Attosecond electron microscopy: the ultimate challenge of temporal and spatial resolution

Produced by: Science Popularization China

Author: Salty Fish in the Sea (Master of Optics from Changchun Institute of Optics and Fine Mechanics, Chinese Academy of Sciences)

Producer: China Science Expo

The hot topic related to attoseconds is that the 2023 Nobel Prize in Physics was awarded to three experimental physicists in the field of attosecond physics. Seconds are already very short in the human world, and attoseconds are even more fleeting and difficult for humans to perceive.

Attoseconds are difficult for humans to perceive and capture, but they are extremely important to many research fields such as physics and information science. Open the door to attoseconds and the wonders of the microscopic world are waiting for us.

Three experimental physicists in the field of attosecond physics

(Photo source: Nobel Prize official website)

Entering the World of Attoseconds

Attosecond is a unit of time, 1 attosecond = 10^-18 seconds. What does this mean? The number of attoseconds in one second is the same as the number of seconds since the birth of the universe.

(Image source: THE ROYAL SWEDISH ACADEMY OF SCIENCES)

To put it more concretely, it takes tens of billions of attoseconds for a beam of light to travel from one end of a normal-sized room to the wall at the other end. In a vacuum, the distance that light can travel in one attosecond is about 0.3 nanometers.

A tiny hummingbird can flap its wings 80 times per second, and to the human eye, the wings of a hummingbird appear as a blur. To take a photo of a hummingbird's wings in flight, high-speed photography and lighting techniques that match the speed are required.

Similarly, in the microscopic world, when electrons move between atoms, their positions and energies change at the order of attoseconds. If you want to explore the motion state of electrons and take videos of them, you can’t do without the help of attosecond laser pulses. Attosecond laser pulses are flashes of light that last at the order of attoseconds.

Snapshot of attosecond flash generation

(Image source: Max Planck Institute of Quantum Optics, Thorsten Naeser)

Its appearance has opened a new door to the microscopic world, which means that people's ability to study the structure of matter has reached a new level, and has also set off a new trend in the research field of basic physics.

At present, there are dark clouds floating over the building of physics, but attosecond pulses may bring some hope.

Electron microscope breaks through the resolution limit of optical microscopy

In order to observe the changes in the electromagnetic field caused by the movement of electrons, not only a fast enough flash light is needed, but also a microscope with a resolution reaching the atomic scale - a transmission electron microscope.

Electron microscopes are able to image the atomic structure of the sample being observed. Currently, the highest resolution electron microscopes can achieve a resolution of 0.5 angstroms (0.05 nanometers).

High-resolution electron microscopy of magnesium samples

(Image source: Wikipedia)

In the early days, people gradually discovered the resolution problem of optical systems while using conventional optical systems for observation.

In 1834, George Biddell Airy discovered the diffraction phenomenon caused by the wave nature of light while observing celestial bodies with an astronomical telescope. In 1835, he proposed the concept of the Airy disk, an optical diffraction limit that limits resolution.

In 1878, Cleveland Abbe pointed out that the resolution of optical microscopes is limited by the diffraction of light waves, and gave a formula to express the resolution limit of microscopes, pointing out that the resolution of microscopes is limited by the wavelength of light. The current traditional optical microscopes have a resolution limit of several hundred nanometers.

In order to break through the limits of optical microscopes, people gradually shifted their attention to high-speed electrons moving in a vacuum in their efforts to find high-resolution microscopes.

In 1924, de Broglie proposed the wave nature of electrons, which showed that the motion of electrons had profound similarities with light waves and laid a theoretical foundation for the establishment and development of electron optics.

At that time, a group of physicists were arguing over the wave-particle duality of light, and de Broglie's theory made the physics community extremely lively. He himself won the Nobel Prize in Physics for discovering the wave nature of electrons and his research on quantum theory.

High-speed electrons can be refracted and focused by axially symmetrical electric or magnetic fields, which means that electric or magnetic fields can be used to make electron lenses, just like glass can be made into lenses that refract light.

With sufficient theoretical foundation, the birth and development of electron microscope was a natural outcome.

In 1931, the first transmission electron microscope came out . It was modified from a cathode ray oscilloscope and the image magnification was only 13 times. In 1939, the first commercial electron microscope was manufactured, with a resolution better than 10 nanometers.

Early transmission electron microscope

(Image source: Wikipedia)

Attosecond electron microscopy: opening new doors in physics

" Attosecond light pulse + transmission electron microscope ", this combination means that the whereabouts of electrons have nowhere to hide under the nose of the attosecond electron microscope!

(Image source: THE ROYAL SWEDISH ACADEMY OF SCIENCES)

In 2023, an article in the journal Nature reported the use of attosecond electron microscopy to observe the movement of electrons on the surface of an object when laser irradiated the object.

(Image source: Reference [1])

In this experiment, scientists used attosecond laser pulses to modulate the electron beam into a series of electron pulses with a duration of attoseconds. The various signals generated by the pulses hitting the sample were filtered out with an energy filter to remove noise, and the electric field photos generated by the electron movement were recorded to obtain the movement state of the electrons.

When a series of these photos are superimposed together, we get a "video" of the electron's motion.

Image of electron energy changing over time, 1fs (femtosecond) = 1000as (attosecond)

(Image source: Reference [1])

Precisely measuring the movement of electrons, understanding their physical properties, and controlling the dynamic behavior of electrons in atoms is one of the important scientific goals that people pursue. With attosecond pulses, we can measure and even manipulate single microscopic particles, and then make more basic and principled observations and descriptions of the microscopic world.

Conclusion

Look up at the vastness of the universe and look down at the abundance of species.

Looking at the development of natural science, it is not difficult to find that human beings have never stopped exploring the microscopic world. With the rapid advancement of technology today, we have more means and tools to open the door to the microscopic world. Although the exploration is still on the way, the excitement of the microscopic world will eventually be known to people.

References:

[1] Nabben, David, Joel Kuttruff, Levin Stolz, Andrey Ryabov, and Peter Baum. "Attosecond electron microscopy of sub-cycle optical dynamics." Nature (2023): 1-5.

[2] Corkum, P. Á., & Krausz, F. (2007). Attosecond science. Nature physics, 3(6), 381-387.

[3] Dai Chen, Wang Yang, Miao Zhiming, Zheng Wei, Zhang Linfeng, and Wu Chengyin. "High-order harmonic generation and applications based on femtosecond laser-matter interaction." Laser & Optoelectronics Progress 58, no. 3 (2021): 0300001-30000114.

[4] Huang Siyuan, Tian Huanfang, Zheng Dingguo, Li Zhongwen, Zhu Chunhui, Yang Huaixin, and Li Jianqi. "Development and application of high temporal and spatial resolution transmission electron microscope." World Science and Technology Research and Development 44, no. 3 (2022): 392.

[5] Dong Quanlin, Jiang Yueling, Wang Jiujiu, et al. A brief review of the development of transmission electron microscopy[J]. Journal of Chinese Electron Microscopy, 2022, 41(6):685-688.

[6] Li, Cheng, Jun-Chi Chen, Xing-Kun Wang, Ming-Hua Huang, Wolfgang Theis, Jun Li, and Meng Gu. "Going beyond atom visualization—Characterization of supported two-atom single-cluster catalysts with scanning transmission electron microscopy." Science China Materials (2023): 1-8.