Continuous-wave Y-band planar BWO with wide tunable bandwidth

A piecewise sine waveguide for terahertz traveling wave tube

In this paper, a piecewise sine waveguide (PWSWG) is proposed as the slow-wave structure (SWS) to develop high-power terahertz (THz) traveling wave tubes (TWTs). The PWSWG is an improvement over the rectangular waveguide wherein its two E-planes simultaneously oscillate up and down along the longitudinal direction. The oscillation curve in the H-plane is a piecewise sine curve formed by inserting line segments into the peaks and troughs of the sine curve. The simulation analysis and experimental verification show that the PWSWG offers the advantages of large interaction impedance and excellent electromagnetic transmission performance. Furthermore, the calculation results of beam–wave interaction show that the TWT based on PWSWG SWS can generate a radiated power of 253.1 W at the typical frequency of 220 GHz, corresponding to a gain of 37.04 dB and an interaction efficiency of 6.92%. Compared with the conventional SWG TWTs, the PWSWG TWT has higher interaction efficiency and shorter saturation tube length. In conclusion, the PWSWG proposed in this paper can be considered a suitable SWS for high-power THz radiation sources.

Self-Winding Helices as Slow-Wave Structures for Sub-Millimeter Traveling-Wave Tubes

We present a transformative route to obtain mass-producible helical slow-wave structures for operation in beam-wave interaction devices at THz frequencies. The approach relies on guided self-assembly of conductive nanomembranes. Our work coordinates simulations of cold helices (i.e., helices with no electron beam) and hot helices (i.e., helices that interact with an electron beam). The theoretical study determines electromagnetic fields, current distributions, and beam-wave interaction in a parameter space that has not been explored before. These parameters include microscale diameter, pitch, tape width, and nanoscale surface finish. Parametric simulations show that beam-wave interaction devices based on self-assembled and electroplated helices will potentially provide gain-bandwidth products higher than 2 dBTHz at 1 THz. Informed by the simulation results, we fabricate prototype helices for operation as slow-wave structures at THz frequencies, using metal nanomembranes. Single and intertwined double helices, as well as helices with one or two chiralities, are obtained by self-assembly of stressed metal bilayers. The nanomembrane stiffness and built-in stress control the diameter of the helices. The in-plane geometry of the nanomembrane determines the pitch, the chirality, and the formation of single vs intertwined double helices.

Free-electron-driven beam-scanning terahertz radiation

A beam-scanning terahertz (THz) radiation mechanism in a free-electron-driven grating system is proposed for THz applications. By loading a period-asynchronous rod array above the grating, the spoof surface plasmon (SSP) originally excited by the electron changes its radiation characteristics owing to the rod-induced Brillouin zone folding effect. The rod array functions as an antenna and converts the SSP into a spatial coherent THz radiation. The radiation frequency and direction can be precisely controlled by the electron energy. The field intensity of the radiation is increased approximately 20 times compared with that of the conventional Smith-Purcell radiation in the same frequency range. In addition, a microwave-band scaling prototype is fabricated and the frequency-controlled radiation is measured. Excellent agreement between the experimental and simulated results is obtained. This study paves the way for the development of on-chip THz sources for advanced communication and detection applications.

Nickel stamp origination from generic SU-8 nanostructure arrays patterned with improved thermal development and reshaping

In this study, we show that rapid, reliable, and scalable custom-input colour patterning and eye-readable data storage can be achieved through high-throughput nanoimprinting-exposure-thermal-treatment (NETT) and thermal development and reshaping (TDR) techniques. The main impediment for commercial realization of high-resolution metasurfaces using NETT and TDR is the cost and speed of stamp origination as well as the quality and durability of the fabricated stamp. In order to accelerate the patterning process, lower the fabrication costs, and obtain patterns with high-resolution, we introduce and optimize a new method for origination of durable Ni stamps by electroplating on an SU-8 master fabricated according to custom-input colour patterns via NETT and TDR. In these processes, laser exposure is used to locally activate the generic RGB pixels fabricated on SU-8 via thermal nanoimprint lithography (NIL), according to the custom design. Upon TDR treatment, the exposed regions crosslink while the unexposed areas flatten. TDR is optimized to find the fastest processing condition that results in minimum nanocone height reduction and maximum diffraction efficiency. AFM results show that the TDR-processed nanocones in all red, green, and blue subpixels witness minimal shrinkage in comparison with the corresponding as-imprinted RGB pixels. Among three different sets of direct baking and ramping temperature TDR experiments, direct 55 °C-10 min TDR is found to be the optimal recipe. As a proof-of-concept, the originated stamp was employed to replicate colour images on PET and glass substrates using UV-thermal NIL. The reproduced colour image, photographed at pre-defined lighting and viewing angles, bears vivid diffractive colours with different RGB ratios that are in good match with the custom-input image. Furthermore, the red, green, and blue diffraction peaks from the TDR-55 °C-baked sample exhibit either trivial or no distinguishable difference as compared to the corresponding peaks in the as-imprinted sample.

A megawatt-level surface wave oscillator in Y-band with large oversized structure driven by annular relativistic electron beam

High power vacuum electronic devices of millimeter wave to terahertz regime are attracting extensive interests due to their potential applications in science and technologies. In this paper, the design and experimental results of a powerful compact oversized surface wave oscillator (SWO) in Y-band are presented. The cylindrical slow wave structure (SWS) with rectangular corrugations and large diameter about 6.8 times the radiation wavelength is proposed to support the surface wave interacting with annular relativistic electron beam. By choosing appropriate beam parameters, the beam-wave interaction takes place near the π-point of TM₀₁ mode dispersion curve, giving high coupling impedance and temporal growth rate compared with higher TM_0n modes. The fundamental mode operation of the device is verified by the particle-in-cell (PIC) simulation results, which also indicate its capability of tens of megawatts power output in the Y-band. Finally, a compact experimental setup is completed to validate our design. Measurement results show that a terahertz pulse with frequency in the range of 0.319-0.349 THz, duration of about 2 ns and radiation power of about 2.1 MW has been generated.