Nobel Prize for Attosecond Physics
Katalin Varjú 1, Péter Dombi 2, Gábor Szabó 3
1PhD, Science Director, ELI ALPS, ELI-HU Non-Profit Ltd., Szeged
Associate Professor, Department of Optics and Quantum Electronics, University of Szeged
2Doctor of the Hungarian Academy of Sciences, Head of Department ELI ALPS, ELI-HU Non-Profit Ltd., Szeged
Research Professor, Wigner Research Centre for Physics, Budapest
3Full member of the Hungarian Academy of Sciences, Managing Director, ELI ALPS, ELI-HU Non-Profit Ltd., Szeged
Professor, Department of Optics and Quantum Electronics, University of Szeged
DOI: 10.1556/2065.184.2023.12.3
The 2023 Nobel Prize in Physics has been awarded for the study of the movement of electrons in atoms, molecules and solids using attosecond spectroscopy. This year’s winners are Ferenc Krausz, Anne L’Huillier and Pierre Agostini, whose experiments produced flashes of light (so-called attosecond light pulses) that proved to be short enough to provide snapshots of the extremely fast motion of electrons.
It now goes without saying that the level of scientific development of an age can be measured by the range of scales over which it can experiment. It is perhaps less obvious, but looking back at the history of science, it is clear that scientific sophistication can also be judged by the speed of processes we are able to study. Therefore, to understand our physical world we require techniques that can take snapshots of the processes in question. These techniques include high-speed photography, or stroboscopy, developed in the second half of the 20th century, which made it possible to study the flight of a hummingbird, for example. The time resolution of one millionth of a second provided by the stroboscope method was sufficient in the macroscopic world. With the development of quantum mechanics, detailed calculations could be made of the movement of atoms within a molecule. However, studying this movement experimentally required a further improvement of one-billionth, i.e. a leap down to the femtosecond (10-15 second) range. This leap was achieved in the last decade of the 20th century, and is perhaps best marked by the 1999 Nobel Prize in Chemistry awarded to Ahmed Zewail for his studies of the transition states of chemical reactions using femtosecond spectroscopy. The motion of atoms can now be studied, but to get the full picture we need to understand the motion of electrons. However, due to their small mass, electrons move a thousand times faster than atoms, so by the turn of the 2000s, the stage was set for the attosecond technique to take off.
Since their discovery in 1960, lasers have played a key role in improving temporal resolution as the shortest available pulse durations have continuously decreased. Femtosecond dye lasers became accessible for researchers already in the 1970s, and important developments in the 1980s allowed for the production of very short, high-energy laser pulses with peak powers orders of magnitude greater than ever before. One such advancement was the principle of chirped pulse amplification (CPA), which was rewarded with the Nobel Prize in 2018. Furthermore, the 1988 discovery of the titanium-doped sapphire crystal (commonly referred to as Ti:sapphire) as laser medium significantly boosted the development of multi-stage laser amplifiers.
Ferenc Krausz recognized the great potential of Ti:sapphire technology during his research at Vienna University of Technology in the early 1990s. He implemented several developments in laser physics to enable these lasers to deliver the shortest possible pulses. This period also saw the invention of the chirped mirror, which is credited to Ferenc Krausz and Róbert Szipőcs. The first such mirrors were produced in 1993 by Kárpát Ferencz at the Institute of Solid State Physics and Optics. This technology also helped Ferenc Krausz to turn the institute in Vienna into a leading research place in laser physics, but these results proved useful in attophysics too.
To understand the fundamental problem of attosecond pulse generation, it is reasonable to take a small detour. Light is an electromagnetic wave which, like all waves, consists of a series of successive crests and troughs. If we want to generate a pulse from such a wave train, we must somehow cut out, say, ten successive wave crests. The shortest pulse length that can be obtained is obviously the duration of a wave crest. The bad news is that lasers typically operate in the visible light range, so this minimum duration is not more than a few femtoseconds. This means that attosecond pulses cannot be directly generated with lasers. To understand the idea needed for the next step, we can use an analogy from acoustics. If a violinist presses the bow too hard on the strings, the result is a distorted sound because over-excitation causes overtones, i.e. multiples of the fundamental frequency to appear. (In music the first overtone or the first harmonic is called octave). After the discovery of lasers, researchers found that strong laser light can make certain materials – typically crystals – behave in a way that the octave of the fundamental light appears. In the following decades, this technology was further developed so that frequency multiplication, typically second or third harmonic generation, became part of practical laser technologies. (Few people realize that when holding a green-light laser pointer in their hand, they take advantage of this phenomenon: the laser operates in the infrared, which is invisible to the naked eye, and the beautiful green light they see is an octave of the fundamental frequency.) The development of frequency multiplication in itself led to far-reaching results, but there was a huge problem: these techniques work only in the range of a few octaves, and the generation of attosecond pulses would need thirty to fifty octaves. The way to the solution was found through research from another approach. Since the 1980s, the steady increase in laser power has made it possible to study light-matter interactions at extreme intensities, in a world, which is different both technically and physically. The first technical difference lies in the fact that ordinary optical materials cannot be used as targets, because they are immediately destroyed by the laser beam upon irradiation. Hence, the only possible targets that remain are gas jets introduced into the vacuum chamber.
Since the late 1980s, the research centre at Paris-Saclay University, where the other two laureates, Anne L’Huillier and Pierre Agostini used to work, has been at the forefront of experiments to study extreme light-matter interactions, including the generation of high harmonics. Experimental experience led to surprising results, the understanding of which laid the foundations for the development of strong-field physics, leading to the birth of attophysics two decades later.
Győző Farkas, a researcher at the Institute of Solid State Physics and Optics in Budapest, was involved in the work of the Saclay Research Centre from the 1970s. Seeing the results of high harmonic generation, he and his then doctoral student Csaba Tóth were the first to formulate – in a remarkable paper in 1992 – the then controversial claim that the temporal shape of harmonic radiation forms a train/sequence of attosecond pulses.
Thanks to the development and widespread application of high intensity lasers, efforts to understand and model the high harmonic generation process were made by several groups in Europe, the USA and Canada in the 1990s. The theories developed indicated that the temporal structure of the full high harmonic radiation as it is generated does not exhibit the ultrashort attosecond pulses. Measurement techniques suitable for the confirmation of the existence of attosecond pulses did not appear before 2001. Then, within the same year, the group of Ferenc Krausz and that of Pierre Agostini managed to successfully characterize the temporal shape of high harmonic radiation using two different techniques, and to detect attosecond pulses in spectrally filtered radiation, in agreement with the theories.
Although the two detection methods seem similar, they are very different in principle, and have therefore become established as distinct techniques in the attosecond toolbox. In Pierre Agostini’s attosecond pulse train, flashes of 250 attoseconds were identified during the first measurement. Using the same technique in 2003, Anne L’Huillier’s group, already in Sweden, managed to shorten this duration to 170 attoseconds. At the same time, Ferenc Krausz and his research group in Vienna were working on a technique that produced a single attosecond pulse, as a result of unique laser developments at the institute. The pulse which they first managed to isolate and measure lasted 650 attoseconds. This duration was shortened to 250 attoseconds thanks to their developments in the following few years. In 2008, Ferenc Krausz entered the Guinness Book of Records for generating 80 attosecond pulses at the Max Planck Institute for Quantum Optics in Garching, Germany. Since then, a new record has been set in Switzerland, and the shortest light pulse ever produced is now 43 attoseconds long.
This article presents the story behind the birth of attosecond science and the background to the Nobel Prize-winning discoveries. The field of science that has emerged from these discoveries is very broad: what started as a rather narrowly focused field of multi-photon processes in atomic physics has now become useful in the field of molecular physics, physical chemistry, condensed matter physics and applied sciences such as materials science, pharmaceuticals, electronics or even biomedical research. The very first example of modern applications is the molecular fingerprinting technique developed by Ferenc Krausz. This technique combines broadband optics, ultrafast laser sources and precision femtosecond-attosecond field resolving technologies, and is able to detect changes in the molecular composition of biofluids. This holds promise as a new in vitro diagnostic analytical technique for the early detection of diseases.
At the end of a piece of writing like this, one might expect a mention of the future. In our case, this is also obvious: as ELI ALPS Laser Research Institute was built in Szeged, Hungary will surely play a central role. As the first letter of the acronym ALPS – Attosecond Light Pulse Source – in the institute’s name indicates, one of the focus areas of the institute’s research profile is attosecond physics. The explicit aim of the research centre is to promote the growth of attoscience by offering experimental opportunities to interested researchers from around the world. For us, the fact that two of the leading researchers in the field, Ferenc Krausz and Anne L’Huillier – both of whom played a major role in the establishment of the institute and with whom we are still in contact – have been awarded the Nobel Prize sends out the message that attosecond science is a major discovery for mankind. In other words, we are on the right track, our “only” task is to support as many scientific breakthroughs as possible.
For more in-depth knowledge in attosecond physics, please see the sources below, which also include links to original publications presenting the scientific results.
References
Dombi Péter (2005): Femtokémiából attofizika? Mafigyelő, a Magyar Fizikushallgatók Egyesületének lapja, 15, 2, 6–7. https://mafihe.hu/wp-content/uploads/2013/11/mafigyelo2005maj.pdf
Farkas Győző (2006): Attoszekundumos időtartamú fényimpulzusok. Fizikai Szemle, 56, 12, 408– 412. http://fizikaiszemle.hu/archivum/fsz0612/FarkasGy.pdf
Krausz Ferenc (2002): Atomok és elektronok mozgásban. Fizikai Szemle, 52, 1, 12. http://fizikaiszemle.hu/old/archivum/fsz0201/krau0201.html
Major Balázs – Kőrös Pál Csaba – Varjú Katalin (2017): Attoszekundumos impulzuskeltés makroszkopikus optimalizációja. Fizikai Szemle, 67, 10, 331–334. http://fizikaiszemle.hu/szemle/
tartalom/31
Varjú Katalin (2008): Attoszekundumos impulzusok, Fizikai Szemle, 58, 3, 87–92. http://fizikaiszemle.hu/archivum/fsz0803/VarjuK.pdf
URL1: https://www.nobelprize.org/prizes/physics/2023/advanced-information/
URL2: https://www.nobelprize.org/uploads/2023/10/popular-physicsprize2023.pdf