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  • June 25, 2020
    "The next 20 years will be as exciting as the last 20 years"

    team — With his colleagues of the research group led by Prof. Dr. Matthias Kling, Dr. Shubhadeep Biswas has published his first paper in Nature Physics called “Probing molecular environment through photoemission delays”. We talked to him about his research, his career and his plans for the future.

    Can you give us a short description of what was your paper in Nature Physics about?

    The publication “Probing molecular environment through photoemission delays” is about trying to visualize the inner territory of a molecule. It has been a universal quest for decades to understand the intriguing detail of how molecules react with each other or with some external perturbation like light, moreover whether these reactions can be manipulated according to your desire. To reach there, it is important to understand the inner microscopic environment of the molecule. To give an example, for a typical organic reaction the role of the functional groups like hydrocarbon is really important and these mostly determine the outcome of the reaction. Now in a very simple picture, this reaction happens because of the electrostatic interactions within the molecule and also with its counterpart. Therefore the entire problem boils down to the fact that you have to understand the electrostatic potential landscape of the molecule to gain knowledge about the chemical reaction and its way of control.

    Were there any difficulties with the measurements?

    In case of molecule, probing the potential landscape is not that easy because of its (unlike with an atom) really complicated anisotropic structure and congestion of huge number of quantum states. For decades scientists have tried to gain understanding about that with electron emission measurements in energy (or momentum) domain. But in most of the cases the difficulty came because of the valance nature of the probed electrons, whose initial state or position is very uncertain. We tried to approach this problem in time domain, with a very simple concept: within the molecule, if you can inject or generate an electron, about whom you are very certain about its initial position and also about the fact that before propagation it is not influenced by the molecular environment, then the electron can roam around within the molecular landscape and pick up the influence of the electrostatic potential.

    Can you give us an analogy for better understanding?

    It is very similar to the traffic situation that we often encounter in large cities: you would have a fair bit of knowledge about the traffic if you roam around within the city and if you encounter more traffic you get late to reach your final destination. In the present work we generated an electron by photoionization from a core shell of iodine atom which is part of a molecule called ethyl iodide. Being a core shell electron it is hardly influenced by the molecular environment, although it is part of the molecule. Then, after generation, it traverses through the rest of the molecular environment and gets influenced by it before being detected finally. By means of attosecond streaking spectroscopy, we could accurately measure a time delay, although extremely small, in electron emission due to this molecular influence in comparison to no such influence for a single iodine atom.

    What perspective do your findings offer?

    This work provides a new way of looking at this grand problem. With this new technique along with angle resolved electron detection capability, one can eventually reach the ultimate goal of mapping the full molecular potential landscape in three dimensions, which would be a big step forward in the fields of molecular physics as well as chemistry.

    It was your first publication in Nature. How does it feel like?

    I feel privileged being part of this kind of big project and wonderful collaboration, which engaged large number of big players in the field of attosecond physics. Personally for me, the most exciting part was the learning part throughout the whole journey of this publication, and obviously we all are happy that its potential is recognized by this kind of top tier scientific journal.

    You come from India. Can you give us a short overview, where you have been born, where you went so school and university?

    Yes, I am from India. So, I grew up at a small place called Tehatta. It is in one of the eastern states of India called West Bengal. My entire schooling was there, predominantly in Tehatta High School. For my bachelor degree in physics I took admission in a college named Ramakrishna Mission Vidyamandira, Belur, which is under University of Calcutta in Kolkata. There, actually I got the first real taste of physics and also bit shifted from my childhood interest in mathematics.

    And where did you get your second taste of physics?

    At the end of 2010, I moved to Mumbai to join Tata Institute of Fundamental Research for my integrated PhD, where I worked in the field of atomic-molecular physics but with accelerators not with lasers. However, over time, I developed deep interest in time-resolved physics from a very basic and also I guess very natural thirst to see how an electron moves in a molecule or how a molecular fragmentation happens in real time. Although I had no prior real experience, to fulfil that dream of working in time-resolved physics in its extreme precision, I could not think of better place than the Max-Planck-Institute for Quantum Optics, Garching and the Ludwig Maximilian University of Munich. So, in 2017, I ended up joining the research group of Prof. Matthias Kling within the division of Prof. Ferenc Krausz, which often is considered to be the birth place of attosecond physics. For making this exciting new journey possible, I should acknowledge the generous support of the Alexander von Humboldt Foundation and the European Union under the Marie Skłodowska-Curie COFUND Action.

    What fascinates you most about light technologies?

    Present day light technology, especially the laser, has many facets. Being interested in time-resolved physics, I am always fascinated by ultrafast laser technology. It has enabled us to generate single light bursts as short as 100 attosecond long routinely. How much is an attosecond? It is 1, 000, 000, 000, 000, 000, 000th of a second. If I compare, it is nearly equivalent to one second with respect to the age of our universe. This number itself is spectacular. To scientists like us this number is even more fascinating because this time scale corresponds to the fastest motion of electrons. Essentially we can take pictures of an event which lasts in the attosecond time scale, e.g. the classical round trip time of hydrogen atom electron motion around the proton. In fact, in the present work we could measure time intervals of about 10 to 50 attoseconds. Other than that the current light technology along with precise use of nonlinear optics gives rise to ultimate control over light pulses and its waveform, that makes the situation even more interesting as this can be utilized as the control parameter of different microscopic phenomena in physics and chemistry.

    You are standing at the beginning of a career as a scientist. Where do you see yourself in 20 years?

    I would like to continue in academia as a scientist. Given that, however, I would also be happy to contribute to the technological development which comes along with basic fundamental science research. I would like to have my own research group in near future which would focus mostly on ultrafast physics aspects in atomic, molecular and novel condensed matter systems; and coherent control of different interesting phenomena within these.

    And what will light technologies look like then?

    Light technology is evolving in a very rapid pace extending its reach to many different novel parameter spaces. If we take lasers only in next 20 years, there would be every possibility of having compact mid-infrared lasers which can generate ultrashort pulses with considerable energy for applications. The efforts in this direction are already in progress. This region of operation enables to extend the current higher harmonic generation based attosecond physics to the x-ray region which would make possible to investigate atomic core shell dynamics, x-ray nonlinear physics, or even perform x-ray pump / x-ray probe experiments in small lab scale. Moreover, as we are able to reach the fundamental time scale of electron driven phenomena, the application of these new laser technologies would increase the chances of uncovering new physics especially in condensed matter systems. Other than the lasers there is another new type of light source coming up, namely attosecond capabilities with free electron lasers like currently available at LCLS at SLAC in California. This would present enormous new opportunities to investigate physics which is experimentally unreachable currently. Therefore, I can easily predict that next 20 years will be as exciting as the last 20 years or even more in terms of new light technology and its applications.