Electromagnetic radiation can be seen everywhere in our lives and is closely related to our lives, such as sunlight and light in visible light band, mobile phone and WIFI signals in microwave band, lithography machine light source in extreme ultraviolet band and X-ray in high-energy band, etc. However, most of the light in nature is incoherent, which has complex frequency, wide spatial direction and chaotic phase. The first coherent light source, laser, was invented in the 1960s. For coherent light, due to the coherence of its spectral components, the phase difference of each component is fixed, so it can achieve the modulation and compression of light pulses, thus obtaining a coherent light source with extremely short duration and high peak power.
Laser, as a coherent light source, has become ubiquitous, from scientific research, industry and military to communication, entertainment and art, as well as our daily life, we can see the important application of laser everywhere. The development and application of laser technology has also led to a number of Nobel Prizes, such as the Nobel Prize in Physics in 2018 awarded to Professor Gerard Mourou and Professor Donna Strickland, who invented chirped pulse laser amplification technology, which has increased laser brightn ess (power density) by about 10 orders of magnitude. It exceeds the brightness of sunlight by about 21 orders of magnitude; This year's Nobel Prize in Physics was awarded to Pierre Agostini, Ferenc Krausz and Professor Anne L 'Huillier, the inventors of attosecond pulse light, who invented a method to generate attosecond pulse light, which is very short enough to capture images of the internal evolution of atoms and molecules.
The key to the generation of coherent light source is phase locking, that is, to make the phase of each microscopic particle involved in radiation the same. The generation of laser is based on the principle of stimulated radiation proposed by Einstein, that is, atoms with inverted population will release outgoing photons with the same phase as the incident photons; Free-electron laser, a super-large scientific device, is based on the micro-bunching effect of electron beams, which ensures that the phase of each electron is consistent. In nature, there is another phase-locking mechanism, shock wave. For example, when a supersonic aircraft flies faster than the speed of sound in the air, an acoustic shock wave is generated because the sound waves generated by the nose of the aircraft at different times spread outward in a spherical wave front, and the phase front along a special angle (Cherenkov angle) is locked. Similarly, if the radiation source is allowed to exceed the speed of light, a new kind of coherent electromagnetic wave radiation, optical shock wave, can be produced. However, it is impossible for the same radiation source to exceed the speed of light in vacuum, because special relativity tells us that no object can move "faster than the speed of light".
In recent years, the research team of Shenzhen University of Technology is vigorously promoting the construction of Chenguang series devices, the first large-scale ultra-intense laser integrated experimental platform (high-power nanosecond-picosecond-femtosecond laser device) in domestic universities. An important research direction of the platform is to develop new coherent radiation light sources and carry out related application research. Recently, based on the basic principle of coherent radiation, the team proposed a new coherent radiation mechanism based on the collective action of electrons: through the interaction between relativistic electron beams and plasma with a slowly rising density gradient, a plasma bubble with a gradually decreasing size can be excited (the size of the bubble is negatively correlated with the plasma density). Plasma electrons at different positions bounce and radiate at the tail end of the cavity. Because the longitudinal size of the cavity is gradually reduced, the collective velocity of the tail end is greater than the velocity of the driving electron beam (close to the speed of light), reaching the "superluminal" condition. Therefore, the radiation generated by different electrons here is coherently superimposed along the Cerenkov angle to form a light shock wave. The radiation source has very unique properties: not only the pulse width is extremely short, reaching the attosecond scale, and the intensity is very high, which is proportional to the square of the propagation distance, but also it has excellent spatial directivity, extremely small angular dispersion, stable carrier-envelope phase and ultra-wide frequency tuning range.
The above work illustrates a new coherent radiation mechanism driven by electron beam, which breaks through the restriction that the size of electron beam is much smaller than the wavelength of radiation in classical coherent radiation theory. At the same time, this work provides a simple and feasible physical experimental scheme for coherent light source generation, which is expected to produce high-quality attosecond sub-cycle laser pulses on the mesa scale, and has an important impact on the application research of attosecond spectroscopy of living tissue cells, ultrafast molecular manipulation and diagnosis, electronic attosecond dynamic measurement, and ultra-high frequency signal processing. 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-field and far-field transformation noise in traditional simulation methods, realized the self-consistent simulation of high-frequency radiation with high spatial and temporal resolution, and also provided a new technical method for the development of new coherent radiation sources.
