The scientific research team of Shenzhen University of Technology has discovered a new mechanism of attosecond pulse coherent radiation

Recently, a team of Professor Ruan Shuangchen and Professor Zhou Cangtao from Shenzhen University of Technology proposed for the first time internationally a physical solution based on the generation of attosecond pulses and subperiodic coherent light shock radiation from a superluminal plasma wake field, and explained a new coherent radiation generation mechanism dominated by collective electron interactions.

The research results were published in the top international journal of physics, Physical Review Letters, under the title of "Coherent subcycle optical shock from a superluminal plasma wave". Team assistant professor Peng Hao is the first author of the paper, and professors Huang Taiwu, Zhou Cangtao, and Ruan Shuangchen are the co corresponding authors of the paper.

Electromagnetic radiation is ubiquitous and closely related to our daily lives, such as sunlight and lighting in the visible light band, mobile phones and WIFI signals in the microwave band, lithography machine light sources in the extreme ultraviolet band, and high-energy X-rays in the ultraviolet band. However, most of the light in nature is incoherent light, which has complex frequencies, wide spatial orientation, and chaotic phases. In the 1960s, humans invented the first coherent light source - laser. For coherent light, due to the coherence of its spectral components and the fixed phase difference of each component, modulation and compression of optical pulses can be achieved, thereby obtaining coherent light sources with extremely short duration and high peak power.

Laser, a coherent light source, soon became ubiquitous, and its important applications can be seen everywhere from scientific research, industry, and military to communication, entertainment, and art, as well as in our daily lives. The development and application of laser technology have also given rise to multiple Nobel Prizes, such as the 2018 Nobel Prize in Physics awarded to Professor Gerard Mourou and Donna Strickland, who invented chirped pulse laser amplification technology. This technology has increased laser brightness (power density) by about 10 orders of magnitude, surpassing solar brightness by about 21 orders of magnitude; And this year's Nobel Prize in Physics was awarded to the inventors of attosecond pulsed light, Pierre Agostini, Ferenc Krausz, and Professor Anne L'Huillier, who invented a method for generating attosecond pulsed light, which is very short enough to capture images of the internal evolution of atoms and molecules.

(a) Natural light sources; (b) Coherent light source created by humans - laser; (c) Acoustic shock waves caused by supersonic aircraft; (d) Schematic diagram of the principle of shock waves generated by radiation sources

The key to the generation of coherent light sources is phase locking, which means that the phase between each microscopic particle participating in radiation is the same. The generation of lasers is based on Einstein's stimulated radiation principle, which means that atoms with inverted particle numbers will release emitted photons with the same phase as the incident photon; The free electron laser, a super scientific device, is based on the micro bunching effect of the electron beam, ensuring that the motion phase of each electron is consistent. In nature, there is another phase locking mechanism of waves - shock waves.

For example, when a supersonic aircraft flies at a speed exceeding the speed of sound in the air, a sound shock wave is generated because the sound waves generated by the aircraft's head at different times diffuse outward in a spherical wavefront, and the phase front is locked along a special angle (Cherenkov angle). Similarly, if the radiation source exceeds the speed of light, a new type of coherent electromagnetic wave radiation - optical shock waves can be generated. However, it is impossible for the same radiation source to travel faster than light in a vacuum, as special relativity tells us that the motion of any object cannot "travel faster than light".

In recent years, the research team of Shenzhen University of Technology has been vigorously promoting the construction of the first large-scale ultra strong laser comprehensive experimental platform (high-power nanosecond picosecond femtosecond laser device) in domestic universities - the Chenguang series device. An important research direction of this platform is to develop new coherent radiation light sources and carry out related application research. Recently, the team proposed a novel coherent radiation mechanism based on the fundamental principle of coherent radiation: by interacting with a plasma with a slowly increasing density gradient through a relativistic electron beam, a gradually decreasing size plasmon bubble (with a negative correlation between the bubble size and plasma density) can be excited. Plasma electrons at different positions rebound at the tail end of the bubble and radiate from it, Due to the gradual reduction of the longitudinal size of the bubble, the collective velocity at its tail end is greater than the driving electron beam velocity (close to the speed of light), reaching the "superluminal" condition. Therefore, the radiation generated by different electrons here coherently overlaps along the Cherenkov angle to form a light shock wave. This radiation light source has very unique properties: not only is the pulse width extremely short, reaching the attosecond scale, but also the intensity is high, proportional to the square of the propagation distance. At the same time, it has excellent spatial directionality, minimal angular dispersion, stable carrier envelope phase, and an ultra wide frequency tuning range.

(a) Schematic diagram of light shock waves generated by relativistic electron beams hitting a plasma at the tail end of a bubble; (b) Light shock radiation at the tail end of a supersonic bubble observed in large-scale supercomputing numerical simulations
The above work elucidates a novel coherent radiation mechanism driven by an electron beam, breaking through the limitation of classical coherent radiation theory that the electron beam size is much smaller than the radiation wavelength. At the same time, this work provides a simple and feasible physical experimental scheme for the generation of coherent light sources, which is expected to generate high-quality attosecond subperiodic laser pulses on a table size, and have a significant impact on the application research of attosecond spectroscopy, ultrafast molecular manipulation and diagnosis, electronic attosecond dynamics, and beat hertz ultra-high frequency signal processing in living tissue cells. In addition, this work developed the first parallel computing program for far-field time-domain coherent radiation in China, which solved the bottleneck problems of numerical dispersion and near-far-field transformation noise in traditional simulation methods, achieved high spatiotemporal resolution self-consistent simulation of high-frequency radiation, and also provided new technical methods for the development of new coherent radiation sources.

Source: Sohu