In a Flash: Professor Dixit’s Research Captures Electrons in Motion

Insight had the opportunity to interview Professor Gopal Dixit who is an Associate Professor in the Department of Physics at IIT Bombay. He is a renowned physicist in the domain of Attosecond Physics, and has been actively working alongside multiple international researchers (including this year’s Nobel laureates in Physics), making significant contributions to the advancement of this field.

Let’s start with a brief introduction about the Nobel Prize in Physics that was awarded this year to three physicists for their contribution to the field of Attosecond Physics.

Nobel Prize in Physics 2023

On October 3^rd, The Royal Swedish Academy of Sciences announced the names of this year’s recipients of the Nobel Prize in Physics. The recipients are Pierre Agostini, Ferenc Krausz, and Anne L’Huillier, who have been awarded the Nobel Prize in Physics 2023 “for experimental methods that generate attosecond pulses of light for the study of electron dynamics in matter”. The Nobel Laureates have demonstrated a way to create extremely short pulses of light that can measure the rapid processes in which electrons move or change energy. 

The journey began in 1987 when Anne L’Huillier of Lund University discovered that many different overtones of light arose when she transmitted infrared laser light through a noble gas. She is the fifth woman ever to have been awarded the Nobel Prize in Physics. 

In 2001, Pierre Agostini succeeded in producing and investigating a series of consecutive light pulses, in which each pulse lasted just 250 attoseconds. An attosecond is 10^-18 seconds. These pulses were too close to each other to be of any use. At the same time, Ferenc Krausz combined laser with high harmonic generation that made it possible to isolate a single light pulse that lasted 650 attoseconds, which broke the 1000-attosecond barrier for the first time.

These experiments culminated in the Nobel Prize in Physics in 2023. The study of the movement of electrons in atoms, molecules and matter in the condensed phase has become possible by the means of attosecond spectroscopy. They allow us to capture the shortest of moments and have enabled the investigation of processes which were otherwise too fast to follow. “We can now open the door to the world of electrons. Attosecond physics gives us the opportunity to understand mechanisms that are governed by electrons. The next step will be utilising them,” says Eva Olsson, Chair of the Nobel Committee for Physics.

For more context on Nobel Prize selections and the physics behind the phenomenon, check out the sections towards the end of the article

How are the Nobel laureates selected?

Physics of the Phenomenon

About Professor Dixit

Professor Gopal Dixit is an Associate Professor in the Department of Physics at IIT Bombay, as well as a Visiting Scientist at Max Born Institute, Berlin and Max Planck Institute for the Physics of Complex Systems, Dresden. He has been researching at IIT Bombay for eight years now. His academic papers have been cited multiple times by the recipients of the Nobel Prize in Physics 2023 in their research in Attosecond Physics. He has worked extensively in the field and has been closely connected with the international community in Attosecond Physics.

He leads the Attosecond Physics Lab at IIT Bombay. His research group currently consists of two students. Eleven past students have graduated who had been a part of this group. His research group focuses on the theoretical aspects and collaborates with experimental groups in Europe and the US. His group is the only theoretical research group in the field of attosecond physics in India. There are two other emerging experimental groups in this field

What fascinates Professor Dixit in his research domain?

His research focuses on the motion of subatomic particles: nuclei and electrons. A nucleus is nearly 2000 times heavier than an electron. The motion of the nucleus can be captured as it occurs on timescales of femtoseconds. However, it is difficult to understand the motion of electrons, as electron processes occur on a timescale of attoseconds. For example, the revolution of an electron around the nucleus in the Hydrogen atom takes 150 attoseconds. That is, the movements of electrons are nearly 1000 times faster than the motion of the nuclei, similar to comparing the motion of fighter jets (electrons) to that of container ships (nuclei). And previously, we did not have the technology to detect such motions for such short periods.

“Nature is dynamic. The microscopic world is constantly changing: matter continuously transforms from one form to another, and these transformations need to be understood for technological purposes. Then, you start asking fundamental questions about these rapid processes, which have no answer. This is what fascinates me.”

Historical Background and Professor Dixit’s Contribution

Using visible light (assuming a wavelength of 800 nm), physical processes up to an order of 2.7 femtoseconds could be observed, and this was considered to be a hard limit for a long time until the 2000s.

Consider the photoelectric effect; this process was always considered instantaneous. That is, when the photon (with a frequency larger than the threshold) collides with an electron on the surface of a metal, the electron is ejected (with relevant kinetic energy) instantaneously, without any delay. This is called photo-ionisation. 

In 2010, Ferenc Krausz (one of the three recipients of the Nobel Prize in Physics for 2023) conducted a photoionisation experiment on Neon gas and observed that the process took 20 attoseconds. This could not be supported by any theoretical explanation. A similar experiment to observe response time in light-matter reaction was conducted by Anne L’Huillier (another recipient of the Nobel Prize in Physics) on Argon gas in 2011-12. The results didn’t match any theoretically calculated values. At this point, the physicist community was in doubt whether there was any truth to the hypothesis that electron processes are not instantaneous.

Professor Dixit’s research group worked on a theory and could explain the two data points out of the three observed for the experiment on Argon gas. In 2014, a similar experiment was conducted by Pierre Agostini (the third recipient of the Nobel Prize in Physics). The catch was that the experiment involved photo-recombination rather than photoionisation. Photo recombination is the reverse process of photo-ionisation in which the electron falls on the surface of the metal and attaches, emitting a photon in the process. Photoionisation and photo-recombination are similar processes and have time-reversal symmetry. The experimental observations were explained entirely by Professor Dixit and his colleagues successfully in this paper. This validation proved that the processes such as photoionisation and photo-recombination require a finite amount of time, and are not instantaneous.

This time period in the history of ultra-fast sciences is called the Attosecond 1.0.

We move next to the phase Attosecond 2.0, which focuses on high harmonic generation from solids and attosecond pulses. Earlier, as we have seen, the attosecond pulses were generated using gases, and the processes were studied. In 2011, a research group at Stanford managed to show the same process in solids. This proved to be an important development, as the solids have richer and well-established physics compared to gases. It helped in getting rid of the bulky setups that were used in the case of gases (particularly, huge cylinders of gases) and were easily replaced by small solid crystals. 

Professor Dixit’s research group also focuses on this aspect of attosecond physics. He has researched using pristine graphene at room temperature to encode and process quantum information. Graphene is a lightweight 2D atomically thin structure. His word and its relevance was also the subject of an article in Forbes Magazine. Stephen Ibaraki, who wrote this article, also invited Professor Dixit for an IEEE TEMS interview about his research. His work on valleytronics in graphene may lead to smaller-sized quantum computers, and these, combined with the high computational speed of quantum computers, will be a huge leap in quantum technology. Its significance can be understood from the fact that the processor speed has not increased since a decade now (stalling at 4 GHz), but the storage space has improved multifold. His research has been mentioned in an article by the MIT Technological Review.

The ultra-fast sciences are currently a popular topic. Professor Dixit’s research focuses on three domains: quantum jump in technologies, photonics and table-top setups, and digital and classical computing in processor speeds. 

Professor Dixit’s post-doctoral mentor, Misha Ivanov, is also an important pioneer in the field of Attosecond physics. He is a Professor at Imperial College London and Max Born Institute, Berlin. He was among the first persons to be able to explain the quantum interpretation of high harmonic generation. Professor Dixit has co-authored multiple research papers with Professor Misha Ivanov (Professor Ivanov co-wrote a milestone paper on Attosecond physics with Ferenc Krausz in 2009) In 2013, Professor Dixit gave a talk at ATTO 2013, an International Conference on Attosecond Physics, alongside Professor Ivanov, other Nobel Laureates and the recipients of the Nobel Prize in Physics 2023.

Professor Dixit’s Perspective on the Research Landscape in India

Professor Dixit talks about the lack of a peer group in India and the hurdles it brings, such as a lack of appreciation for his work or limited growth opportunities. He also remarks that students are more keen on following the standard route rather than charting a path of their own or discovering fields worth looking at. 

Being a theoretical research group, they do not require funding from the institute. A problem that he highlights is the scarcity of students’ interest in this domain, along with a decreasing number of PhD admissions in the institutes in India. He would like it if more undergraduate and postgraduate students pursue niche research areas, not only because they are fast-moving but also because the scope for original research is very high. He currently requires 4-5 PhD students in his research group. 

He opines that in research, “When you don’t know the answer, you are on the right path. Because if you already know the answer, what are you looking for anyway?”

When the Nobel Prize in Physics 2023 was announced, he couldn’t sleep for two nights. In his words, “This was the first time that a Nobel Prize had been awarded in the field of my research, and the people involved were all my close international colleagues. I was very excited, but there was no one with whom I could celebrate this due to a lack of knowledge about this field in India. The Nobel Prize reassured me that what I had been working on was finally being recognised internationally.” 

India is still trying to catch up with international research. There is a lack of continuous investment in research. The percentage of GDP that India spends on research is very low compared to that of the developed countries. While well known research domains like materials science have well established labs and experimental groups in India, the experimental groups in niche areas like quantum technology and photonics which require huge funding for expensive experiments are very few. A few groups that began setting up their laboratories way back are still in the process. In IIT Bombay, there are two quantum technology groups that have been setting up their labs for many years now: one is in the Department of Physics, and another is in the Department of Electrical Engineering. The results obtained by these groups are encouraging, but the procedure for the establishment of the laboratories is still slow. Even though there is a significant amount of funds given by alumni, they are usually received in chunks by the respective institutes and are allotted to multiple divisions by the administration, and not only to research in some specific area. Running laboratories requires continuous funding. In his experience, he has never seen any institute going out of its way to support research in a niche area. Same goes for the industries, as they tend to invest only once the products are developed. None of the industries invest in the research so that a lab can be established and a new product can be built in a few years and be brought to the market. India is still behind in culminating research output into products that can be brought to the market and contribute to the GDP. Majority of the equipment continues to be imported at expensive prices from abroad, and there are very few indigenous products being manufactured.

Important academic papers of Professor Dixit cited by Nobel laureates over the years of research in Attosecond physics:

Paper 1: Attosecond time delay in valence photoionization and photorecombination of argon: a TDLDA study

1. Attosecond interferometry: techniques and spectroscopy (Reference No.: 73)

2. How can Attosecond pulse train interferometry interrogate electron dynamics? (Reference No.: 19)

Paper 2: Time Delay in the Recoiling Valence Photoemission of Ar Endohedrally Confined in  C-60

1. Attosecond High-Harmonic Spectroscopy of Atoms and Molecules Using Mid-Infrared Sources (Reference No.: 279)

2. Atomic delay in helium, neon, argon and krypton (Reference No.: 26)

3. Measurements of relative photoemission time delays in noble gas atoms(Reference No.: 29)

Paper 3: Imaging electronic quantum motion with light

1. Attosecond Science Comes of Age (Reference No.: 39)

How are the Nobel laureates selected?

The Royal Swedish Academy of Sciences selects the winners of Nobel Prizes in Physics and Chemistry. It appoints the Nobel Committee to manage the process of selection. There is also an option to award no prizes for the year, but this is seldom employed.

In September of the year before the prize, confidential nomination forms are sent out to about 3000 people in the scientific community in Physics and Chemistry. The completed forms should reach the committee before 31st January. The forms are screened, and about 300 candidates are nominated. During the period of March-May, the committee decides one or two domains in which it wishes to award the prize. For the 2023 Nobel Prize, the topics considered prominently were Attosecond Physics and Dark Matter. The committee consults the experts in these chosen fields to assess the candidates’ work. It then compiles a report during June – August and submits it to the Academy with recommendations on the final candidates in September. This report is discussed in two meetings in a larger body called the Physics Class of the Academy. The laureates are selected and announced in October, and the prize is received in December.

Physics of the Phenomenon

When you switch on the fan in your room, the motion of the blades of the fan becomes increasingly blurred as you increase the speed using the knob until it seems that you can directly see the ceiling through the blades of the fan. The phenomenon behind this is the persistence of vision. For human eyes, the persistence of vision is 1/16th of a second. For this reason, movies show 24 frames or images per second to project motion to viewers.

The same problem arises in studying nature. The deeper (and smaller) you go into the microscopic world, the binding energies between particles increase. As a consequence, the time span of the processes decreases rapidly (due to energy-time uncertainty). As explained earlier, the electron processes occur at an attosecond time scale, which was difficult to observe earlier due to limitations in generating light with higher frequencies. Generating light pulses which last a few attoseconds makes it convenient to trace back from observations the processes taking place. This is analogous to using a high-speed camera to capture sports movements.

Let’s now focus our attention, like a laser, on the science behind the phenomenon. Meet the term: “attosecond”, which is a unit of time. It is equal to 10^-18 seconds. If you divide a second into attoseconds, it is approximately the same number of seconds since the creation of the universe.

Light is a wave that consists of synchronised oscillating electric and magnetic fields. Superimposing waves or overtones produce pulses of light, that is, light waves, but more localised. (Never think that quantum mechanics will ever leave your tail 🙂 ) When laser light of a certain frequency is shined on the particles of the gas, the light interacts with its atoms and causes overtones – waves that complete a number of entire cycles for each cycle in the original wave. That is, higher frequencies, other than the frequency of the laser light, are observed in the detectors. This was observed by Anne L’Huillier during her experiment on Argon gas in 1987, which led her to further research on ways to improve the pulses generated, and thus increase the intensity of high harmonics.

When laser light enters the volume of the gas and interacts with the atoms, it distorts the electric fields that hold electrons around the nuclei. As a result, an electron may escape the atom via quantum tunnelling, leaving behind a positive ion. But suppose the wave is at the right frequency. In that case, its rapidly oscillating fields will immediately reverse their direction and push the electron back towards the ion before it has the time to go anywhere else. The incoming electron often has more energy than the amount needed to ionise the atom in the first place, and that extra energy is released as higher-frequency photons, majorly lying in the ultraviolet region.

Much research was done in the 1990s, but the breakthrough occurred in 2001. Pierre Agostini and his research group in France succeeded in producing and investigating a series of consecutive light pulses. They used a unique trick: a prism was used to split the laser beam into two beams. One of them interacts with the gas, and the emitted light is in the form of a train of pulses. They put this “pulse train” together with the delayed part of the original laser pulse, to see how the overtones were in phase. This procedure also gave them a measurement for the duration of the pulses in the train, and they could see that each pulse lasted just 250 attoseconds.

At the same time, Ferenc Krausz and his research group in Austria were working on a technique to select a single pulse from this “pulse train”. The pulse they succeeded in isolating lasted 650 attoseconds, and the group used it to track and study a process in which electrons were pulled away from their atoms.

These experiments demonstrated that attosecond pulses could be observed and measured, and that they could also be used in new experiments.

References:

nobelprize.org/prizes/physics/2023/press-release/

nature.com/articles/d41586-023-03047-w

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