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 an 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 an attosecond time scale, as well as processes in atoms and solids under the influence of femto and attosecond 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 prediction of 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 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 is why the following problem appears: Is it possible the obtain dark states in the 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 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 the 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 a 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 the 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 the electrical field. The question of what kinds of radiation forces exist for laser pulses is still open. Recently we obtain an exact analytical expression for longitudinal radiation force of a laser pulse propagating in dielectric media. This force is proportional to the second derivative of the pulse time envelope. That is why the force vanishes in the CW regime, while in the femtosecond region leads to the 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 the 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 detail. The processes into the target and the ablation plume dynamics and its composition will also be studied experimentally. This knowledge will open 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 a single filament in air, the process was explained by the optical rectification mechanism. The initial spectrally limited laser pulse at 800 nm reaches a broad-band spectrum during the filamentation process and frequencies at second harmonics (SH) are also generated. The combined action of main frequency and SH by an optical rectification mechanism generate a 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 the 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 a new nonlinear parametric conversion χ(3) mechanism, leading to an 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 a 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 by 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 the 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 microenvironments 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 a resolution of 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 the investigation of the application of femtosecond pulses in eye surgery and dentistry by introducing a 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.