Team Competence
of the Bulgarian partners in the ELI-ERIC consortium

The specific directions of research in each area are listed below.

1. (λ³) diffraction of phase-modulated femtosecond laser pulses and managing of an electron mirror.

Phase modulated femtosecond pulses with time duration 20−30 fs diffract in λ³ regime, depending from the value and the sign of the chirp parameter. By changing the chirp's value, we can manage the parabolic curvature and additionally, from the sign of the chirp parameter, it is possible to inverse the parabola of the intensity profile. Thus we can obtain converging or diverging electron mirrors from the generated electron band. This cannot be obtained from spectrally limited attosecond pulses.

   

2. Characterization and change of the form of femtosecond impulses and bundles with the means of non-linear and singular optics

The singular optics is a rapidly growing branch of physical optics that has been the subject of research in optic bundles with phase dislocations. These two-dimensional phase dislocations are known as optical vortices. On a spiral phase front, Pointing's vector has azimuthal component. Optical vortex solitons can be formed with optical vortices, which, for example, in photorefractive environments, can store two-dimensional waveguide structures subject to erasure and reconfiguration through full-optical processes. The members of the project team have significant experience in the generation of singular bundles - optical vortices, optical vortex dipoles, one-dimensional dark bundles and ring-shaped dark waves, as well as their hybrid formations.

   

3. Investigation of femto- and attosecond dynamics in atomic systems and condensed media

Progress nowadays in the building of laser sources with pulses in femto- and attosecond-time scales provides new opportunities for expanding the time scale in the research of molecules, clusters and solids. In addition to pioneering research in the field of generating and amplification femto- and atoscene pulses, the team has also explored the mechanisms of interaction of high-intensity laser pulses with atoms and molecules where there is significant deformation of the electronic cloud. Under these conditions, the induced charge exchange allows for the study of redistribution of the electronic density with sub-fs resolution and the role of this redistribution in the breakdown of chemical bonds. With the use of the recently developed time-dependent quantum Monte Carlo method (TDQMC), the task of solving the Schrodinger equation for N bodies is a numerical solution to a number of related 3D non-stationary Schrodinger equations for individual guiding waves and individual equations for packet movement. TDQMC is an essential parallel technique for numerical calculations of correlated major states of complex quantum systems, as well as for tracking the temporal evolution of systems exposed to electromagnetic radiation (eg light pulse). The main scientific goal in this direction is to simulate the effects of electronic exchange and correlation on the formation and modification of chemical bonds in attosecond time scale, as well as processes in atoms and solids under the influence of femto and atto-second laser pulses tracking the time evolution of systems.

In the field of femtosecond photonics, the team has a number of internationally recognized achievements, published in the most renowned journals (Nature, Science, Proc Nat., Acad Sci USA)

   

4. Dark states in nonlinear optics

The ability to create matter from light is the most interesting predictions of the quantum electrodynamics (QE). The simplest mechanism by which pure light can be transformed into matter, Breit-Wheeler pair production (γγ→e⁺e^-), has never been observed in the laboratory. Other mechanisms on the base of the contemporary QE request generation of dark state (time dependent boson) before creating of electron-positron pairs.To investigate nonlinear processes in gases and isotropic materials do not request such high laser intensities as these for pair creation. That why the following problem appears: Is it possible the obtaining dark states in regime of nonlinear wave propagation? Obviously, the answer is negative in the frame of laser geometry. The pulse spectrum is situated near one plane wave and the divergence of the Pointing vector determines the direction of propagation and light detection. To be transformed into a dark state the Pointing vector of the localized wave must be transformed into a circulated vector with zero divergence. In this way the electromagnetic energy will exist in a volume, but can't be seen and measured. Such possibility really exists in the frame of Amplitude Nonlinear Maxwell - Dirac system of equations and localized light packet near a spherical wave. Thus appear the problem of engineering design of such types of spherical wave resonators.

   

5. Theoretical and numerical study on the feasibility of particle acceleration and generation of coherent radiation based on cyclotron autoresonance interaction in laser-generated plasma and in vacuum using petawatt femtosecond lasers

ELI Beamlines offers many opportunities for an interdisciplinary research on various fundamental physical phenomena, underlying the interaction of intense femtosecond laser beams with matter, as well as on their applications in novel technologies. Among them are advanced concepts for laser acceleration of charged particles and for generation of coherent radiation. A promising mechanism for both of these techniques is the electron cyclotron autoresonance maser (CARM) interaction, which takes place in a strong magnetic field at relativistic velocities of the beam electrons. These techniques have already proved very promising for acceleration and generation using the currently available powerful lasers and electron sources. We believe that the next generation of extremely powerful (petawatt level) lasers that are under development as a part of the ELI project will open many new (possibly even unsuspected) opportunities for further advancement of such methods. Although the concept of laser-driven acceleration based on the cyclotron authoresonance has been extensively studied recently, its feasibility in the case of petawatt femtosecond lasers remains unexplored. The generation of coherent radiation using high-quality laser accelerated electron beams is also a promising research topic but is even less explored. This motivates us to formulate a study, which could help to evaluate the potential of these approaches and to assist prospective proof-of-the principle experiments.

The research topic formulated above corresponds to the theoretical background and the practical experience of the members of the Bulgarian research team demonstrated pursuing several research projects that involve modelling and simulation of beam-wave interaction in various gyro-devices operating at electron cyclotron resonance.

   

6. Generation of white continuum in optical glasses, air and gaseous media from infrared femtosecond laser pulses by nonlinear parametric conversion mechanisms.

The absence of ionization and observation of white continuum in the initial moment of filamentation of powerful femtosecond laser pulses, propagating in silica glasses, as well as the filamentation without plasma channels observed in the experiments in air, forced us to look for other nonlinear mechanisms of description the above mentioned effects. For this reason we investigate new parametric conversion mechanism for asymmetric spectrum broadening of femtosecond laser pulses towards the higher frequencies in isotropic media. This mechanism includes cascade generation with THz spectral shift for solids and GHz spectral delay for gases, proportional to the three time carrier to envelope frequency. The process works simultaneously with the four-photon parametric wave mixing.

   

7. Soliton solutions and soliton interactions

In a number of phenomena in nonlinear optics, plasma physics there appear stable localized nonlinear waves known as solitons. Part of one-dimensional multi-component nonlinear Schrodinger equations (MNLSE) investigated in nonlinear optics are integrable system by Inverse Scattering Method (ISM), but most of the equations are not integrable by this method, and in addition also have stable soliton solutions. These solutions interact and by a self-confinement generate more complicated objects as mixed states of solitons.

   

8. Longitudinal radiation force of ultra-short laser pulses

As it was demonstrated by Ashkin in 1970, it is possible to trap particles by lasers, working in CW regime. The analytical expression of the radiation force is obtained in dipole approximation and as it well known, is proportional to the transverse gradient of the square of electrical field. The question what kinds of radiation forces exist for laser pulses is still open. Recently we obtain exact analytical expression for longitudinal radiation force of a laser pulse propagating in dielectric media. This force is proportional to the second derivative of pulse time envelope. That is why the force vanish in CW regime, while in the femtosecond region leads to trapping of particles into the pulse envelope. The moving particles admit an unexpected nonlinear response and the result is new nonlinear evolution of the laser pulses.

   

9. Ultrashort laser ablation: from fundamentals to applications

The research in this field is focused on the fundamental understanding of the interaction of ultrashort laser pulses with solid material and all involved processes that lead to ablation. It is based on development of complex methods for simulation of the material's response including heating and atomic motion dynamics. The processes of material removal and involved processes realized at extreme laser intensities also will be studied in details. The processes into the target and the ablation plume dynamics and its composition will also be studied experimentaly. This knowledge will open a ways of fabrication of novel materials with application in photonics.

   

10. Nonlinear mechanisms of merging and energy exchange between light filaments

The recent experiments with high power Ti: Sapphire laser pulses demonstrate that it is not possible to produce a homogeneous beam pattern. Hot zones are situated across the beam cross section. Each hot zone self-focuses into a filament if the intensity and the power are high enough. Each of the multiple filaments has a core intensity clamped down to that of a single filament of the order of 0.5−5 TW/cm² where the ionization rates are negligible. That is why we will investigate different types of nonlinear interaction mechanisms between collinear femtosecond laser pulses with power slightly above the critical for self-focusing Pcr.

   

11. Spectrum management of coherent THz generation by two color laser pulses

In the first studies on coherent THz generation from single filament in air the process was explained by optical rectification mechanism. The initial spectrally limited laser pulse at 800 nm reach broad-band spectrum during the filamentation process and frequencies at second harmonics (SH) are also generated. Combined action of main frequency and SH by an optical rectification mechanism generate coherent THz signal. This mechanism requests strong SH with energies at least 10-20 percent from the initial pulse. However, such strong SH are not observed in the experiments. The process of effective generation of THz emission via optical rectification process was really realized, but by two color schemes of mixing of two beams, one at main frequency and second on SH. Nevertheless, two main questions still exist: 1) why coherent THz generation could appear in a filament with negligible SH? 2) it is possible that this process could be managed by χ⁽³⁾ mechanisms only? The purpose of our investigation is to present new nonlinear parametric conversion χ⁽³⁾ mechanism, leading to asymmetrical spectral broadening and coherent THz generation. In addition, by separating the initial laser pulse spectrally into two frequencies separated in the spectrum pulses, with nm distances between the spectral maximums, we expect significant increase of the THz signal and the possibility for its spectral management.

   

12. Multiphoton excitation (MPE), Coherent Anti-Stokes Raman (CARS), second- and third- harmonic generation (SHG and THG) time-resolved spectroscopy and microscopy of biological objects.

Second Harmonic Generation (SHG), Third Harmonic Generation (THG), Coherent anti-Stokes Raman (CARS), and multiphoton excitation (MPE) techniques are now applying for optical imaging of cells and tissues. Time-resolved fluorescent techniques allow to increase the diagnostic accuracy for discrimination of pathological changes, and to evaluate the influence of microenvironment - pH, pK, temperature, presence of different bioactive drugs, enzyme activity, which allow to investigate and modulate the cell-drug interactions working in the field of development of novel pharmaceutical compounds development and control. FLIM and TCSPC techniques based on ultrafast light and laser sources are used for time-resolved imaging of biological objects. Ultra-fast laser mass spectrometry of biological molecules could be used combining ultrashort laser ablation with time-of-flight mass spectroscopy (TOF-MS). The sensitivity of TOF-MS for the identification of the biomolecular species in the ablated material from hard (dental, bone) tissues is used for evaluation of the alterations related to development of pathologic changes, biochemical and mineral alterations.

A strong emphasis has been on developing such spectroscopic methods as quantitative tools not just for basic science research, but also as future diagnostic tools for clinical applications. The submicron resolution afforded by optical wavelengths allows investigating the primary processes and influence of microenvironment on a cellular and sub-cellular level. In contrast, applied in clinical practice clinical tomographic modalities such as computed tomography, magnetic resonance imaging, and positron emission tomography (PET) are limited to resolution about 1 mm. This capability is particularly relevant given the size scales of cells/tissues, which display architectures ranging from ~50 nm for organelles to tens of microns for whole cells, and alterations on these scales in the 3D tissue environment accompany many diseases. That is why we investigate the specific cellular and sub-cellular structures, excitation-emission properties and applicability of nonlinear imagines and spectroscopy techniques to resolve and differentiate normal from abnormal cells and microstructures.

   

13. Laser ablation of hard and soft biological tissues; ultra-short laser microprocessing and modification of materials - biopolymers, synthetic polymers and ceramics for development of biomimetic materials.

Ultra-short laser ablation of biological tissues is used for investigation of application of femtosecond pulses in eye surgery and dentistry by introducing minimally invasive concept for treatment and modification of the tissues. Laser processing of innovative biomaterials; physical, functional and biochemical characterization of the laser micro processed biomaterials; monitoring of cell proliferation, migration, differentiation on thin films and 3-D matrices of biocompatible materials, engineering tissues, polymer scaffolds and ceramics.

   

Senior participants:

1. Prof. Dr. Sci Ivan P. Christov
   Research Area: B, C, G

Ivan Christov is a full Professor at the Faculty of Physics, Sofia University "St. Kl. Ohridski" and Doctor of Sciences. His research interests are centered on generation, propagation and interaction of highly coherent optical radiation in atomic and molecular media. More recently he have become interested in developing advanced quantum Monte Carlo methods for efficiently solving the long standing problem of many-electron systems in different time-evolving environments. His research has led to better understand the intimate mechanisms for generation of ultra-short (femtosecond duration) optical pulses directly from the laser source, the so called space-time focusing in solid-state femtosecond lasers with compensated intra-cavity dispersion, which explained the direct generation of sub-10 fs optical pulses for the first time (I.P.Christov et al, Opt.Lett., 21,(1996), pp. 1493-1495). He and his collaborators have demonstrated both theoretically and experimentally that using sub-100 fs laser pulses may significantly enhance the generation of high harmonics, which has led to a rapid progress in the research towards generation of coherent x-ray radiation with unique properties (I.P.Christov et al, Phys.Rev.Lett., 77, (1996), pp. 1743-1746). His research has proposed a new method for generation of isolated attosecond-duration x-ray pulses by using a few-cycle laser pulse in the infrared, interacting with gaseous media (I.P.Christov,et al, Phys.Rev.Lett., 78,(1997), pp. 1251-1263). He and his collaborators have demonstrated for the first time the possibility to control the x-ray emission through controlling the phase of the incoming laser field (x-ray coherent control).(I.P. Christov, et al, Phys. Rev. Lett. 86, (2001) pp. 5458-5461).The experiment was published in Nature. A novel method for quasi-phase matched amplification of high-harmonics and attosecond pulses in hollow waveguides was proposed (I. P. Christov, et al, Opt. Express 7, 362-367 (2000)) which was next realized experimentally and published in Nature and Science. Since 2006 he has been developing novel methods and algorithms to better understand the correlated dynamics of electrons in atoms, molecules and solid state. The new time-dependent quantum Monte Carlo method would allow to reveal the mechanisms which control the behavior of quantum particles in various environments and interactions (I. P. Christov, Opt. Express 14, 6906 (2006), J. Opt. Soc. Am. B 34, 1817 (2017)).He has published a total of 90 research papers, six of those in Phys. Rev. Letters, three in Science, and two in Nature.

   

2. Prof. Dr. Sci Alexander Dreischuh,
   Research Area: B, G

Alexander Dreischuh is DSci, full Professor at the Faculty of Physics, Sofia University "St. Kl. Ohridski". He has been Head of the Department of Quantum Electronics (2003-2011) and Dean of Faculty of Physics (2011-2019). His research interests are in the fields of nonlinear optics (self-phase modulation and cross-phase modulation, four-wave mixing, spatial solitons and soliton interactions, symbiotic mode of nonlinear beam propagation, all-optical interactions), singular optics (phase dislocations, optical vortices, fractional vortex dipoles, singular femtosecond optics), dark and bright spatial solitons, self-focusing, photorefractive nonlinear optics, coherent white light generation), ultrashort laser pulse generation (dispersion control, adaptive control, pulse characterization, above-threshold ionization, high-harmonic generation). He has been Fellow of the Alexander von Humboldt Foundation and Fellow of the Max Planck Society at Max Planck Institute of Quantum Optics, Garching (M?nchen, Germany). Prof. Dreischuh is author and co-author of 114 scientific papers in peer-reviewed international journals (76 with IF, the rest - with SJR), 4 papers in Bulgarian Journal of Physics, 12 full text conference contributions and 79 poster contributions at international conferences, and 27 invited talks. His works are cited more than 1400 times (as of June 2019).

   

3. Prof. Dr. Sci Lachezar Avramov,
   Research Area: L, M

Prof. Dr. Sci. Lachezar Avramov is Director of the Institute of Electronics of Bulgarian Academy of Sciences. He has over 30 years of experience in the research of the interaction of laser radiation with biological structures. He is the founder of the new scientific field - Biomedical photonics for Bulgaria. He is the founder and manager of the first private high-tech company OPTELLA Ltd. in Bulgaria (1990), winner of the Pythagoras Award of Ministry of Education and Science for the most successful company in the field of research and their application, 2009. He is the founder and manager of the Medical Center "Integrative medicine" - a medical institution with the best results in the country in the treatment of cancer. Prof. Lachezar Avramov is the only Eastern European scientist nominated for the 2004 Descartes Award for Excellence in Cancer Diagnostics. He is leading in the development of a new generation of laser medical systems, some of which have been clinically tested and implemented in the country's healthcare network.

   

4. Prof. Dr. Sci Lyubomir Kovachev,
   Research Area: A, D, F, G, H, J, K

Prof. Dr. Sci. L. M. Kovachev is Head of Nonlinear and Fiber Optics Laboratory in Institute of Electronics, BulgarianAcademy of Sciences (BAS). He graduated from SofiaUniversity in 1981 and starts to work in BAS. From 1988 to 1991 he was PhD student in Institute of General Physics (Prokhorov Institute), Moscow and obtains PhD degree in 1991. Up to now he works in Institute of Electronics, BAS, initially on position as physicist and now as full professor and Head of Laboratory. His scientific interests are in the fields of the nonlinear optics, filamentation, optical solitons and nonlinear field theory. Prof. Kovachev is author of 114 papers in scientific journals and Conference Proceedings cited by more of 320 other authors and holds one patent. He is reviewer in the OSA journals and also in some journals of nonlinear optics.

   

5. Assoc. Prof. Dr. Svilen Sabchevski
   Research Area: E

   

6. Assoc. Prof. Dr. Ekaterina Borisova,
   Research Area: L. M

Assoc. Prof. Dr. Ekaterina Borisova is the Head of Biophotonics Laboratory of the Institute of Electronics at the BulgarianAcademy of Sciences (IE-BAS). She received her M.Sc. degrees in Medical Physics and Laser Physics in 2000 from Physics Faculty of Sofia University, Bulgaria. She defended her PhD degree in Physics of wave processes with the PhD thesis entitled "Laser-induced fluorescence and reflectance spectroscopy of biological tissues" in 2005 in Institute of Electronics, BulgarianAcademy of Sciences. In 2007 became an associated professor in IE-BAS and continuously work in IE-BAS till nowadays. From April 2012 to July 2018 she was a Scientific Secretary of IE-BAS and from July 2018 became a Deputy Director of the Institute. Since 2000 she was involved in 2 FP EC projects, 3 COST Actions, 7 international and 10 bilateral projects, 12 nationally funded projects and 9 projects with other national organizations, being a coordinator of 4 international and 7 nationally funded projects in the field of biomedical optics, lasers in medicine and biology, spectroscopy and imaging of oncological diseases, photophysical properties of novel synthesized photosensitizers for photodiagnosis, photodynamic therapy and inactivation, spectral analysis of biological samples, bio-molecules and composite materials. Dr. Borisova was awarded for her scientific investigations in the field of biophotonics with "The best youngest scientist of Bulgarian Academy of Sciences in the field of Physics for 2004", she was a member of the team-finalist for the EU Descartes prise in 2004, awarded with the highest prize of Bulgarian Ministry of Education in Sciences of Bulgaria "Pythagoras" as the best young scientist in Bulgaria for 2012 and with National award of L'Oreal-UNESCO "For Women in Science" for 2014. In 2014 she also became a Senior member of SPIE in recognition of her significant achievements within the optics and photonics community. She is a co-author in more than 140 publications with IF and SJR, 6 book chapters, 4 patents (2 national and 2 European ones) and has more than 600 citations.

   

7. Assoc. Prof. Dr.Todor Petrov,
   Research Area: F, I, K

Assoc. Prof. Dr. T. Petrov is the Head of Metal Vapor Lasers Laboratory of the Institute of Solid State Physics (ISSP) at the Bulgarian Academy of Sciences (BAS). He received his M.Sc. degree in physics from Sofia University, Sofia, Bulgaria. He obtained his Ph.D. degree at the Metal Vapor Lasers Laboratory, ISSP-BAS, Bulgaria in 1993. The topic of his dissertation was "The frequency conversion of copper bromide vapor laser radiation". Dr. Petrov is at the ISSP-BAS since 1991. Since then he was involved in several international and national projects. He was the lieder of two national and three international projects. In 1997 he was invited by Prof. Mentel to join the AEEO, Ruhr Universit?t Bochum in Germany. His investigation program on UV lasing in capacitively coupled RF-exited He (Ne) Copper vapor discharge was supported by a Volkswagen Fellowship. Since 2002 he was visiting regularly the Institute for Laser Science, University of Eelctrocommunications (UEC), Chofu, Tokyo. The research there was related mainly with different aspects of femtosecond lasers and ultrafast laser - mater interaction. Todor Petrov has published 43 papers, cited by 197 other authors and holds one patent.

   

8. Prof. Dr. Sci. Nikolay Nedialkov,
   Research Area: F, I, K

Prof. Dr. Sci. Nikolay Nedyalkov has expertise in laser-matter interactions. He is Head of Micro- and Nanophotonics Lab at the Institute of Electronics, Bulgarian Academy of Sciences (BAS). He received MS degree from Sofia University in 1998. In 2005 he obtained PhD degree in Physics (Physics of wave processes) at the Institute of Electronics. He is actively working in the field of laser mater interaction, laser ablation plume dynamics, and interaction of ultrashort laser pulses with metals. The research activities comprise both experimental and theoretical work. He has strong background in molecular dynamics simulation of ultrashort laser pulses - matter interaction. Recent works are also focused on laser nanostructuring, optical properties of metal and composite nanostructures, and ultrashort laser pulse propagation in transparent materials. Prof. Nedyalkov is author of more than120papers in scientific journals and conference proceedings with more than 800 citations.