On October 3, the Nobel Prize Committee announced that the 2023 Nobel Prize in Physics would be awarded to Pierre Agostini, Ferenc Krausz and Anne Luriel (Anne L Huillier), three physicists in the field of attosecond lasers. In recognition of their contribution to "an experimental method for the generation of attosecond light pulses for the study of electron dynamics in matter". This is also another reward for the breakthrough achievements of laser experimental technology, especially ultrafast laser technology.
Observing the dynamic characteristics of microscopic particles from the time domain
A flash process can be described as a light pulse. The so-called attosecond light pulse refers to a light pulse whose duration is only in the order of attoseconds, that is, 10 billionths of a billionth (10-18) of a second. People say it is "as fast as lightning", but in fact, the fastest lightning lasts only about one hundredth to one thousandth of a second. Therefore, it is hard to imagine how short and fast the attosecond is with people's macro experience. The distance that light can travel in one attosecond is only 0.3 nanometers, which is equivalent to the diameter of three hydrogen atoms. If one attosecond is compared to one second, it is equivalent to one second to the age of the universe of about 14 billion.
At present, using the most advanced electron microscope, people have been able to see the image of atoms, which is a great progress in natural science from the space domain to study physical phenomena and laws. It is known that the molecules, atoms and electrons that make up the material world are in absolute motion, so how to observe the motion of these particles from the time domain, that is, the dynamic characteristics of the micro-world, is another important aspect for people to study and understand physical phenomena and laws. In 1999, the Egyptian scientist of California Institute of Technology, A. Professor Zewail of the H. is awarded the Nobel Prize in Chemistry for his first study of chemical reaction dynamics using femtosecond pulses. Generally speaking, the vibration of molecules and the motion of atoms are in the order of picoseconds to femtoseconds, and femtosecond pulses can be used to study the dynamic processes of different molecules and atoms. However, for electrons moving around the nucleus in an atom, the time is in the order of attoseconds, for example, the time for an electron to circle the nucleus in a hydrogen atom is about 152 attoseconds. Atomic structure model is an important cornerstone of modern physics and an important achievement of quantum mechanics, but there has been a lack of experimental measurement of this theoretical model. For the first time, attosecond light pulses provide an unprecedented means for people to measure and control the motion of electrons. As the operation of extranuclear electrons is a basic scientific problem faced by physics, chemistry, biology and other disciplines, the emergence of attosecond optical pulses has important scientific significance and practical value in the frontier research and application of many disciplines.
Ushering in the Attosecond Era of Ultrafast Science
The generation of attosecond optical pulses is closely related to high-order harmonic generation (HHG). When the laser acts on the nonlinear crystal, it will produce the second harmonic under certain conditions, that is, the so-called frequency doubling. In general, this effect needs to satisfy the phase matching condition and requires a certain intensity of the laser. Different from conventional laser, ultrashort pulse laser has high peak power and intensity, so it can excite many nonlinear effects with different characteristics, such as multi-photon effect, above threshold ionization, self-phase modulation and so on, when it interacts with matter even without phase matching. In 1988, A. of the Institute of Atomic and Surface Physics, France. L 'Huillier and M. Ferray et al. Observed extreme ultraviolet HHG with wavelength extending to nearly 30 nm by using Nd: YAG laser with pulse duration of 36 PS and peak power of about 1 GW to interact with Ar, Kr and other atomic gases on the basis of previous nonlinear effect studies. It is found that the harmonics have two typical characteristics: one is that the harmonics are odd multiples of the incident laser frequency, and the other is that the harmonics are composed of three parts: the falling region, the plateau region and the cut-off region. The generation of this EUV coherent radiation provides a compact and economical new scheme for obtaining short-wavelength lasers with high photon energy. Soon after, G. of the Hungarian Institute of Solid State Physics. Farkas and C. Toth et al. Pointed out that HHG is a feasible way to generate attosecond pulses through theoretical analysis, and it is expected to obtain 100 attosecond ultrafast laser. In 1993, P. of the National Research Institute of Canada. Professor Corkum proposed a three-step model theory to explain its physical mechanism reasonably and perfectly, and in subsequent studies, he further concluded that high-order harmonics have the characteristics of attosecond pulse width.
So how to realize attosecond pulse by HHG? At that time, people were faced with complex technical problems, one is that the efficiency of HHG is extremely low, the other is that it needs to be transmitted in vacuum, and the third is that there is no mature time measurement technology. Therefore, in physical studies based on experimental results, it is impossible to prove that the generated HHG has an attosecond pulse width. Until 2001, under the leadership of Pierre Agostini, a joint research team from France and the Netherlands first used a 40 FS Ti: Sapphire amplified laser to interact with Ar gas to generate HHG, and then used a time-of-flight electron spectrometer (TOP) and a microchannel plate (MCP) to measure the photoionization electrons changing with the delay. An attosecond pulse train with a pulse width of 250 attoseconds and an interval of 1.35 femtoseconds between adjacent pulses is obtained. However, this kind of attosecond pulse train has great limitations in application, and isolated single attosecond pulse is needed for practical application research. Not long after, by the time of the Vienna University of Technology, F. A more powerful research team led by Professor Krausz and P. Corkum et al., based on the production of HHG in Ne gas driven by a 7 FS femtosecond Ti: sapphire amplified laser, the momentum distribution of photoelectrons produced by filtered extreme ultraviolet light in Kr gas was measured by cross-correlation, the time-resolved measurement ability of 150 attoseconds was demonstrated, and a single isolated attosecond pulse of 650 attoseconds was obtained. It has been successfully used to measure the electron motion in Kr atom. The realization of this result marked the advent of the attosecond era of ultrafast science, and was selected as one of the top ten scientific advances of the year by Nature and Science magazine the following year.
Origin of Innovation and Development of Future High-tech Industry
With the emergence of isolated attosecond pulses, different gating and measurement techniques have been further developed, and the development of the shortest attosecond pulses has been promoted. For example, Professor Chang Zenghu's team at the University of Central Florida in the United States broke the world record of attosecond laser pulses twice in 2012 and 2017, and obtained isolated attosecond pulses of 67 attoseconds and 53 attoseconds respectively. The European Union has also built ELI-ALPS, an extreme light science facility based on attosecond pulse lasers in Hungary. The appearance of attosecond light pulse opens the door of higher resolution research for ultrafast science. By measuring and controlling the dynamic characteristics of electrons, it provides unprecedented means for further promoting the innovation and development of atomic and molecular physics, condensed matter physics, chemistry, biology and many other disciplines, and is expected to achieve new understanding of many physical phenomena. Such as the mechanism of superconductivity, the nature of spin exchange in magnetism, and the mechanism of charge transfer between electrons and holes in semiconductors. Because the essence of many phenomena in physics, biology, chemistry and other fields comes from the movement of electrons in atoms, in the field of medicine, attosecond pulse laser will help people to fundamentally understand the microscopic causes and formation processes including diseases, and will be used to guide the development of new drugs. In the field of energy, it is expected to improve the efficiency of photovoltaic cells by studying the electron motion process in materials with attosecond pulses. In the field of information, the switching rate of PHz has been achieved by controlling the electrical properties of materials with attosecond pulses. Compared with the current CPU clock frequency of GHz, it is expected to bring about a revolution in information processing and computing. These exciting works are not only the focus of frontier basic science, but also closely related to the future high-tech industry. Of course, at present, attosecond pulse is still in the laboratory development stage, and further development needs the joint efforts of scientists and engineers.
