Fast, accurate, and clear! This "video recorder" can record atoms and molecules!

Author: Luo Huiqian, Researcher, Institute of Physics, Chinese Academy of Sciences

Light is probably one of the most familiar substances. Because of light, the universe is full of energy, the world is vivid and colorful, and everything appears in various sizes, shapes and colors. Nowadays, we are used to using cameras to take photos or videos to record macroscopic things. Scientists also have a super-fast and super-powerful recorder of the microscopic world, which is called a free electron laser device.

So what is a free electron laser? How is it different from an ordinary laser? Why can it record microscopic molecules or atoms?

Hello everyone! I am Luo Huiqian, a researcher at the Institute of Physics, Chinese Academy of Sciences. Today we will talk about the "fourth generation light source" besides synchrotron radiation - free electron laser device.

Light is the most important tool for humans to capture information about the world. Microscopes and telescopes based on visible light allow humans to see tiny microorganisms and distant stars. However, to gain further insight into microscopic information such as the composition and arrangement of atoms inside materials, we must rely on X-rays with shorter wavelengths. Since 1947, scientists have created four generations of synchrotron radiation sources, continuously improving the brightness of light, especially the "photographing" resolution of X-rays has reached unprecedented accuracy. However, the X-rays of synchrotron radiation sources are not coherent light sources, and their performance is much lacking if used to "shoot videos" of atoms and molecules.

Scientists thought of the best coherent light source - laser. Conventional lasers originate from quantum stimulated radiation produced by electrons on atoms, molecules or solid energy levels after the particle number flips, which is also a bound electron laser, but this mode cannot produce extremely short wavelength X-rays. To turn X-rays into a coherent laser beam, high-speed free electrons are needed.

The basic principle of free electron laser

The electron beam is accelerated to near the speed of light using a linear accelerator, and then placed in a periodically changing transverse magnetic field (also called an undulator) to oscillate and continuously emit spontaneous radiation. The spontaneously radiated light is repeatedly coupled with the electron beam itself, and the light of a specific energy is selected to achieve continuous gain until it reaches saturation and output. This is a beam of coherent free electron laser. Introducing high-order harmonic generation as seed light can further improve the coherence of the free electron laser.

High-harmonic beams

In April 1977, researchers at Stanford University in the United States built the first free electron laser oscillator, which realized far-infrared free electron laser based on the accelerator method. In the early 21st century, scientists at the Hamburg Electron Collider (DESY) in Germany developed an X-ray free electron laser device, whose brightness is equivalent to 10 million times the intensity of natural light. At present, there are many free electron laser devices built internationally, such as the soft X-ray free electron laser device (FLASH) of the German Electron Synchrotron Laboratory, the linear accelerator coherent light source (LCLS) of the National Accelerator Laboratory of the United States, the European X-ray free electron laser device (European-XFEL), the Pohang free electron laser device (PAL-XFEL) in South Korea, and the Swiss free electron laser device (SwissFEL).

The free electron laser FELIX in the Netherlands

As early as 1994, my country built the Beijing Free Electron Laser Facility (BFEL) in the mid-infrared band; the Dalian Coherent Light Source (DCLS) in the extreme ultraviolet band, built in 2016, is the brightest extreme ultraviolet light source in the world; in 2017, the High Average Power Terahertz Free Electron Laser Facility (CTFEL) was built in Chengdu, and in the same year, the Shanghai Soft X-ray Free Electron Laser Experimental Facility officially emitted light and will be further upgraded to a user device (SXFEL); in 2018, the Shanghai Hard X-ray Free Electron Laser Facility (SHINE) started construction, and the user device will be installed in September 2024.

Schematic diagram of the distribution of hard X-ray free electron laser facilities in Zhangjiang, Shanghai

The hard X-ray free electron laser facility located in Shanghai Zhangjiang Science City is 3.11 kilometers long and contains an 8 GeV superconducting linear accelerator, three undulator lines, three optical beam lines, a 100 PW ultra-intense ultra-short laser system, and the first batch of 10 experimental stations. It will have both nanometer-level ultra-high spatial resolution and femtosecond-level ultra-fast time resolution.

Free electron laser combines the coherence of ordinary lasers with the high energy, high brightness, and high resolution of synchronous light sources. It can be said to be an ultra-strong, ultra-fast, and ultra-high-energy "flashlight video recorder" in the microscopic world, and has cutting-edge applications in many disciplines such as energy, life, materials, physics, and chemistry. It can capture the atomic motion during the chemical bond breaking process, the strange physical states of atoms and molecules under extreme conditions, the charge transport process of complex molecules, and the ultra-fast dynamics of electrons or atoms during phase change. For tiny protein crystals or nanostructured materials, ultra-short and ultra-strong femtosecond X-ray pulses can obtain the 3D structure and even electronic state information of the crystal before damaging the sample, and the video taken can be described as "fast, accurate, and clear."

Free electron laser technology is still in a stage of vigorous development. In the future, it will greatly promote the development of basic scientific research and the invention of cutting-edge technologies, and bring human understanding of the material world into a new stage.

This article is a work supported by the Science Popularization China Creation Cultivation Program

Author: Luo Huiqian

Reviewer: Ji Yang, Researcher, Institute of Semiconductors, Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.