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ELI ALPS had as a guest Csaba Tóth, forerunner of attosecond laser pulse generation

With fellow physicist Győző Farkas, he predicted the generation of attosecond laser pulses in 1992. He contributed to building the dinosaur of laser physics dubbed T-Rex in the US and was consulted on the technical concepts of all three ELI institutes. Physicist Csaba Tóth spent a month at ELI ALPS.

ELI ALPS had as a guest Csaba Tóth, forerunner of attosecond laser pulse generation

 

Were you born a physicist or did you evolve into one?

I could say that I was fascinated by physical phenomena already in my childhood, but it would not be entirely true. I was mainly interested in prisms, mirrors and lenses. I used them to build telescopes, so I was somewhat attracted to astronomy, but nothing more. My primary school physics teacher gave me the biography of Ányos Jedlik as a present. I read it and found it inspiring, but at the beginning of secondary school I turned to mathematics, which intrigued me more at the time. I studied in a class specialized in maths at Széchenyi István Secondary School in Sopron, where I realized over time that I was more interested in tangible sciences. I applied to the physicist degree programme at Eötvös Loránd University, Budapest, where I had fantastic teachers. But it was the university dormitory life that had the biggest impact on me. In the afternoons, we would go through the material we had heard in the morning lectures with the many bright young people around. I think the best way to deepen your knowledge is to take a critical approach to what you have heard, to discuss your doubts with your classmates. After graduation, I applied to the doctoral school of the Central Research Institute for Physics (KFKI), where my two mentors were Győző Farkas and Zoltán Horváth. They were both confident that a physicist can only work with conclusions based on experimental, measured data.

 

With Győző Farkas, you proposed a method for generating attosecond laser pulses back in 1992. Such pulses were experimentally verified in 2001, and the birth of attosecond science is in fact associated with this year. Where did the idea come from?

Győző Farkas, who had already been working on special laser construction techniques for two decades, played a key role in this. I joined his group as a summer intern in 1981, and immediately became involved in the generation and measurement of mode-locked, picosecond laser pulses. The basis of our concept, published in 1992, had been known in laser technology: the combination of light components of different frequencies can produce much shorter light pulses. This technique worked in the nano- and picosecond range. We thought we could transfer the method to the much shorter pulse range, as mathematically, there is no obstacle to the generation of attosecond light pulses. I did the necessary calculations on a Commodore 64 computer. We supplemented the article with concrete measurement data, backing up our ideas with international laboratory measurements from previous years. At the time, fellow physicists said that the theory was all well and good, but they were sceptical about the practical implementation. Over the next decade, experiments brought scientists closer and closer to realization. Finally, the three winners of the 2023 Nobel Prize in Physics – Pierre Agostini, Ferenc Krausz and Anne L’Huillier – proved that attosecond laser pulses can be generated under the right experimental conditions.

 

 

You learned from your mentors, Győző Farkas and Zoltán Horváth, that apart from building a laser it is equally important to start experimenting with it as soon as possible, because it may yield results that can subsequently be useful for the construction of another laser. Why didn’t you try to produce attosecond laser pulses in 1992?

The answer is simple: we didn’t have the equipment. At that time, the shortest laser pulses we could work with at KFKI were in the picosecond range. Although dye lasers that could produce six femtosecond laser pulses were already available – in Hungary, the first physicists using such lasers were the researchers of the University of Szeged, i.e. Zsolt Bor, Gábor Szabó, Béla Rácz and Sándor Szatmári, – KFKI could not afford such equipment back then. It is no coincidence that Ferenc Krausz continued his laser research at the Vienna University of Technology, where everything was given for femtosecond experiments.

However, I would not say that we completely abandoned research in this direction. In the second half of the 1990s, we had joint measurements with Pierre Agostini’s laboratory group in France on surface harmonics and multiphoton electron excitation processes. We did publish a few papers, but this was not our focus area. Győző Farkas remained close to attosecond physics until the end of his life. He continued to work in this topic with Sándor Varró, but mainly on a theoretical level. In the experimental field, Győző Farkas primarily collaborated with Péter Dombi and Norbert Kroó in surface nonlinear optics research, which by that time had become intertwined with plasmonics. In the meantime, my life took twists and turns, as I spent more and more time abroad.

 

Between 1992 and 1997 you worked alternatingly in Budapest and Houston. Your stay in Houston was also supported by a recommendation from the Szeged laser group. Why did you go to the United States?

So that I can return with new experience. First, I participated in a one-year-long project: from the vapour of molten salts we generated a new class of ionic excimer molecules to develop new vacuum ultraviolet light-emitting lasers. I already possessed half the knowledge I needed for this work, and acquired the other half in Houston. In general, I think that when one changes research topics or jobs, the new field should not be completely new, but should not be 90 per cent the same as the old one either. I believe the healthy overlap is around fifty percent.

The Houston project was successful, but the money ran out. I returned to Budapest, but then my American professor invited me back to work on X-ray laser development for another two years. I again found myself at Rice University, but soon funding dried up there too. At that point, I seriously considered returning to Hungary for good, but I came across an exciting job advertisement: a group in the San Diego campus of the University of California was looking for a physicist with combined experience in ultrashort pulses and experiments in the X-ray field. I successfully applied for the job, and I was finally able to work in a group with many more resources than ever before. After three years in San Diego, where I further developed my practical knowledge of femtosecond laser pulses and the CPA (Chirped Pulse Amplification) technology, and published some X-ray diffraction experimental results which aroused great interest, I had to change again. This time because of the untimely death of Professor Kent Wilson, the researcher who had kept the group together and supported it financially. In 2000, I was thus forced to look for a job again, which led me to start working on a new, emerging topic at Berkeley: particle acceleration with plasma waves. Once again, I found myself in world-class conditions – it was then and there that we decided that our family would stay in the US for good, as I still felt insecure about funding for basic research at home.

At that time, only three or four people were working on this new topic in the group founded and led by Wim Leemans at Lawrence Berkeley National Laboratory.  Laser-driven electron acceleration has since become a rapidly developing discipline with a wide range of applications. Today, between 1,000 and 1,500 researchers in nearly two dozen laboratories around the world are involved in particle acceleration with lasers. Again, what attracted me in this last career change was that I already possessed half of the skills I needed, but had to acquire the other half on the spot.

 

 

The construction of the 100 TW T-Rex laser, the dinosaur of laser physics, began around 2002, and in its 13 years of operation it produced many outstanding results. In 2013, the world’s first relatively high repetition rate, high-power PW laser, known as BELLA (Berkeley Lab Laser Accelerator), was completed. What were/are these instruments capable of?

In 2004, we achieved our first major internationally acclaimed experimental result with our 10 TW peak power laser nicknamed Godzilla, which preceded the T-Rex. We demonstrated that by using ultrashort laser pulses and appropriate plasma targets we can produce electron beams with energies up to 80–100 megaelectronvolts (MeV), comparable to the energy of particle beams produced by conventional methods. Subsequently, in 2006, we produced electron beams with an energy of 1 GeV (gigaelectronvolt) with the newly completed 100 TW T-Rex laser. Building on this success, and with funding from the US Department of Energy (DoE), we could start designing and building the world’s first petawatt laser system with a repetition rate of 1 Hz in 2009. The BELLA laser was completed in 2013. Already that year, we set up a world record: we accelerated electrons to 4.2 GeV. In 2018, we beat our own previous record by reaching 8 GeV. A group from Texas took over the first place with 10 GeV around 2022. In the meantime, we also succeeded to produce an electron beam of around 10 GeV – a feat yet to be published –, but with the added advantage that we can now produce 7 to 8 GeV electron beams with significantly more stable beam parameters, and with much less laser energy.

 

How did you react to the news that Szeged would be home to a laser centre that the professional community had only dreamed of before?

As an external consultant, I threw myself into this joint work with great enthusiasm. I was consulted throughout the design and construction process of the Szeged, Prague and Bucharest based facilities alike. I am proud that these three research institutes have been built in Central Europe.

 

How does the world look at our institute?

With envy. ELI is known and recognized by the high intensity laser community in the United States. A few years ago, US based laser physicists held an important meeting to set the direction for laser development in the future, and one of the key messages was to watch out, because the world was passing us by. At that time, Europe was mentioned alongside China:  in relation to the Old Continent, the three ELI institutes were cited as examples to follow. The conclusion was that if the US did not want to be left behind, it should spend money in this area. As a result, the construction of new high repetition rate petawatt devices has taken off across the pond, too.

 

When can we see you next at ELI ALPS?

I can’t give you a date, but it is very likely that I will stay in close contact with the institute. Thanks to the ESF (Embassy Science Fellows) programme of the US Embassy in Budapest, I could spend a month at the Szeged based Laser Research Institute. Although I am now returning to the US and will continue to be based in Berkeley, California, I will also continue to work with my European colleagues, for example, in the evaluation of proposals for ELI user calls.

 

Profile:

Csaba Tóth is a physicist. He graduated from Eötvös Loránd University (ELTE), then worked at the Institute of Solid State Physics of the Hungarian Academy of Sciences. From 1993, he worked in the USA, first at Rice University in Houston, then at the San Diego Laser Laboratory of the University of California. Since 2000, he has been working in Berkeley, California.

 

Photos: Gábor Balázs

 

Author: Zoltán Ötvös

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