SZTU Research Team Discovers New Mechanism Of Coherent Radiation in Attosecond Pulses

Recently, the team of Prof. Ruan Shuangchen and Prof. Zhou Cangtao from Shenzhen University of Technology (SZUT) has proposed for the first time in the world a physical scheme for generating attosecond pulses and subcycle coherent optical shock from a superluminal plasma tail field, and explained a new mechanism for generating coherent radiation which is dominated by the collective action of electrons. The research results were published in the top international physics journal Physical Review Letters under the title of "Coherent subcycle optical shock from a superluminal plasma wake". Assistant Professor Hao Peng is the first author of the paper, and Professors Taiwu Huang, Cangtao Zhou and Shuangshen Ruan are the co-corresponding authors.
Electromagnetic wave radiation can be seen everywhere in our lives and is closely related to our lives, such as sunlight and lights in the visible band, cell phones and WIFI signals in the microwave band, photolithography light sources in the extreme ultraviolet band and X-rays in the high-energy band. However, most of the light in nature is non-coherent light, which has complex frequencies, very wide spatial pointing and chaotic phases. The first coherent light source, the laser, was invented in the 1960s. For coherent light, because of the coherence of the spectral components it contains, the phase difference of each component is fixed, so it is possible to realize the modulation and compression of light pulses, so as to obtain a coherent light source with a very short duration and very high peak power.
Coherent light sources such as lasers became ubiquitous soon after their introduction, and important applications of lasers can be found everywhere, from scientific research, industry and the military to communications, entertainment and the arts, as well as in our daily lives. The development of laser technology and its applications have also given rise to a number of Nobel Prizes, such as the 2018 Nobel Prize in Physics awarded to Gerard Mourou and Prof. Donna Strickland for the invention of chirped-pulse laser amplification, which has increased laser brightness (power density) by about 10 orders of magnitude, exceeding the brightness of sunlight by about 21 orders of magnitude; while this year's The Nobel Prize in Physics was awarded to Pierre Agostini, Ferenc Krausz and Prof. Anne L'Huillier, inventors of attosecond pulses of light, which are short enough to capture images of the internal evolution of atoms and molecules.

(a) Light source in nature; (b) Coherent light source created by human - laser; (c) Acoustic excitation caused by supersonic airplane; (d) Schematic diagram of the principle of generating excitation by radiation source.
The key to the generation of coherent light sources is phase-locking, that is, so that the phase between each microscopic particle involved in the radiation is the same, the creation of the laser is based on the principle of stimulated radiation put forward by Einstein, that is, the number of particles reversed atoms will be released with the incident photon phase consistent with the incident photons of photons; and free-electron laser, such a mega-scientific device is based on the micro-aggregation of the beam of electrons beam effect, which ensures that the movement of each electron is in the same phase. In nature, there exists another phase-locking mechanism for waves - excitations. For example, acoustic excitations are generated when a supersonic airplane travels faster than the speed of sound in air, because the phase front along a particular angle (the Cherenkov angle) is phase-locked when the sound waves generated by the airplane's head at different moments spread outward in a spherical wave front. Similarly, if a radiation source is allowed to exceed the speed of light, a new type of coherent electromagnetic wave radiation, optical excitation, can be produced. However, it is impossible to make the same source of radiation exceed the speed of light in a vacuum, because special relativity tells us that the motion of any object cannot "exceed the speed of light".
In recent years, the research team of Shenzhen University of Technology is vigorously promoting the construction of the first large-scale super-intense laser comprehensive experimental platform (high-power nanosecond-picosecond-femtosecond laser device)-Chenguang series of devices in domestic universities. An important research direction of this platform is to develop new coherent radiation light source and carry out related application research. Recently, the team has proposed a new coherent radiation mechanism based on the collective action of electrons from the basic principle of coherent radiation: through the interaction of a relativistic electron beam with a plasma with a slowly varying upward density gradient, a plasma vacuole of gradually decreasing size can be stimulated (the size of the vacuole is negatively correlated to the density of the plasma), and plasma electrons at different positions rebound at the end of the vacuole and radiate at the end of the vacuole, due to the longitudinal size of the vacuole. The plasma electrons at different locations bounce off the end of the bubble and radiate there. As the longitudinal size of the bubble gradually decreases, the collective speed of its tail is greater than the speed of the driving electron beam (close to the speed of light), which achieves the condition of "superluminal", and thus the radiations of the different electrons generated here are coherently superimposed to form optical excitations along the Cherenkov angle. The radiation light source has very unique properties: not only the pulse width is extremely short, reaching the attosecond scale, and the intensity is very high, proportional to the square of the propagation distance, but also has excellent spatial directivity, very small angular dispersion, stable carrier envelope phase, and ultra-wide frequency tuning range.

(a) Schematic diagram of a relativistic electron beam hitting a plasma and generating an optical excitation wave at the tail end of a vacuole; (b) Optical excitation wave radiation at the tail end of a superluminous vacuole as seen in a large supercomputing numerical simulation.
The above work illustrates a new coherent radiation mechanism driven by an electron beam, which breaks the limitation of the classical coherent radiation theory that requires the electron beam size to be much smaller than the radiation wavelength. Meanwhile, this work provides a simple and feasible physical experimental scheme for coherent light source generation, which is expected to generate high-quality attosecond subperiodic laser pulses at the tabletop size, which will have an important impact on attosecond spectroscopy of living tissues and cells, ultrafast molecular manipulation and diagnosis, electronic attosecond dynamics metrology, and ultrahigh-frequency signal processing of the beat-hertz frequency, and other applied research. In addition, this work has developed the first parallel computing program for far-field time-domain coherent radiation in China, solved the bottleneck problems of numerical dispersion and near- and far-field transformation noise in the traditional simulation methods, and realized high time-space-resolved self-consistent simulation of high-frequency radiation, as well as provided a new technological method for the development of new coherent radiation sources.
This result is another important breakthrough in electron beam-driven coherent radiation generation made by the high energy density physics research team of Shenzhen University of Technology, following the December 2021 and May 2023 publications in Physical Review Letters. It is worth mentioning that Portuguese scientists proposed a similar physical mechanism and scheme almost simultaneously with the team, and the related work was accepted by Nature Photonics, a journal under Nature.
This research was funded and supported by the Key Research and Development Program of the Ministry of Science and Technology of China, the National Natural Science Foundation of China (NSFC), the Shenzhen Key Laboratory Establishment Program, and the Shenzhen Outstanding Youth Fund Program. The simulation work was done on the near-trillion times/second supercomputing simulation platform of the Advanced Materials Testing Technology Research Center of Shenzhen University of Technology.