At about 17:45 Beijing time on October 3, 2023, the 2023 Nobel Prize in Physics was awarded to French scientist Pierre Agostini, Hungarian-Austrian scientist Ferenc Krausz, and French/Swedish scientist Anne L’Huillier, in recognition of their “experimental method of generating attosecond light pulses for the study of electron dynamics in matter.”
Pierre Agostini received his PhD from the University of Aix-Marseille in France in 1968 and is currently a professor at The Ohio State University in the United States.
Ferenc Krausz was born in Mol, Hungary in 1962. He received his PhD from the Technical University of Vienna, Austria, in 1991. He is currently the director of the Max Planck Institute for Quantum Optics in Garching, Germany, and a professor at Ludwig Maximilian University in Munich, Germany.
Anne L'Huillier was born in Paris, France in 1958. He received his PhD from Pierre and Marie Curie University in Paris, France, in 1986, and is currently a professor at Lund University in Sweden.
electrons in light pulses
This year's winner experimentally created flashes of light short enough to take snapshots of extremely fast electron motions. Anne L'Huillier discovers new effects of laser interaction with atoms in gases. Pierre Agostini and Ferenc Krausz demonstrated that this effect can be used to generate shorter light pulses than before.
A tiny hummingbird can beat its wings 80 times per second, yet to us we can only perceive a buzzing sound and vague wing movements. Rapid motion becomes blurry to the human senses, and extremely short events are invisible to the human senses—we need special techniques to capture or depict these very brief moments. Using high-speed photography and flash, we can capture the physical appearance of fleeting phenomena. If you want to take a high-definition photo of a hummingbird in flight, you need an exposure time much shorter than a single wingbeat of a hummingbird. If you want to capture faster events, you need to shoot faster.
The same principle applies to all methods used to measure or describe fast motion processes: any measurement must be faster than the time when the target system changes significantly, otherwise only vague results will be obtained. This year's Nobel Prize winner in physics demonstrated in experiments a way to generate pulses of light that are short enough to capture images of processes inside atoms and molecules.
The natural time scale of atoms is very short. In molecules, atoms can move and rotate in quadrillionths of a second (femtoseconds), and these movements can be studied using extremely short pulses generated by lasers. But when entire atoms move, the time scale is determined by their large, heavy nuclei, which move extremely slowly compared to the light, flexible electrons. When electrons move inside atoms or molecules, they move so fast that they cannot be clearly described on the femtosecond scale. In the electronic world, position and energy change at rates ranging from one to hundreds of attoseconds, with attoseconds being 10-18 seconds.
An attosecond is so short that the number of attoseconds in one second is the same as the number of seconds that have elapsed since the creation of the universe 13.8 billion years ago. To give an example closer to home, we can imagine a beam of light being emitted from one end of a room to the opposite wall - this takes 10 billion attoseconds.
Femtoseconds have long been considered the limit of flash that can be produced. Simply improving existing techniques wasn't enough to see electrons moving on extremely brief timescales—scientists needed something entirely new. And this year’s winners have opened up a whole new realm of attosecond physics.
Electrons move very fast in atoms and molecules, measured on the attosecond scale. An attosecond in a second is as short as a second in the age of the universe.
Shorter pulses at higher harmonics
Light consists of waves (fluctuations in electric and magnetic fields) that travel faster than anything else in a vacuum. Light of different wavelengths appears as different colors of light. For example, red light has a wavelength of about 700 nanometers, about one hundredth the width of a human hair, and vibrates about 430 trillion times per second. We can think of the shortest light pulse as the length of a single cycle in the light wave, that is, a cycle in which the light wave rises to the peak, falls to the trough, and returns to the starting point. In this case, the laser wavelengths used in common laser systems could never go below the femtosecond range, so in the 1980s this was seen as a hard limit on the shortest light pulses.
According to the mathematics of waves, we can construct arbitrary waveforms if we use enough waves with the right wavelength, frequency, and amplitude (the distance between wave crests and troughs). The trick with attosecond pulses is to generate shorter pulses by combining more and shorter waves.
Electrons move extremely fast, so if you want to observe electron motion at the atomic scale, you need short enough light pulses, which means combining short waves of many different wavelengths.
To produce the shortest wavelength of light ever produced, we need more than just a laser. Crucially, we need to understand a phenomenon that occurs when laser light passes through gas. When the laser interacts with atoms in the gas, it creates a type of harmonic wave—waves that complete multiple complete cycles for each period of the original wave. We can compare harmonics to overtones, which give a sound a specific character. Overtones allow us to hear the difference between the same notes played on guitar and piano.
In 1987, Anne Lullier and her colleagues at a French laboratory demonstrated the generation of harmonics using an infrared laser beam passing through a noble gas. The infrared light produces more harmonics and is stronger than the shorter wavelength lasers used in previous experiments. In this experiment, they observed many harmonics with approximately the same light intensity.
Overtones have multiple cycles for every cycle in the fundamental tone. Harmonics work in light waves similarly to overtones.
In the 1990s, Lullier continued to explore this effect with a series of articles published at Lund University. Her findings help theoretically understand this phenomenon and lay the foundation for the next experimental breakthrough.
Escaped electrons produce harmonics
When laser light enters a gas and affects its atoms, it causes electromagnetic oscillations that distort the electric field of electrons around the nucleus, causing electrons to escape from the atoms. However, the laser's electric field is constantly oscillating, and when it changes direction, loose electrons can rush back into the nucleus. As the electron moves, it gains a large amount of extra energy from the laser's electric field. To return to the ground state near the nucleus, the electrons must release excess energy in the form of pulses of light. These light pulses from the electrons create the harmonics seen in the experiment.
Laser interacts with atoms in gas
Experiments have discovered the mechanism by which lasers produce harmonics. How does it work?
1. The electron combined with the nucleus usually cannot escape from the atom. It does not have enough energy to pull itself out of the potential well formed by the atomic electric field.
2. When atoms are affected by laser pulses, their electric fields will be distorted. When an electron is confined by only a narrow potential barrier, quantum mechanics allows it to tunnel and escape.
3. The free electrons are still affected by the laser electric field and gain some additional energy. When the electric field turns and changes direction, the electrons are pulled back.
4. In order to reattach to the nucleus, the electron must shed the extra energy it gained on its way out. This energy is emitted in the form of ultraviolet light, whose wavelength is related to the wavelength of the laser field and varies depending on how far the electrons have traveled.
The energy of light is related to its wavelength. The energy in the harmonics emitted by the experiment is comparable to that of ultraviolet light, and its wavelength is shorter than that of visible light. Since the energy comes from the oscillations of the laser, the harmonic oscillations will be in elegant proportion to the wavelength of the original laser pulse. The result of light interacting with many different atoms is a set of different light waves of a specific wavelength.
Once these harmonics appear, they interact with each other. When the peaks of light waves overlap, the light produced becomes stronger, but when the peaks of one light wave overlap with the troughs of another, the light produced becomes less intense. Under the right circumstances, the harmonics coincide, giving rise to a series of UV light pulses, each with a period of several hundred attoseconds. Physicists understood the theory behind it in the 1990s, but the real breakthrough came in 2001, when scientists actually identified and tested the pulse.
Explore the world of electrons with the shortest pulses of light: When a laser passes through a gas, the atoms in the gas produce harmonics of ultraviolet light. Under the right conditions, these harmonics may be synchronized. When their periods coincide, a concentrated attosecond pulse is formed. Example of experimental setup: The laser is split into two beams, one of which is used to generate a series of attosecond pulses. This pulse sequence is then added to the original laser pulses, and the combination is used to perform extremely fast experiments.
Pierre Agostini and his research team in France succeeded in creating and studying a series of continuous light pulses like a train with multiple carriages in series. They used a special trick of placing this "pulse train" alongside delayed portions of the original laser pulse to see how the harmonics synchronized with each other. They also measured the duration of pulses in the "pulse train" and found that each pulse lasted only 250 attoseconds.
Meanwhile, Ferenc Krausz and his research group in Austria were working on a technique that could pick out individual pulses—like uncoupling one carriage on a train and switching it to another track. They successfully isolated a pulse that lasted 650 attoseconds, which the team used to track and study the process of electrons breaking free from their atomic confines.
These experiments demonstrate that attosecond pulses can be observed and measured, and that they can also be used in new experiments.
Now that the attosecond world is within reach, these short pulses of light can be used to study the movement of electrons. It is now possible to generate pulses as low as tens of attoseconds, and the technology is evolving all the time.
The movement of electrons becomes easier to understand
Attosecond pulses measure the time it takes for an electron to be pulled away from an atom, and examine how tightly the electron is bound to the nucleus determines how long that time takes. We can reconstruct the distribution of electrons in atoms and materials so that they oscillate from one side to the other, or from one position to another; until now, the position of an electron could only be measured as an average.
Attosecond pulses can be used to test the internal processes of a substance and identify different events. These pulses have been used to explore the details of atomic and molecular physics and have potential applications in electronics, medicine and other fields.
For example, attosecond pulses can be used to push molecules, thereby emitting a measurable signal. Signals from molecules have a special structure, a fingerprint that can reveal their "identity" and could have potential applications in areas such as medical diagnosis.