches is verified by the comparison of the dispersion curves. Using the proposed methods, the pulse compression of terahertz wave inside a strong dispersive waveguide is verified in principle. [ABSTRACT FROM AUTHOR]

In this paper, a displacement sensor with an electrically extremely small size and high sensitivity is proposed based on an elaborately designed metamaterial element, i.e., coupled split-ring resonators (SRRs). The sensor consists of a feeding structure with a rectangular opening loop and a sensing structure with double-layer coupled SRRs. The movable double-layer structures can be used to measure the relative displacement. The size of microwave displacement sensors can be significantly reduced due to the compact feeding and sensing structures. By adjusting the position of the split gap within the resonator, the detection directions of the displacement sensing can be further expanded accordingly (along with the x- or y-axis) without increasing its physical size. Compared with previous works, the extremely compact size of 0.05λ0 × 0.05λ0 (λ0 denotes the free-space wavelength), a high sensitivity, and a high quality factor (Q-factor) can be achieved by the proposed sensor. From the perspective of the advantages above, the proposed sensor holds promise for being applied in many high-precision industrial measurement scenarios. [ABSTRACT FROM AUTHOR]

The rapid rise of two-dimensional (2D) materials has aroused increasing interest in the fields of microelectronics and optoelectronics; various types of 2D van der Waals heterostructures (vdWHs), especially those based on MoS 2 , have been widely investigated in theory and experiment. However, the interfacial properties of MoS 2 and the uncommon crystal surface of traditional three-dimensional (3D) metals are yet to be explored. In this paper, we studied heterostructures composed of MoS 2 and metal(001) slabs, based on the first-principles calculations, and we uncovered that MoS 2 /Au(001) and MoS 2 /Ag(001) vdWHs reveal Schottky contacts, and MoS 2 /Cu(001) belongs to Ohmic contact and possesses ultrahigh electron tunneling probability at the equilibrium distance. Thus, the MoS 2 /Cu(001) heterostructure exhibits the best contact performance. Further investigations demonstrate that external longitudinal strain can modulate interfacial contact to engineer the Schottky-Ohmic contact transition and regulate interfacial charge transport. We believe that it is a general strategy to exploit longitudinal strain to improve interfacial contact performance to design and fabricate a multifunctional MoS 2 -based electronic device.

The vortex beam provides a promising alternative for next-generation wireless communication, but it is a long-standing challenge to generate a multi-mode and robust vortex beam. In this Letter, a multi-mode vortex beam emitter is introduced and experimentally verified based on spoof surface plasmon polaritons (SSPP). The SSPP on a helical grating carries multi-mode orbital angular momentum and can be converted into a high-purity vortex beam via the diffraction of a ring array. The operation frequency and topological charge are determined by that of the SSPP. This emitter can achieve the function of beam-scanning in each radiation band. The beam-scanning and vortex characteristics are experimentally verified. The designed emitter is compact and robust, and we are confident that this work will have great application prospects in communication systems.

A broadband vortex beam generator provides a promising solution for various applications. Since the space-charge wave of the free-electron bunch inherently covers a wide frequency range, the free-electron-driven devices can be utilized to generate broadband radiation. This work presents a wideband tunable multi-mode vortex beam generator based on the Smith-Purcell radiation (SPR) from a helical grating. The generated broadband vortex radiation presents prominent flexibility in the manipulation of frequency, topological charge, and radiation direction. The frequency range and the topological charge depend on the operating harmonic order, operating voltage, and structural parameters. The radiation direction varies with the operating frequency and covers a wide range in each band. Moreover, the proposed principle can be scaled to the terahertz frequency band. This work advances the application of the vortex beam in the millimeter wave-terahertz communication system.

Recently, ultrathin localized spoof surface plasmon (LSSP) resonators are found to have intrinsic defects of relatively low quality factors (Q-factors) because of unavoidable material and radiation losses. In this paper, multilayer structures of planar-circular-grating resonators and their magnetic-coupling schemes are proposed to achieve effective excitation of high-Q LSSPs modes. By adopting the multilayer structures with air between the layers, the power dissipation effected by both material and radiation losses is significantly suppressed. Experimental results show that the Q-factors could reach more than 200 and the excitation efficiencies could reach more than 90%. Numerical simulations show the distribution of the electromagnetic field and illustrate the principle of magnetic coupling. Besides, the Q-factors of resonators with different structural parameters were measured and analyzed. This study aims to provide some inspirations on planar gyro-devices and to improve the performance of existing applications, such as sensors and filters.