Progress in Research On Electron Acceleration Of Terahertz Waves

Recently, the team of Li Ruxin, Tian Ye and Song Liwei from Shanghai Institute of Optics and Precision Machinery (SIPM) of the Chinese Academy of Sciences (CAS) has made important progress in the field of electron acceleration at terahertz waves. Based on the new generation of ultra-intense ultra-short pulse laser integrated experimental device of SIPM, the team utilized ultra-intense ultra-short laser to drive the silk waveguide to generate millijoule-level terahertz surface waves, and used the surface waves for electron acceleration, which solved the problems of high-energy terahertz wave generation as well as the low efficiency of free-space terahertz wave-to-waveguide energy coupling. The study integrates the generation, transmission and coupling of terahertz wave into waveguide, and realizes the highest 1.1 MeV electron energy gain and 210 MV/m average acceleration gradient at a distance of 5 mm in the waveguide, which is nearly an order of magnitude higher than the current world record of electron energy gain for terahertz wave acceleration and opens up a brand-new pathway for the research of all-optical integrated electron gas pedal.
The miniaturized and integrated electron gas pedal will promote its application in frontier science and technology. The use of terahertz wave-driven electron acceleration, as an emerging acceleration technology developed in the last decade, can provide higher acceleration gradients than traditional RF acceleration, and is one of the reliable ways to realize miniaturized, low-cost acceleration devices, which is expected to extend the use of gas pedals to more application scenarios, including small-scale laboratories, hospitals, and so on.
The current development of terahertz electron acceleration is based on free-space terahertz source technology. Terahertz waves are generated, collected, transmitted, polarization converted, and then focused onto a waveguide structure used to accelerate the electrons. Experimentally, in order to maximize the terahertz acceleration gradient inside the waveguide, a terahertz source is required to provide enough energy to compensate for the energy losses from scattering, reflection, and mode conversion in the optical path. Common terahertz sources, such as those based on optical crystals, usually require the collection and guiding of terahertz radiation through optical elements and mode conversion through segmented waveplates or phase-shifted plates, which inevitably results in energy loss. Compared with terahertz radiation in free space, optical surface waves bound to the surface of a medium, such as surface plasmon polaritons (SPPs), provide a completely new way of thinking about terahertz guidance and mode conversion.
The team's long-term exploration in the fields of miniaturized laser accelerated electron sources and radiation light sources has led to the discovery of a coherent amplification mechanism for terahertz surface plasmon polaritons, which enables the realization of high-power surface plasmon polaritons coherent radiation sources. Based on the Sommerfeld wave property of terahertz surface iso-polarized excitations on axisymmetric metallic cylindrical waveguide, and on the low-dispersion fundamental transverse magnetic (TM) modes, the team further coupled this high-power terahertz surface iso-polarized excitations directly to the accelerating waveguide, and achieved 85% coupling efficiency, which can effectively couple the millijoule-level terahertz energy generated by femtosecond laser pumping the metallic cylindrical waveguide with the electron beam, and Ultimately in the 5mm length of the electron to obtain the highest 1.1 MeV energy gain and 210 MV / m of the average acceleration gradient, will be the current international terahertz wave-driven electron energy gain of the best results to enhance the nearly an order of magnitude.
In the future, the team will further develop the integrated all-optical electron acceleration technology based on this new scheme of terahertz surface wave mode-driven electron acceleration, and expand its cross-applications in the fields of small-scale radiation source and material detection.
The relevant research results were published in Nature Photonics under the title Megaelectronvolt electron acceleration driven by terahertz surface waves. The research was done in collaboration with Shanghai Institute of Optical Machinery, Beijing University of Aeronautics and Astronautics and Zhangjiang Laboratory. The research was supported by the National Key Research and Development Program of China, the Strategic Pilot Project of the Chinese Academy of Sciences (Class B), the Shanghai Basic Research Special Zone Program, the National Natural Science Foundation of China, the Young Innovators Association of the Chinese Academy of Sciences, and the Shanghai Science and Technology Inspiration Star Sail Program.

Figure 1. Schematic diagram of terahertz surface wave-driven electron acceleration experiment.

Figure 2: Experimentally measured maximum electron energy gain results

Figure 3. Comparison of the electric field strength inside the accelerating waveguide (c) in the terahertz-coupled state of free space (a) and metal cylindrical waveguide (b)