An attosecond equals one billionth of a billionth of a second, or 10–18 s. This timescale has lent its name to a new, active field of science: attosecond physics, which deals with ultrafast light-matter interactions. One of the international centres in this area of interest is the ELI ALPS Research Institute in Szeged, which provides researchers investigating atomic, molecular and optical processes with attosecond pulses of unique intensities. Papers on the generation and characterization of attosecond pulses were published by Dr. Balázs Major and Dr. Katalin Varjú (researchers from the University of Szeged and ELI ALPS) in the journal Optica in 2021 and in the American Physical Society's Physical Review A in 2023, in collaboration with the National Research and Development Institute for Isotopic and Molecular Technologies in Cluj-Napoca and the Max Born Institute in Berlin.
Dr. Balázs Major, physicist, assistant professor at the Department of Optics and Quantum Electronics of the University of Szeged (USZ) was interviewed by USZ staff.
How is an attosecond light pulse produced? What is the novelty in your recent publication?
We tried to generate an attosecond pulse in a range of laser intensities that had previously been unfeasible. It was formerly thought that a well-defined attosecond light pulse could be produced by lasers with intensities of 1014–1015 W/cm2. In our experiment, we used a laser pulse with an intensity that was one to two orders of magnitude higher than this, by focusing the beam from the laser source into a noble gas jet using a lens with a focal length of about 1 metre. In this case, the electric field of the laser induces a so-called three-step process. If we look at just one atom of the noble gas, the phenomenon is simplified as follows: the atom is ionized by the electric field, i.e. an electron leaves the atom, accelerates and then recombines with its atomic core. During this process, the energy gained in the laser field is released by photon emission, i.e. through the generation of an ultrashort extreme ultraviolet (XUV) pulse. The three-step model describes the interaction of a single atom and the laser light, but the gas jet introduced into the path of the laser light contains a billion times billion atoms. It does matter how the generated XUV pulses add up: a certain phase-matching is required to ensure that the emitted photons do not "cancel out" each other, but rather constructively interfere with each other, and enough photons are produced to form the output, the attosecond light pulse.
Dr. Balázs Major, physicist (USZ, ELI ALPS). Photo: Ádám Kovács-Jerney
What is the research significance of a flash of light that lasts for a billionth of a billionth of a second?
This is the very advantage of the attosecond pulse: it can be used to study small-scale, ultrafast processes. The smaller a thing is, the faster the processes occur. The Bohr atomic model, familiar from secondary school, is a good example. Although outdated in many aspects, it vividly describes what happens in reality. In this simplified model, the nucleus is made up of positively charged protons and neutral neutrons, while negatively charged electrons orbit the nucleus like the planets orbit the Sun. The single electron of a hydrogen atom moves around the nucleus in about 200 attoseconds, which means it completes a billion times billion circles in one second. To capture this phenomenon on a photograph, you need to use a very short flash duration to avoid having a blurred image. This light flash is the attosecond pulse. ELI ALPS offers an unprecedented, high-power attosecond source to researchers to study very fast atomic processes in real time.
Let's clear up a misunderstanding! Can an attosecond light pulse be considered a laser? Or is the laser source here only used to produce ultrashort light?
Laser is light amplification by the stimulated emission of radiation. Attosecond pulses, on the other hand, are generated with a well-focused laser in the three-step process mentioned above. At the same time, attosecond pulses, similarly to the extremely short, albeit non-attosecond laser pulses, form coherent beams with low divergence. If we are clever, we can also produce attosecond light in the X-ray range to achieve low-divergence X-ray beams, different from those used in medical devices, where the X-ray light is scattered at a relatively large angle.
What materials can be tested with attosecond light?
All materials from gases to solids. In the ELI laboratories, attosecond light is used mainly for research in atomic, molecular and optical physics, and first of all for the study of very fast dynamic processes in the gaseous state. One of these methods is called XUV transient absorption spectroscopy, where an attosecond light pulse is used to induce a very fast atomic or molecular change in the studied sample, lasting a fraction of a second, and this change can be imaged or filmed in real time. The word transient means that we want to investigate something that exists for a very short time and is not permanent. The attosecond pulses that we produce are most often used by researchers in so-called pump-probe experiments. This means that a sample is illuminated by two subsequent beams, the first (the pump beam) triggers the phenomenon and the second (the delayed probe beam) investigates it; the delay is in the order of attoseconds.
Are there unsolved problems in attophysics?
Yes, for example, we don't yet know for sure whether there is a delay during the ionization of gases when two electrons leave the atom simultaneously. Many scientists have been trying to find the answer for a long time, but the measurements still show large variations. The ionization of gases involves illuminating atoms in a gaseous target and extracting electrons from them. The question is: if two electrons leave the atom during ionization and these electrons are removed from two different atomic orbitals, do they leave at the same time or is there a delay between them? All the indications are that there is a delay, and it is only a few attoseconds; however, we do not yet know the exact value.
At ELI ALPS, the place for attosecond research. Photo: Ádám Kovács-Jerney
What is the significance of the fact that a train of attosecond light pulses could be produced over a larger range of laser intensities?
The main advantage is that it can be used to generate a so-called broad continuous spectrum of attosecond pulses from the upper ultraviolet to the lower X-ray frequency range. This broad continuous spectrum can cover all processes. It doesn’t matter where photon emission or absorption occurs in the spectrum during the process; we are able to investigate it because we can illuminate the sample with the full spectrum. Another important message of the paper is that, during the illumination of the gas jet, the high-intensity laser pulse is transformed into a lower-intensity beam as it propagates through the gas, and this process becomes self-sustaining. In this scheme many parameters are self-setting and do not require external means to control them. This also has a practical advantage. Traditionally, attosecond pulses are generated in noble gas atoms, and the ideal laser intensity has been determined for each noble gas (typically neon, argon, krypton and xenon). When different frequencies of attosecond light pulses are needed in the XUV and soft X-ray range, different noble gases are used. As our process is self-regulating, we do not need to change the parameters of the generating laser beam, we only need to replace the gas cylinder containing the noble gas with another one, and we get the ideal attosecond pulses for our purposes.
How are light-matter interactions simulated in theoretical calculations?
With the simulations we modelled not only the single-atom case, but also the optimal combination of XUV pulses emitted from the myriads of atoms of the gas jet, i.e. the phase-matching process. For this purpose, we developed a software to combine the single-atom and the many-atom case and to optimize the emitted light. We do not have a dedicated software engineer, the numerical computational models are written by us, researchers. Here, for example, the propagation of light in the many-body case was described by Maxwell's wave equations, in which we took into account the most critical effects. Of course, there are some effects which are not relevant in our case, and were therefore neglected. If we wanted to make a model that took everything into account, we would never complete it, and there would be no computer that could process it. In physics, simplifications are necessary.
Dr. Balázs Major, physicist (USZ, ELI ALPS). Photo: Ádám Kovács-Jerney
Are quantum phenomena involved in light-matter interactions?
There is also a quantum aspect to the process, the significance of which is being investigated by theoretical research groups at the University of Szeged and ELI. In the aforementioned three-step model, the electron exits the ionizing atom and enters the laser field through a typical quantum phenomenon, i.e. tunnelling. Otherwise, the rest of the process can be interpreted in an almost entirely classical way. The study of quantum phenomena is currently an emerging field of research in attophysics. One such research topic is the study of the quantum entanglement of emitted electrons, which also has a tradition at the Department of Theoretical Physics at the University of Szeged. An interesting question is whether the two electrons leaving the noble gases during ionization are individual or quantum entangled particles, and the observation of one electron affects the motion of the other even from a large distance. This is basic research, meaning that practical results can only be expected in the longer run, but in the future this research area could be linked to quantum informatics. The present is about understanding the processes. This is how physics works: we first understand the phenomena and then ask how we can control them. But the path from understanding to controlling may take us in a very different direction than we originally thought.
For more articles on this topic, see Fizikai Szemle:
Attosecond pulses – by Prof. Dr. Katalin Varjú (March 2008; available in Hungarian)
Author: Sándor Panek (University of Szeged)
Cover photo: Dr. Balázs Major, physicist at the ELI ALPS attosecond research facility. Photo: Ádám Kovács-Jerney