Bulgarian physicist Tzveta Apostolova would be delighted to come back to ELI ALPS

Associate Professor Tzveta Apostolova from the New Bulgarian University and the Institute for Nuclear Research and Nuclear Energy in Sofia and two MSc students conducted experiments at ELI ALPS for eight days. The Bulgarian physicist thinks that the Szeged based laser centre is indeed a world-class research facility.

Bulgarian physicist Tzveta Apostolova would be delighted to come back to ELI ALPS

Tzveta Apostolova in the middle

Did you always want to be a physicist, or did you consider other careers?

Initially, I was interested in biophysics, as my father worked as a reactor physicist and my mother as a radiologist. However, there was no independent biophysics institute in Bulgaria, so eventually I earned a degree in physics. I had just graduated from Sofia University when I heard that PhD positions in the United States were opening up for us too. I did well in the rigorous entrance exam (GRE), and the University of Connecticut offered me a place. I worked as a postdoctoral fellow at the University of New Mexico for a year, and then joined one of the research laboratories of the US Air Force as a National Research Council fellow and later in a permanent position. Yet, after five years, I returned home to the astonishment of my American colleagues. I continued my career at the Institute for Nuclear Research and Nuclear Energy in Sofia. Currently, I also teach at the New Bulgarian University, where I am the head of the Institute for Advanced Physical Studies.

How did you get in touch with the laser research centre in Szeged?

It’s a long story. Upon my return to Bulgaria, I learnt that three laser centres were being built in the Czech Republic, Hungary and Romania. At that time, around 2012, Bulgaria was hoping that a fourth research centre could be established there. One of the leading Bulgarian physicists personally knew Professor Gérard Mourou, the initiator of the ELI laser infrastructure, who was awarded the 2018 Nobel Prize in Physics for the invention of the technique called chirped pulse amplification. ELI is important to us, and we would like to join ELI ERIC, the Extreme Light Infrastructure European Research Infrastructure Consortium, which will assume responsibility for the operation and financing of the ELI facilities in the coming decades. We are following the work of the Consortium as observers in the hope of becoming full members in the foreseeable future. I will do my best to help achieve this. Until a decision is made in this respect, we Bulgarian physicists continue to conduct experiments at the three ELI facilities.

What research topic did you bring to Szeged?

With my two students, I worked on the MIR laser system under the guidance of Bálint Kiss, head of the Mid-Infrared (MIR) Laser Group, for eight days, from morning to evening every single day. We made the most of every moment with Dimitar Velkov and Miles Thoma. The MIR laser emits extremely short pulses of light at a wavelength of about three micrometres at a very high, 100 kHz repetition rate. An important feature of the system is that the phase of the electric field is nearly identical for each shot and can be adjusted to the desired value with the help of a precision control system. We used the MIR laser system to investigate the response of crystals, such as zinc oxide, to intense laser irradiation.


Bálint Kiss, Zoltán Attila Kis, Tzveta Apostolova


What practical significance can the experiment have?

Our research can facilitate the understanding and modelling of ultrafast electron dynamics and other non-equilibrium phenomena in solids. High spatial resolution imaging can provide new information about the internal structure of solid materials and help us understand their intrinsic properties.

What do you think of the colleagues and the research environment?

Problems may occur at any time during an experiment. For me, the excellence of a place is shown by how quickly a challenge is addressed. In our experiment campaign, we also encountered unexpected problems, and part of them were solved with the help of the institute’s researchers. The eight days we spent here were a success, and we saw the results we had anticipated in our theoretical work.

How did your students enjoy their stay in Szeged?

I am convinced that these few days at ELI ALPS were a defining experience for them. They are currently working for their MSc degrees so that they can start their PhD programme afterwards. I hope that they have realized that the working conditions can be good in our region too. Most young researchers, especially those who have graduated from foreign universities, do not return to their home countries. Many Bulgarian professionals envisage a career abroad. Such a research opportunity could provide the necessary motivation to keep our highly qualified human resources in the region.


 Dimitar Velkov, Tzveta Apostolova and Miles Thoma


Will you return to Szeged if you can?

I’ve been to many places around the world, so I’m not just saying that ELI ALPS is an excellent place of research. Here, physicists are supported in the use of world-class facilities by trained, helpful and friendly research staff. Yes, I am planning to return to Szeged to complete our experiments.



The MIR laser system

The MIR laser’s 3.2-micrometre (relatively long) wavelength allows for special experiments, e.g. the generation of high photon and electron energy for high-harmonic spectroscopy or light-induced electron diffraction (LIED). Thanks to the very high, 100 kHz repetition rate, a large amount of data can be collected in a short period of time, which can then be statistically interpreted and analyzed. Due to the stability of the system, processes with a low probability of occurrence can be also measured and investigated. This laser system is well suited for the excitation of semiconductor crystals and for the investigation of solid materials.


Photos: Gábor Balázs 

Author: Zoltán Ötvös