Scientists have increased the energy of terahertz waves to accelerate electrons by nearly an order of magnitude

On July 13, Nature Photonics published the important progress made by the team of Li Ruxin, academician of the Chinese Academy of Sciences and researcher of the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences (hereinafter referred to as the Shanghai Institute of Optics and Mechanics), in the field of terahertz wave electron acceleration.

Based on the new generation of ultra-strong ultra-short pulse laser comprehensive experimental device of Shanghai Institute of Optics and Mechanics, the team uses ultra-strong ultrashort laser to drive the silk waveguide to generate millijoule-level terahertz surface waves, and uses surface waves for electron acceleration, which solves the problems of high-energy terahertz wave generation and low coupling efficiency of free-space terahertz wave-to-waveguide energy.


Illustration of a terahertz surface wave-driven electron acceleration experiment. Photo courtesy of Shanghai Optics Machinery

The researchers integrated the generation, transmission and coupling of terahertz waves into the waveguide, and achieved an energy gain of up to 1.1 megaelectron volts and an average acceleration gradient of 210 megavolts per meter at a distance of 5 mm in the waveguide, which is nearly an order of magnitude higher than the current world record for energy gain of accelerated electrons with terahertz waves, and opens up a new way for all-optical integrated electron accelerator research.

Miniaturized and integrated electron accelerators will greatly promote their wide application in cutting-edge science and technology. The use of terahertz waves to drive electron acceleration as an emerging acceleration technology developed in the past decade can provide a higher acceleration gradient than traditional RF acceleration, which is one of the reliable ways to achieve miniaturized and low-cost acceleration devices, and is expected to promote the application of accelerators to more application scenarios including small laboratories and hospitals.

The current development of terahertz electron acceleration free space based terahertz source technology, after terahertz wave generation, after collection, transmission, polarization conversion, and then focused to the waveguide structure used to accelerate electrons. Experimentally, in order to maximize the terahertz acceleration gradient inside the waveguide, the terahertz source is required to provide enough energy to compensate for the energy loss of scattering, reflection, and mode conversion in the optical path. Common terahertz sources, such as terahertz radiation based on optical crystals, usually need to be collected and guided by optical components, and mode conversion is carried out by segmented waveplates or phase-shifters, inevitably resulting in energy loss. Compared with the terahertz radiation in free space, optical surface waves bound to the surface of the medium, such as surface plasmonizon (SPP), provide a new idea for terahertz guidance and mode conversion.

In recent years, the research team has been exploring the fields of miniaturized laser-accelerated electron sources and radiation sources, and recently discovered the coherent amplification mechanism of terahertz surface plasmonization, which can realize high-power surface plasmonized excite coherent radiation sources. Focusing on the Somofi wave properties of terahertz surface plasmons on axisymmetric metal cylindrical waveguides, and the low-dispersion base transverse magnetic (TM) mode, the research team further directly couples this high-power terahertz surface plasmonization exciton with the accelerated waveguide, achieving a coupling efficiency of 85%, which can effectively interact with the millijoule-level terahertz energy generated by the femtosecond laser-pumped metal cylindrical waveguide with the electron beam, and increase the best electron energy gain of the current international terahertz wave driven by nearly an order of magnitude.

In the future, based on this new solution, the research team will further develop the integrated all-optical electronic acceleration technology, and expand its cross-application in the fields of small radiation sources and material detection. (Source: China Science News, Zhang Shuanghu, Huang Xin)

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