# Tzveta ApostolovaBulgarian Academy of Sciences | BAS · Neutron and Reactor Physics

Tzveta Apostolova

PhD, associate professor

## About

61

Publications

8,192

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500

Citations

Introduction

Additional affiliations

September 2006 - present

September 2006 - present

June 2001 - July 2006

## Publications

Publications (61)

This book collects the lectures presented in the first COST TUMIEE Training School held in Greece in 2019, supplemented with specific applications that illustrate how the multi-scale approach is implemented in specific cases of interest. The book is intended both as a reference in the field and as a textbook for people becoming interested or enteri...

We investigate the high harmonic generation in bulk silicon irradiated by intense near-infrared laser pulses with pulse duration $$\le $$ ≤ 100 fs. For peak field strength of the applied laser is below 1 V/Å, the spectral intensity of the emitted harmonics follows the prediction of perturbative nonlinear optics—the frequency comb consists of a seri...

We investigate photoinization, energy deposition, plasma formation and the ultrafast optical breakdown in crystalline silicon irradiated by intense near-infrared laser pulses with pulse duration τ≤ 100 fs. The occurrence of high-intensity breakdown was established by the sudden increase of the absorbed laser energy inside the bulk, which correspond...

We investigate photoinization, energy deposition, plasma formation and the ultrafast optical breakdown in crystalline silicon irradiated by intense near-infrared laser pulses with pulse duration $\tau \le $ 100 fs. The occurrence of high-intensity breakdown was established by the sudden increase of the absorbed laser energy inside the bulk, which c...

Diamond laser engineering is of great importance for designing devices, which find applications in radiation sensing and quantum technologies. A review of the present state of the art of experimental and theoretical studies on ultrashort laser irradiation of diamond is presented. For a wide range of laser parameters, the optimization of laser-induc...

We present the generation of high order harmonics in crystalline silicon subjected to intense near-infrared 30fs laser pulse. The harmonic spectrum extends from the near infrared to the extreme ultraviolet spectral region. Depending on the pulsed laser intensity, we distinguish two regimes of harmonic generation: (i) perturbative regime: electron-h...

We present the generation of high order harmonics in crystalline silicon subjected to intense near-infrared 30fs laser pulses. The harmonic spectrum extends from the near infrared to the extreme ultraviolet spectral region. Depending on the pulsed laser intensity, we distinguish two regimes of harmonic generation: (i) perturbative regime: electron-...

We investigate the non-linear response and energy absorption in bulk silicon irradiated by intense 12fs near-infrared laser pulses. Depending on the laser intensity, we distinguish two regimes of non-linear absorption of the laser energy: for low intensities, energy deposition and photoionization involve perturbative three-photon transition through...

We investigate the time-evolution of electron excitation and high-harmonic generation in diamond bulk subjected to intense 800 nm laser pulse. We report the conversion efficiency of near-infrared to ultraviolet radiation and discuss aspects of the transient photoelectron distribution.

We present calculation of photoexcitation and high-harmonic generation in bulk diamond induced by linearly-polarized intense laser pulse of photon energy 1.55 eV, having a time duration of 15 fs and peak laser intensity $I =$ 10 TW/cm$^2$. Following the excitation, the laser pulse creates electron-hole pairs in a localized region near the Brillouin...

Intense femtosecond laser pulses are routinely used to stimulate ultrafast transformations in the properties of the dielectric materials. Understanding the connection of the microscopic response with the macroscopic properties of the photoexcited material, requires application of space and time resolved experimental techniques and development of pr...

Ultrafast plasmonics of novel materials has emerged as a promising field of nanophotonics bringing new concepts for advanced optical applications. Ultrafast electronic photoexcitation of a diamond surface and subsequent surface plasmon-polaritons (SPPs) excitation are studied both theoretically and experimentally - for the first time. After photoex...

We investigate the high-harmonic generation (HHG) from bulk diamond induced by intense 15 fs laser pulse and photon energy 1.55 eV. For laser intensity in the range $I \in [1,50]$ TW/cm$^2$, we find that HHG spectra from diamond exhibits two plateaus with high harmonics extending beyond the 50th order. Consistently with experimental observations, w...

Direct laser writing of conductive paths in synthetic diamond is of interest for implementation in radiation detection and clinical dosimetry. Unraveling the microscopic processes involved in laser irradiation of diamond below and close to the graphitization threshold under the same conditions as the experimental procedure used to produce three-dim...

Ultrafast intense photoexcitation of a silicon surface is complementarily studied experimentally and theoretically, with its prompt optical dielectric function obtained by means of time-resolved optical reflection microscopy and the underlying electron-hole plasma dynamics modeled numerically, using a quantum kinetic approach. The corresponding tra...

A quantum kinetic approach based on the Boltzmann equation is employed to describe the response of dielectric and semiconductor materials to high electronic excitation induced by laser irradiation. The formalism describes from the initial photo-ionization inter-band processes through free carrier absorption inducing additional impact ionization to...

We investigate numerically the photionization of diamond irradiated by 800 nm intense 30fs laser pulse. We find that ionization depends sensitively on the characteristics of the substrate band structure and that the electron yield changes non-monotonically with the increase of the laser intensity. For low intensity of the light pulse I ∼ 10 12 W/cm...

We investigate numerically the photionization of diamond irradiated by 800 nm intense 30fs laser pulse. We find that ionization depends sensitively on the characteristics of the substrate band structure and that the electron yield changes non-monotonically with the increase of the laser intensity. For low intensity of the light pulse I ∼ 10 12 W/cm...

The electron dynamics and surface ablation of silicate glass excited by single UV femtosecond pulses have been studied by photoluminescence and contact ultrasound diagnostics. At low energy densities of the laser radiation, the photoluminescence signal reveals the linear character of the photogeneration of the electron–hole plasma, accompanied by i...

The dynamics of many-electron system under both dc and infrared fields is separated into a center-of-mass and a relative motion. The force-balance equation is employed for the center-of-mass motion and the scattering equation is used for the ultrafast relative-scattering motion. This allows us to include the anisotropic energy-relaxation process. I...

In this work femtosecond laser photo-excitation of GaAs is studied numerically. The transient plasma densities photogenerated during the pumping IR fs-laser pulses were evaluated having in mind experimental data of time-resolved reflectivity measurements of transient bandgap shifts. Theoretical modeling employing quantum kinetic formalism based on...

We calculate the binding energy of on- and off-center hydrogenic impurities in a parabolic quantum dot subjected to an intense high-frequency laser field. An exactly solvable model that replaces the actual Coulomb interaction with the donor by a non-local separable potential is introduced for calculating the binding energy. The separable potential...

Theoretical modeling of photo-induced electron-hole plasma and bandgap
dynamics in GaAs at high femtosecond laser intensities
(˜TW/cm2) employing a quantum kinetic formalism based
on a generalized Boltzmann-type equation, predicts for the first time
against expectations, the saturation of plasma densities despite the
strong direct bandgap narrowing...

Theoretical
modeling of photo-induced electron-hole plasma and bandgap dynamics in GaAs at high femtosecond laser intensities (∼ TW/cm2) employing a quantum kinetic formalism based on a generalized Boltzmann-type equation, predicts for the first time, against expectations, saturation of plasma densities despite of the strong direct bandgap narrowi...

A space and time-dependent quantum-kinetic theory has been formulated based on previous theoretical approaches to study the spatio-temporal microscopic carrier dynamics in laser excited semiconductor materials that accounts for the effects of inhomogeneous excitation and structural inhomogeneities due to bulk filamentation damage and micro/nano str...

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A generalized non-local Fokker-Planck type kinetic equation obtained on the basis of quantum-mechanical formalism is used to study the conduction electron dynamics in model semiconductors excited by spatially inhomogeneous pulsed laser irradiation. In this approach the electrons are assumed to undergo a drift-diffusion motion in the energy-position...

The materials used in the first wall of ITER and fusion power plants will be required to withstand neutron bombardment produced in fusion plasmas. The development of models describing the structural defects in fusion materials has become an important element in the development of a prototype of a fusion power reactor. Modeling of structural defects...

The kinetic time-dependent Fokker-Planck type equation for conduction electron energy distribution function is solved for bulk semiconductors and dielectrics in the presence of source terms such as laser field absorption, impact ionization and non-radiative recombination. The numerically calculated electron distribution function, time dependent ele...

This paper reports results from the theoretical and experimental study
of the energy transfer during CO2 laser beam welding of 10 mm
low-alloyed steels square butt joints. The laser beam power and the
welding speed are varied systematically from 9 kW to 32 kW and from 1
m/min to 5 m/min, respectively. The observed weld depth penetration is
up to 10...

A summary of theoretical investigations of short pulse (τL ≤ 1 ps) laser interaction with dielectrics, semiconductors and semiconductor heterostructures is presented. The time-dependent kinetic Fokker-Planck type equation for excited conduction electrons is used to describe this interaction in SiO2 and is systematically derived for GaAs. The energy...

At the Space Vehicles Directorate of the Air Force Research Laboratory we are interested in the use of detectors in space for surveillance and situational awareness missions. Our primary interests are in observations of objects both on earth and in space, each of which has very different background requirements. In addition, the space environment i...

A successive four-step model is proposed for spatially selective laser cooling of carriers in undoped semiconductor quantum wells. The four physical steps include the following processes: (1) cold electrons with nearly zero kinetic energy are initially excited across a bandgap in a coherent and resonant way by using a weak laser field; (2) the indu...

The ability to reconfigure a sensor in order to enhance performance or perform multiple missions is a very desirable attribute for future sensor systems. If a sensor could reconfigure itself to exploit signals at various wavelengths from the UV through the IR and into the millimeter-wave regimes, that system could support situational awareness miss...

Session 1, New Sensors and New Approaches, was NATO PfP (Partners for Peace) unclassified and started with a presentation by T. Apostolova on new ideas for tuning the wavelength response of detectors throughout the IR, and possibly into the terahertz region, using applied electric and magnetic fields and unconventional biasing methods. A device con...

A four-step laser cooling model according to time sequence is proposed for AlGaAs/GaAs semiconductor quantum wells. The four steps are: (i) cold electrons are pumped coherently by a laser beam close to the edge of the conduction band; (ii) the photo-induced cold carriers are heated to higher energy levels via inelastic phonon scattering; (iii) the...

An approach for carrier cooling in undoped and contactless semiconductors is proposed by using thermal-drag effects in comparison with other methods, such as direct resonant tunneling, nonresonant thermionic, and junction-tunneling cooling, as well as indirect optothermionic and thermoelectric cooling, of carriers in doped and contacted semiconduct...

In the Advanced Detectors Research Group of the Air Force Research Laboratory's Space Vehicles Directorate, we work to enhance existing detector technologies and develop new detector capabilities for future space-based missions, most often using photonic techniques. To that end, we present some ideas we are presently investigating: (i) tuning the w...

Coupled energy-drift and force-balance equations that contain a frictional force for the center-of-mass motion of electrons are derived for hot-electron transport under a strong dc electric field. The frictional force is found to be related to the net rate of phonon emission, which takes away the momentum of a phonon from an electron during each ph...

A generalized nonlocal energy-balance equation for excited carriers and phonons is established for studying the laser cooling of a lattice of a wide-band-gap semiconductor such as AlN using a He-Ne laser through a three-photon nonlinear excitation process. The power-exchange densities of the system are calculated and compared for different strength...

The use of a Boltzmann transport equation with a drift term is physically incorrect for optical-field frequencies. Also, the use of a simple energy-balance equation is found to lead to an inaccurate estimation of electron temperature. Therefore, we have established a Boltzmann-scattering equation for the accurate description of the relative scatter...

An energy-balance equation for excited carriers and phonons is
established for studying the laser cooling of wide-bandgap
semiconductors using three-photon excitation process. The power-exchange
densities of the system are calculated for different strengths of the
excitation filed. When the power-exchange density is positive, it
implies laser cooli...

A nonlocal energy-balance equation is derived for the optical absorption, photoluminescence and inelastic electron-phonon scattering, which determines the electron and hole temperatures for any given lattice temperature. The evolution of the lattice temperature is found to be determined by the difference between the power-loss density due to photol...

A nonlocal energy-balance equation is derived for the optical absorption, photoluminescence and inelastic electron-phonon scattering, which determines the electron and hole temperatures for any given lattice temperature. The evolution of the lattice temperature is found to be determined by the difference between the power-loss density due to photol...

We obtain an energy-balance equation, which determines the electron and hole temperatures for any fixed lattice temperature. The lattice temperature evolution depends on the difference between the power-gain density due to optical absorption of pump laser and the power-loss density due to photoluminescence, as well as the initial lattice temperatur...

The dynamics of a many-electron system under both dc and infrared fields is separated into a center-of-mass and a relative motion. The first-order force-balance equation is employed for the slow center-of-mass motion of electrons, and the Fokker-Planck equation is used for the ultrafast relative scattering motion of degenerate electrons. This appro...

In the presence of a normally incident mid-IR pulsed laser field, phonon-assisted photon absorption by both intrasubband and intersubband phonon scattering of conduction electrons in GaAs/AlGaAs quantum wells are predicted. The novel non-resonant and non-linear intersubband absorption is found by including the photon-induced phonon scattering proce...

A many-body density-matrix theory is derived by including quasiparticle renormalization of kinetic energy and dipole coupling to an external electromagnetic field, as well as the screening and quantum-interference effects. This theory is applied to a three-level resonant asymmetric double-quantum-well system in which the ground subband is coupled t...

In the presence of a normally incident intense mid-IR pulsed laser field, both photon-absorption-induced intrasubband and intersubband phonon-scattering of electrons are found by including the photon-assisted phonon-scattering process in a Boltzmann equation for phonon energies that are smaller than the energy separation between two electron subban...

By including the effect of local fluctuations in the electron kinetic energies, the kinetic Fokker-Planck-type equation for excited conduction electrons in bulk semiconductors (such as GaAs, Si, etc.) is systematically derived in the presence of a pulsed laser beyond the classical limit. A new contribution from an antidiffusion process is found as...

By using a first-order time-dependent perturbation theory and including the effect of energy drift from electron transport on the intraband transitions of electrons due to phonons, the kinetic Fokker-Planck equation for conduction electrons in semiconductors is systematically derived in the presence of a laser pulse. A contribution from the anti- d...

By including the effect of energy drift on the electron intraband transitions due to phonons, the kinetic Fokker-Planck equation for conduction electrons in semiconductors is derived in the presence of a laser pulse. An anti-diffusion term is found as a result of the coupling between the spontaneous-phonon emission and the energy drift of electrons...

Theoretical study of ultrafast laser induced damage by short pulses (τ < 1 ps) is carried out on large-band-gap dielectric in an effort to understand the complex physical processes involved. The numerical method of solving a general time-dependent Fokker-Planck type equation for free electron production is discussed in detail. The calculation shows...

Theoretical study of ultrafast laser induced damage by short pulses (τ<1 ps) is carried out on large-band-gap dielectric in an effort to understand the complex physical processes involved. The numerical method of solving a general time-dependent Fokker–Planck type equation for free electron production is discussed in detail. The calculation shows t...

Theoretical study of ultrafast laser induced damage (LID) by short pulses (τ < 1 ps ) is carried out on large-band-gap dielectric in an effort to understand the complex physical processes involved. The numerical method of solving a general time-dependent Fokker-Planck type equation is discussed in detail and some of the difficulties in the numerica...

Within the itinerant electron picture, full potential LASTO calculations were carried out for Fe_4N. Neutron diffraction measurements yield magnetic moments of 2.98 and 2.01 muB for the inequivalent FeI and FeII sites respectively. The calculated magnetic moment of FeI exhibits saturation around 2.9 mu_B, while the smaller moment of FeII increases...

In the Advanced Detectors Research Group of the Air Force Research Laboratory's Space Vehicles Directorate, we explore innovative ways to enhance existing detector technologies and to develop new detector capabilities for future space-based missions, such as space sensing and situational awareness. Space-based sensing needs will be met with the emp...

## Projects

Project (1)

The ATLANTIC project is aiming at developing a network of advanced theoretical modeling of laser-matter interaction that will foster the development of novel theories by bridging the mathematical descriptions within a consortium of scientific groups who have pioneered these formalisms.