In this paper, a novel V-shaped microstrip meander-line slow-wave structure (SWS) is proposed for use in a low-voltage high-efficiency wide-bandwidth miniature millimeter-wave traveling-wave tube (TWT). The electromagnetic characteristics and the interaction between the sheet electron beam and slow wave in this SWS are obtained by utilizing the CST Microwave Studio and Particle Studio codes, respectively. From our calculations, it is predicted that, at a beam voltage of 3.7 kV and a beam current of 100 mA, an output power greater than 30 W can be obtained ranging from 75 to 100 GHz, and this V-shaped microstrip meander-line TWT will be helpful for a W-band millimeter-wave power module.

A Novel V-Shaped Microstrip Meander-Line Slow-Wave Structure for W-band MMPM

Slow-wave structure (SWS) is a core part of a traveling-wave-tube (TWT) amplifier. In this letter, the planar microstrip meander SWS on a dielectric substrate at $V$ -band (50–70 GHz) is studied. The basic electromagnetic parameters of the SWS are calculated. The SWS circuits are designed and fabricated, and good transmission characteristics are measured. The SWS developed is characterized by a wide bandwidth and a relatively-large slow-wave factor, and is suitable for a low-voltage TWT with the sheet electron beam.

Planar Microstrip Slow-Wave Structure for Low-Voltage V-Band Traveling-Wave Tube With a Sheet Electron Beam

In this paper, we propose and demonstrate a switchable terahertz metamaterial absorber with broadband and multi-band absorption based on a simple configuration of graphene and vanadium dioxide (VO2). The switchable functional characteristics of the absorber can be achieved by changing the phase transition property of VO2. When VO2 is insulating, the device acts as a broadband absorber with absorbance greater than 90% under normal incidence from 1.06 THz to 2.58 THz. The broadband absorber exhibits excellent absorption performance under a wide range of incident and polarization angles for TE and TM polarizations. Moreover, the absorption bandwidth and intensity of the absorber can be dynamically adjusted by changing the Fermi energy level of graphene. When VO2 is in the conducting state, the designed metamaterial device acts as a multi-band absorber with absorption frequencies at 1 THz, 2.45 THz, and 2.82 THz. The multi-band absorption is achieved owing to the fundamental resonant modes of the graphene ring sheet, VO2 hollow ring patch, and coupling interaction between them. Moreover, the multi-band absorber is insensitive to polarization and incident angles for TE and TM polarizations, and the three resonance frequencies can be reconfigured by changing the Fermi energy level of graphene. Our designed device exhibits the merits of bi-functionality and a simple configuration, which is very attractive for potential terahertz applications such as intelligent attenuators, reflectors, and spatial modulators.

Switchable and tunable terahertz metamaterial absorber with broadband and multi-band absorption

A design study for a low-voltage, high-efficiency, and wide-bandwidth W-band traveling-wave tube using a symmetric double V-shaped microstrip meander-line slow-wave structure combined with a sheet electron beam is described in this paper. The electromagnetic characteristics including the dispersion characteristics, interaction impedance, and transmission characteristics of this structure are presented, and the beam-wave interaction is calculated using particle-in-cell algorithms. Our study shows that, when the design voltage and current of the sheet electron beam are set to 4570 V and 100 mA, respectively, this miniature millimeter-wave power amplifier is capable of delivering several tens of watts output power, and the peak output power is about 110 W with a corresponding gain of 31.4 dB and an averaged electronic efficiency of 12% at 94 GHz.

Symmetric Double V-Shaped Microstrip Meander-Line Slow-Wave Structure for W-Band Traveling-Wave Tube

The emerging role of photoacoustic imaging in clinical oncology

In this paper, a nested dual-arm spiral antenna (NDASA) is proposed to generate multiple orbital angular momentum (OAM) modes at the same operating frequency. The NDASA consists of the feeding network and three nested dual-arm spiral structures with different sizes. Meanwhile, a metal reflector is located between the spiral structures and the feeding network to enhance the radiation directivity, which can also reduce the impact of the radio frequency (RF) coaxial cables. Both the experimental and simulated results show OAM waves with topological charges of $l = 1$ , $l = 3$ and $l = 5$ can be independently generated at $f = 3$ GHz, and their maximum gains are 6.94dBi, 6.76dBi and 5.49dBi, respectively. In addition, the OAM modes multiplexing and demultiplexing technology are investigated in simulations and experiments. The isolations between the OAM modes of $l = 1$ , $l = 3$ and $l = 5$ and the other different OAM modes are greater than 13.8dB, 13.2dB and 25.1dB, respectively, which proves the potential ability of the proposed NDASA for multiplexing and demultiplexing multiple OAM modes.

/pdf/generating-multiple-oam-based-on-a-nested-dual-arm-spiral-8mw6lypsug.pdf

Generating Multiple OAM Based on a Nested Dual-Arm Spiral Antenna

With balanced spatial resolution, imaging depth, and functional sensitivity, photoacoustic tomography (PAT) hold great promise for human brain imaging. However, the strong acoustic attenuation and aberration of the human skull (∼8 mm thick) are longstanding technical challenges for PAT of the human brain. In this work, we numerically investigated the impacts of the stratified human skull on photoacoustic wave propagation (i.e., the forward model) and PAT image formation (i.e., the inverse model). We simulated two representative transcranial PAT implementations: photoacoustic computed tomography (PACT) and photoacoustic macroscopy (PAMac). In the forward model, we simulated the detailed photoacoustic wave propagation from a point or line source through a digital human skull. The wave attenuation, refraction, mode conversation, and reverberation were thoroughly investigated. In the inverse model, we reconstructed the transcranial PACT and PAMac images of a point or line target enclosed by the human skull. Our results demonstrate that transcranial PAMac suffers mainly from wave reverberation within the skull, leading to prolonged signal duration and reduced axial resolution. Transcranial PACT is more susceptible to the skull’s acoustic distortion, mode conversion, and reverberation, which collectively lead to strong image artifacts and deteriorated spatial resolutions. We also found that PACT with a ring-shaped transducer array shows more tolerance of the skull’s adverse impacts and can provide more accurate image reconstruction. Our results suggest that incorporating the skull’s geometry and acoustic properties can improve transcranial PAT image reconstruction. We expect that our results have provided a more comprehensive understanding of the acoustic impact of the human skull on transcranial PAT.

Acoustic impact of the human skull on transcranial photoacoustic imaging

A study on a miniature, low voltage, wide bandwidth, and high RF efficiency V-shaped microstrip meander-line slow-wave structure is carried out in this paper. The fabrication of this structure is very convenient by utilizing the 2-D micro-fabrication techniques. Radio-frequency characteristics including dispersion properties, interaction impedance and S-parameters are analyzed. Based on these results, nonlinear large signal performances of the V-shaped microstrip meander-line traveling-wave tube are calculated by 3-D particle-in-cell code. This study shows that this miniature millimeter-wave power amplifier is capable of delivering several tens of watts output power ranging from 127 GHz to 145 GHz.

140-GHz V-Shaped Microstrip Meander-Line Traveling Wave Tube

Several novel spin-dependent bi-functional metasurfaces consisting of different-sized rotary silicon nanobricks have been proposed and numerically investigated based on the Pancharatnam-Berry phase and structural phase simultaneously. Here, a transmission mechanism is strictly deduced, which can avoid crosstalk from the multiplexed bi-functional metasurface. Four kinds of high-efficiency bi-functional devices have been designed successfully at infrared wavelengths, including a spin-dependent bi-functional beam deflector, a spin-dependent bi-functional metalens, a bi-functional metasurface with spin-dependent focusing and deflection function, and a spin-dependent bi-functional vortex phase plate. All of the results demonstrate the superior performances of our designed devices. Our work opens up new doors toward building novel spin-dependent bi-functional metasurfaces, and promotes the development of bi-functional devices and spin-controlled photonics.

https://www.mdpi.com/2079-4991/9/12/1744/pdf

High-Efficiency and Broadband Near-Infrared Bi-Functional Metasurface Based on Rotary Different-Size Silicon Nanobricks.

Study on U-shaped microstrip meander-line slow-wave structure for a low voltage, wide bandwidth millimeter traveling-wave tube is presented. The electromagnetic characteristics and the sheet beam-wave interaction of this structure are carried out. The simulation results predicts that this millimeter-wave power amplifier is capable of delivering hundreds of watts output power in the frequency range of 29–38 GHz, and the peak power is about 200 watts with the correspond-ding gain of 33 dB at 35 GHz.

Fei Shen

Papers

Generating Multiple OAM Based on a Nested Dual-Arm Spiral Antenna

Acoustic impact of the human skull on transcranial photoacoustic imaging

140-GHz V-Shaped Microstrip Meander-Line Traveling Wave Tube

High-Efficiency and Broadband Near-Infrared Bi-Functional Metasurface Based on Rotary Different-Size Silicon Nanobricks.

U-shaped microstrip meander-line slow-wave structure for Ka-band traveling-wave tube