Science and technologies based on terahertz frequency electromagnetic radiation (100 GHz–30 THz) have developed rapidly over the last 30 years. For most of the 20th Century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to 'real world' applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2017, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 18 sections that cover most of the key areas of THz science and technology. We hope that The 2017 Roadmap on THz science and technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies.

The 2016 terahertz science and technology roadmap

The Child–Langmuir Law (CL), discovered a century ago, gives the maximum current that can be transported across a planar diode in the steady state. As a quintessential example of the impact of space charge shielding near a charged surface, it is central to the studies of high current diodes, such as high power microwave sources, vacuum microelectronics, electron and ion sources, and high current drivers used in high energy density physics experiments. CL remains a touchstone of fundamental sheath physics, including contemporary studies of nanoscale quantum diodes and nano gap based plasmonic devices. Its solid state analog is the Mott–Gurney law, governing the maximum charge injection in solids, such as organic materials and other dielectrics, which is important to energy devices, such as solar cells and light emitting diodes. This paper reviews the important advances in the physics of diodes since the discovery of CL, including virtual cathode formation and extension of CL to multiple dimensions, to the ...

100 Years of the Physics of Diodes

Ab initio Quantum-Mechanical methods are well-established tools for material characterization and discovery in many technological areas. Recently, state-of-the-art approaches based on density-functional theory and many-body perturbation theory were successfully applied to semiconducting alkali antimonides and tellurides, which are currently employed as photocathodes in particle accelerator facilities. The results of these studies have unveiled the potential of ab initio methods to complement experimental and technical efforts for the development of new, more efficient materials for vacuum electron sources. Concomitantly, these findings have revealed the need for theory to go beyond the status quo in order to face the challenges of modeling such complex systems and their properties in operando conditions. In this review, we summarize recent progress in the application of ab initio many-body methods to investigate photocathode materials, analyzing the merits and the limitations of the standard approaches with respect to the confronted scientific questions. In particular, we emphasize the necessary trade-off between computational accuracy and feasibility that is intrinsic to these studies, and propose possible routes to optimize it. We finally discuss novel schemes for computationally-aided material discovery that are suitable for the development of ultra-bright electron sources toward the incoming era of artificial intelligence.

Ab Initio Quantum-Mechanical Predictions of Semiconducting Photocathode Materials

A pseudospark (PS) discharge can generate an electron beam with a high current density. The electron beam can be self-focused by an ion channel and transported over a long distance without the need for an external magnetic field. Such features make it attractive to drive millimeter-wave/THz interaction circuits for the generation of high-power radiation from a compact device. This paper presents the experimental results on the generation and transportation of the PS-sourced beam with different cross-sections, as well as the differences of the PS-sourced beam with and without post acceleration. Its application in millimeter-wave/THz sources was demonstrated by the operation of extended interaction oscillators (EIOs) at different frequencies is presented.

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Pseudospark-sourced beam and its application in high-power millimeter-wave generation

High photocurrent density cathodes that enable small cross-section electron beams are required for high-power terahertz vacuum devices. Multi-alkali antimonide photocathodes may be well suited for generating sub-mm electron beam sources. This paper involves the repeatability, stability, uniformity, and linearity experiments of the multi-alkali antimonide photocathodes electron emission operations under a continuous-wave 450 nm laser with a bias voltage of 5000 V. The effect of heat, electric contact, and cathode surface roughness to emission characterizations is analyzed. The methods to maintain the high-current-density emission and avoid the fatigue of the photocathode are verified. The emission can be repeated with increased optical power. The stable photocurrent density of near 1 A/cm2 and maximum current density of near 1.43 A/cm2 is recorded. The continuous photocurrent density is significantly improved compared to the current density reported in traditional applications. It is found that the current curves measuring at different areas of the photocathode differ greatly after the laser power of 800 mW. The increase in current for some areas may be attributed to the conductive current caused by built-in electric fields between the emission rough area and its adjacent areas.

High Photocurrent Density and Continuous Electron Emission Characterization of a Multi-Alkali Antimonide Photocathode

High current density photocathode for CW terahertz photoconductive vacuum devices

Embodiments of the invention disclose a terahertz vacuum triode and a manufacturing method thereof. The terahertz vacuum triode includes an anode, a vacuum channel layer and a photocathode; the vacuumchannel layer includes a first insulating material layer, a gate control electrode and a second insulating material layer; the vacuum channel layer is provided with a vacuum channel; the vacuum channel penetrates the first insulating material layer, the gate control electrode and the second insulating material layer; the gate control electrode is arranged between the first insulating material layer and the second insulating material layer; the photocathode and the anode are arranged on the two ends of the vacuum channel; the first insulating material layer is arranged between the gate controlelectrode and the anode; the second insulating material layer is arranged between the gate control electrode and the photocathode; and sealing cavities are formed between the photocathode, the vacuumchannel layer and the anode. Therefore, the service life of the terahertz vacuum triode can be prolonged.

Terahertz vacuum triode and manufacturing method thereof

The embodiment of the invention discloses an array type terahertz vacuum triode device and a manufacturing method thereof. The array type terahertz vacuum triode device comprises a substrate and a plurality of terahertz vacuum triodes which are arranged on the substrate in an array mode. Each terahertz vacuum triode comprises an anode; a vacuum channel layer and a photocathode, the anode is arranged on the substrate; the vacuum channel layer comprises a first insulating material layer; a gate control electrode and a second insulating material layer, the vacuum channel penetrates through the first insulating material layer, the gate control electrode and the second insulating material layer; the gate control electrode is arranged between the first insulating material layer and the second insulating material layer, the photocathode and the anode are arranged at the two ends of the vacuum channel, the first insulating material layer is arranged between the gate control electrode and the anode, and the second insulating material layer is arranged between the gate control electrode and the photocathode. According to the array type terahertz vacuum triode device and the manufacturing method thereof provided by the embodiment of the invention, the service life of the array type terahertz vacuum triode device is prolonged.

Array type terahertz vacuum triode device and manufacturing method thereof

Embodiments of the invention disclose a terahertz vacuum diode and a manufacturing method thereof. The terahertz vacuum diode includes a photocathode, a vacuum channel layer and an anode; the vacuum channel layer is provided with a vacuum channel; the vacuum channel penetrates the vacuum channel layer; the photocathode and the anode are arranged on the two ends of the vacuum channel; sealing cavities are formed between the photocathode, the vacuum channel layer and the anode; the vacuum channel is in a circular truncated cone shape; and the contact area of the photocathode and the vacuum channel is less than that of the anode and the vacuum channel. A novel terahertz vacuum electronic device can be provided through the provided terahertz vacuum diode and the manufacturing method thereof.

Dai Jun

Papers

High Photocurrent Density and Continuous Electron Emission Characterization of a Multi-Alkali Antimonide Photocathode

High current density photocathode for CW terahertz photoconductive vacuum devices

Terahertz vacuum triode and manufacturing method thereof

Array type terahertz vacuum triode device and manufacturing method thereof

Terahertz vacuum diode and manufacturing method thereof