arXiv subjects

Boyan Obreshkov

Publications and source records attributed to Boyan Obreshkov.

11 recordsLinked to original sources

arXiv2025

Orientation-dependent high harmonic generation in ZnO

We theoretically and experimentally investigate orientation-dependent high-harmonic generation (HHG) in zinc oxide subjected to intense femtosecond mid-infrared (MIR) laser pulses. In agreement with past measurements from literature, we observe non-perturbative harmonic spectra with harmonics extending beyond the material band gap. The spectra depend sensitively on the orientation of the crystal with respect to the laser polarization direction. To benchmark the measurements, we performed detailed theoretical calculation of the orientation-dependent harmonic yields. The theory predicts a three-fold angular modulation of the intensity of odd-order harmonics in the (0001)-plane, which can be deduced from the symmetry of the hexagonal lattice alone, in contrast a four-fold symmetry is observed in the experiment. The theory points out the essential role of the anisotropy of the multiple band structure affecting the dynamics of electron-hole pair excitation and solid-state HHG.

physics.optics↗

arXiv2024

High-harmonic generation in zinc oxide subjected to intense mid-infrared femtosecond laser pulse

We theoretically investigate photo-excitation of electron-hole pairs and high harmonic generation in the bulk of zinc oxide (ZnO) subjected to intense femto-second laser pulses with mid-infrared wavelength. The main microscopic mechanism of solid-state HHG is identified by separating resonant from non-resonant non-linear optical responses in the photo-excited solid. It allows us to obtain an effective description of the light-matter interaction in which electrons become subject to weak atto-second pulse train with the second harmonic of the drive laser frequency being the repetition frequency in the train. Under a condition of constructive interference between electronic transitions at each half-cycle of the drive laser pulse, resonant-like excitation of electron-hole pairs occurs, analogously to above threshold ionization in a gas phase. The inter-band motion of charge carriers creates rapidly oscillating electric dipole moment, which emits radiation in the form of high-order harmonics of the drive laser frequency. We also discuss the importance of the pulse envelope for producing clean frequency combs as observed in experiments. Good semi-quantitative agreement with the experimental data is found: clean and well defined odd-order harmonic peaks extending well beyond the band edge of ZnO are exhibited for laser linearly polarized at right angles to the optical axis of the crystal.

physics.optics↗

arXiv2021

Femtosecond optical breakdown in silicon

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 corresponds to threshold energy fluence $\Phi_{th} > $ 1 J/cm$^2$. The optical breakdown is accompanied by severe spectral broadening of the transmitted pulse. For the studied irradiation conditions, we find that the threshold fluence increases linearly with the increase of the pulse duration, while the corresponding laser intensity threshold decreases. The effect of the high plasma density on the stability of diamond lattice is also examined. For near threshold fluences, when about 5 \% of valence electrons are promoted into the conduction band, the Si-Si bonds are softened and large Fermi degeneracy pressure arises (with pressure up to 100 kbar). The mechanical instability of the diamond lattice suggests that the large number of electron-hole pairs leads directly to ultrafast melting of the crystal structure.

cond-mat.mtrl-sci↗

arXiv2020

Blueshift of high-order harmonic generation in crystalline silicon subjected to intense femtosecond near-infrared laser pulse

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-hole pairs born during each half-cycle of the laser pulse via multiphoton and tunnel transitions are accelerated in the laser electric field and gain kinetic energy; the electron-hole pairs then recombine in the ground state by emitting a single high-energy photon. The resultant high harmonic spectrum consists of sharp peaks at odd harmonic orders. (ii) non-perturbative regime: the intensity of the harmonics increases, their spectral width broadens and the position of harmonics shifts to shorter wavelengths. The blueshift of high harmonics in silicon are independent on the harmonic order which may be helpful in the design of continuously tunable XUV sources.

physics.atom-ph↗

arXiv2019

Ultrafast energy absorption and photoexcitation of bulk plasmon in crystalline silicon subjected to intense near-infrared ultrashort laser pulses

We investigate the non-linear response and energy absorption in bulk silicon irradiated by intense 12-fs 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 the direct bandgap of silicon. For laser intensities near and above 10$^{14}$ W/cm$^2$, corresponding to photocarrier density of order 10$^{22}$ cm$^{-3}$, we find that absorption at near-infrared wavelengths is greatly enhanced due to excitation of bulk plasmon resonance. In this regime, the energy transfer to electrons exceeds a few times the thermal melting threshold of Si. The optical reflectivity of the photoexcited solid is found in good qualitative agreement with existing experimental data. In particular, the model predicts that the main features of the reflectivity curve of photoexcited Si as a function of the laser fluence are determined by the competition between state and band filling associated with Pauli exclusion principle and Drude free-carrier response. The non-linear response of the photoexcited solid is also investigated for irradiation of silicon with a sequence of two strong and temporary non-overlapping pulses. The cumulative effect of the two pulses is non-additive in terms of deposited energy. Photoionization and energy absorption on the leading edge of the second pulse is greatly enhanced due to free carrier absorption.

physics.optics↗

arXiv2016

Systematic effects in the measurement of the negatively charged pion mass using laser spectroscopy of pionic helium atoms

The collision-induced shift and broadening of selected dipole transition lines of pionic helium in gaseous helium at low temperatures up to $T=12$ K and pressure up to a few bar are calculated within variable phase function approach. We predict blue shift of the resonance frequencies of the $(n,l)=(16,15) \rightarrow (16,14) $ and $(16,15) \rightarrow (17,14)$ unfavored transitions and red shift for the favored transition $(17,16) \rightarrow (16,15)$. The result may be helpful in reducing the systematic error in proposed future experiments for determination of the negatively charged pion mass from laser spectroscopy of metastable pionic helium atoms.

nucl-th↗

arXiv2016

High harmonic generation from bulk diamond driven by intense femtosecond laser pulse

We present theoretical results on the high-harmonic generation (HHG) in bulk diamond induced by intense laser pulse of wavelength 800 nm and duration 15 fs. For laser intensity in the range $1 \le I \le 50 $ TW/cm$^2$ above bandgap harmonics are generated after the pulse peak. We find that the intensity of individual harmonics increases non-linearly with the peak laser intensity, following a non-perturbative trend. For moderate intensity the HHG spectrum exhibits a primary plateau with noisy odd order harmonic structure and a cutoff. For increased laser intensity a secondary plateau emerges with quasi-continuous spectrum of harmonics extending beyond the 50th order. Consistently with experimental observations, we find that the cutoff energy for HHG scales linearly with the peak field strength and derivation of the cutoff law is provided.

physics.atom-ph↗

arXiv2016

Collisional shift and broadening of the transition lines in pionic helium

We calculate the density shift and broadening of selected dipole transition lines of pionic helium in gaseous helium at low temperatures up to T=12 K and pressure up to a few bar. In the approximation of binary collisions the shift and broadening depend linearly on the density; we evaluate the slope of this linear dependence for a few spectral lines of known experimental interest, and also investigate its temperature dependence. We find a blue shift of the resonance frequencies of the $(n,l)=(16,15) \rightarrow (16,14)$, $(17,16) \rightarrow (17,15)$, and $(16,15)\rightarrow(17,14)$ unfavored transitions, and a red shift for the favored one $(17,16) \rightarrow (16,15)$. The results are intended to significantly increase the efficiency of the laser spectroscopy investigations of pionic helium and help the interpretation of the experimental data.

physics.atom-ph↗

arXiv2014

Attosecond streaking of photoelectrons emitted from metal surfaces

We numerically investigate attosecond streaking time delays in the photoemission of valence and 2p core electrons of aluminum surface. We find that electron emission from the core level band is delayed by $\Delta \tau =100$ attoseconds relative to the release of electrons from the valence band. We show that this relative time offset in electron emission is caused by the screening of the streaking laser field by conduction electrons.

quant-ph↗

arXiv2014

Field emission from metal surfaces in the Thomas-Fermi-von-Weizsäcker model

We evaluate the electron emission current density from jellium metallic surfaces in the Thomas-Fermi-von-Weizsäcker approximation. We implement the weighted density approximation (WDA) for description of the exchange and correlation energy of interacting electrons. We find the emission mechanism exhibits crossover from quantum to classical over-barrier escape of electrons from the surface. Well below a surface-specific threshold field strength $E_d$, electron tunneling is mainly affected by the change of the metal workfunction, and is less sensitive to the detailed shape of the surface barrier. In particular, since the position of the image charge plane for electrons leaving the surface does not precisely coincide with the centroid of the induced screening charge at the surface in response to the applied electric field, we find an effective increase in the metal workfunction by $ΔW \sim 0.01$ eV, which decreases the dark current.

cond-mat.mtrl-sci↗

arXiv2010

Local phase invariance of the free-particle Schrodinger equation in momentum space

The local phase-invariance of the momentum-space Schrödinger equation for free-particle has been used to construct quantum kinematics that describes a motion of the particle in external U(1) background gauge field. The gauge structure over the momentum space of the particle is interpreted in terms of helicity and spin carried by the particle. As a by-product an effective one-particle Schrödinger equation of motion for the helicity-carrying particle in external potential field is derived. An effect of screening of the external potential is predicted, that can affect quantization and splittings of energy levels of the particle.

quant-ph↗