Search

We report on the reconstruction of ultrashort laser pulses from computer-simulated and experimental second harmonic generation-frequency resolved optical gating (SHG-FROG) spectrograms. In order to retrieve the spectral amplitude and phase we use a convolutional neural network trained on simulated SHG-FROG spectrograms and the corresponding spectral-domain fields employed as labels for the network, which is a complex field encompassing the full information about the amplitude and phase. Our results show excellent retrieval capabilities of the neural network in case of the simulated pulses. Although trained only on computer generated data, the method shows promising results regarding experimentally measured pulses. [ABSTRACT FROM AUTHOR]

We report on the reconstruction of ultrashort laser pulses from computer-simulated and experimental second harmonic generation-frequency resolved optical gating (SHG-FROG) spectrograms. In order to retrieve the spectral amplitude and phase we use a convolutional neural network trained on simulated SHG-FROG spectrograms and the corresponding spectral-domain fields employed as labels for the network, which is a complex field encompassing the full information about the amplitude and phase. Our results show excellent retrieval capabilities of the neural network in case of the simulated pulses. Although trained only on computer generated data, the method shows promising results regarding experimentally measured pulses.

• Reduction is not depending on pulse intensity but on the second order dispersion. • Silver nanoparticles dimension reduction dependents on second order dispersion. • Nanoparticle dimension became smaller for negative second order dispersion. The potential application of metallic nanoparticles had been attracting attention and interest from different areas of academia and industry. The nanoparticles properties and applications depend heavily on their dimension and shape, thus special interest is aimed to controlling the nanoparticles sizes. Ultrashort laser pulses are known to change metallic nanoparticles characteristics, but the interaction mechanism is still not completely understood. In this work we reduced the dimension of silver nanoparticles with ultrashort pulses and demonstrated that there is a dependence of the particles size on the second order dispersion introduced in the pulses, which became smaller as the dispersion becomes more negative. Based on the results, we propose that the Coulomb explosion that reduces the nanoparticles size is predominantly initiated by multiphotonic ionization for the intensities used (1014 W/cm2). [ABSTRACT FROM AUTHOR]

We investigated theoretically the time dependence of ultra-short laser pulse scattering by an atom at the high-frequency limit for the spectral and total probability of the process using new expression which we derived in this paper. We established that the time dependence of spectral scattering is presented by the curve with the maximum for sufficiently large detuning of scattering frequency from the carrier frequency of the pulse, while the total scattering probability is always the monotonically increasing function of time. We also studied the dependence of scattering probability on pulse duration at the long-time limit. It was shown that, at the long-pulse limit, the scattering probability is a linear function of pulse duration, while in the opposite case, it is a function with maximum. The position of this maximum is determined by the detuning of the scattering frequency from the carrier frequency of the pulse.

The paper is devoted to the investigation of the absorption of ultrashort laser pulses on atoms in plasmas, accounting for the different broadening mechanisms of atomic resonant transitions. The analysis is made in terms of the absorption probability during the entire interaction between the laser pulse and atom. Attention is mainly given to dependence of probability upon the pulse duration and the carrier frequency of the pulse. The results are presented via dimensionless parameters and functions describing the effect of finite pulse duration on atomic spectra for different broadening mechanisms, namely Doppler, Voigt, Holtsmark and their combinations, as well as the Stark line broadening of Rydberg atomic lines.

In this manuscript, some effects such as nonlinear Kerr, stimulated Raman, and plasma generation effects lead to obtaining the nonlinear refraction index of the air along an intense ultra-short laser pulse based on one-dimensional propagation analysis. The variations in the pulse frequency by the self-phase modulation effect are investigated for achieving the functionality of the refractive index to the frequency. The mentioned functionality allows implementing Kramers-Kronig relations to measure the absorption coefficient. Results indicate that the front of the laser pulse faces a high rate of the energy loss whereas the back of the pulse experiences a gain. The implementation of Kramers-Kronig relations for a theoretical calculation of absorption coefficient variation along a laser pulse propagating in the air in which we have simultaneously taken into account the three above-mentioned effects distinguishes our work from other studies. [ABSTRACT FROM AUTHOR]

With a fully classical ensemble model, we investigate the nonsequential double ionization (NSDI) of Xe by linearly polarized (LP) and elliptically polarized (EP) ultra-short laser pulses respectively. For these two cases, the momentum distributions of correlated electron in the direction of parallel to the laser field (for LP) and along the long axis of the laser polarization plane (for EP) show significant difference, but along perpendicular to the laser field (for LP) and the short axis of the laser polarization plane (for EP), the momentum distributions often emit into the opposite hemisphere. By back analyzing the classical trajectories of NSDI, it shows that the recollision is still the underlying mechanism. For LP, the recollision often occurs at the first returning of tunneled electron, but for EP, all of the successful NSDI come from the “exit collision” [PRA 81, 023409 (2010)] which is responsible for the momentum distribution of correlated electron. Moreover, for LP, we divide the trajectories of NSDI into four cases based on the time delay between singly ionization and recollision, and find the ionization mechanism is different for them, so the momentum distribution of correlated electron exhibits a strongly distinction. [ABSTRACT FROM AUTHOR]

We discuss in detail the multiphoton absorption theory for wide bandgap dielectrics that we have implemented in the framework of the time-dependent Nth order perturbation theory. In particular, we have carried out calculations on the N-photon absorption coefficient KN and interband transition rate WN for a perfect sapphire crystal (α-Al2O3). Furthermore, we derive an expression on the laser-pulse induced seed electron density n0. Application of this theory on sapphire shows that n0, induced by the ultrashort laser pulse with pulse duration of 30 ps, wavelength of 1 μm, and peak intensity of 5 × 1011W/cm2 at the focus point, cannot lead to multiphoton-based ablation. Indeed, our calculations show that the laser pulse in the N = 7 order absorption process generates a conduction electron density n0 ≈ 6 × 1013 cm-3 that is far too small compared with the required critical density ncr ≈ 1019 - 1021 cm-3. This is in agreement with the extremely small value of the probability (Pind ≈ 2 × 10-9) that we have estimated for a valence electron to be transferred to the conduction band under the influence of the abovementioned laser-pulse conditions. Therefore, we need to seek other mechanisms than multiphoton absorption, to explain ablation in sapphire. However, we show that by using larger photon energies and smaller order N values, it is possible to induce large enough multiphoton absorption excited electron densities, which will lead to ablation in sapphire, even for the abovementioned laser beam intensity. Finally, we discuss the Keldysh adiabatic parameter y and its interpretation in the case of our experimental pulse-laser setup. [ABSTRACT FROM AUTHOR]

Highlights: [•] Surface modifications of Cu are initiated with single laser pulse (1030nm, 6.7ps) at fluence close to modification threshold. [•] Amplified energy absorption is observed on surface topography features. [•] Objects like spikes with spherical endings, extracted spheres, and thin membranes are products of surface micro-eruptions. [•] SEM, TEM, and EBSD techniques are used to investigate the surface modifications. [•] Amorphous phase is not present on the surface, however, sub-surface voids and twins are apparent. [ABSTRACT FROM AUTHOR]

Abstract: The transient response characteristics of HgCdTe photoconductive detector under the radiation of ultra-short laser pulse have been discussed in detail. Specifically, the transient effect of pulse width to the temperature of electronics and crystal lattice, and corresponding resistance changes of detector are mainly discussed. Based on traditional drift-diffusion model, considering that the temperature of electronics and crystal lattice are different under the ultra-short laser pulse, the double-temperature equation is joined to describe the semiconductor carriers’ dynamics features. Using the numerical method, the transient response characteristics of detector in the case of ultra-short pulse have been worked out. The calculation results show that: When the pulse width is greater than nanosecond pulse, the temperature of electronics will be equal to which of crystal lattice. If the pulse width is less than nanosecond pulse, the temperature of electronics is higher than which of the latter. After the end of a pulse, the rebound resistance of detector will be higher than the dark resistance because of the thermal effect. The heat effect is more obvious when a pulse with narrower width and higher energy density incident to the detector. [Copyright &y& Elsevier]

A theoretical model for electrons in the conduction band intend to investigate the optical breakdown threshold in femtosecond laser pulse-fused silica interaction is presented. The model is derived from a rate equation that includes the avalanche and multi-photon ionization processes of Thornber and Keldysh, respectively, and also the three-body and exciton recombination mechanisms. In addition, the time evolution of electron mean energy is also considered through the energy balance equation. The mean energy acts as a trigger for the avalanche mechanism. The evolution of electron density profiles is calculated and discussed with respect to the ionization and recombination mechanisms. The results for the fluence threshold as a function of the pulse duration fall in good agreement with the experimental data reported in the literature. [ABSTRACT FROM PUBLISHER]

The scattering of ultra-short laser pulses by a metallic nanosphere embedded in a dielectric matrix was studied theoretically in the frame of the dipole approximation. Calculations were performed for the corrected Gaussian shape of the incident laser pulse, which enabled us to consider both multi-cycle and sub-cycle pulses. Analysis of the influence of the scattering process on the shape of scattered pulse is made for various durations and carrier frequencies of the incident pulses. [ABSTRACT FROM PUBLISHER]

Abstract: Numerical modeling of electronic excitation processes induced by ultra-short laser pulses in dielectric materials is performed. The developed model is based on a detailed kinetic description and accounts for the absence of equilibrium in the electronic subsystem. The photoionization process is first analyzed for different laser intensities. It is shown that the probability of this process depends strongly on the Keldysh parameter and effective ionization potential, which are calculated as a function of the laser field. Electron energy distributions are calculated and the average energy is analyzed as a function of laser parameters. For laser intensities corresponding to the maximum of this dependency, only around 1% of electrons are shown to achieve the energy required for the impact ionization. [Copyright &y& Elsevier]

Abstract: The theory of sound generation and heat transfer in matrix-embedded metal nanoparticles under ultra-short laser irradiation has been developed. The shape and time dependence of acoustic waves generated by a sharp change in the pressure of electrons have been investigated for a single nanoparticle and for the group of nanoparticles located on a 2-D flat matrix plane. The dependence of the electronic temperature and the temperature of the interface between the dielectric matrix and nanoparticle as a function of time has been derived. [Copyright &y& Elsevier]

Abstract: We review the emerging potential of femtosecond laser-ablation inductively-coupled-plasma mass spectrometry (fs-LA-ICP-MS) for the direct analysis of solid materials. Although LA-ICP-MS has been widely explored and is highly accepted for the determination of major and trace elements, as well as for the measurement of isotope ratios, it still suffers from several limitations that restrain its development as a potential alternative to other, more established techniques. The occurrence of non-stoichiometric effects in the transient signals, defined as elemental fractionation, together with the lack of reference materials for the wide variety of samples of interest often restricts the quantitative analysis to a few elements of identical fractionation behavior. In this sense, significant improvements in the processes responsible for fractionation effects have been recently observed with the use of ultra-short laser pulses. The confinement of pulse energy guarantees better performances in spatial resolution and improves the analytical figures of merit. This review aims to summarize the main differences between the LA mechanisms of short (>1 ps) and ultra-short (<1 ps) laser pulses based on fundamental understanding of the LA process and the most relevant parameters governing the quality of analysis. In order to show the state of the art in fs-LA-ICP-MS, we present a variety of examples for elemental and depth-profiling analysis of solid samples in biological, geological, and materials applications. [Copyright &y& Elsevier]

We investigated theoretically the time dependence of ultra-short laser pulse scattering by an atom at the high-frequency limit for the spectral and total probability of the process using new expression which we derived in this paper. We established that the time dependence of spectral scattering is presented by the curve with the maximum for sufficiently large detuning of scattering frequency from the carrier frequency of the pulse, while the total scattering probability is always the monotonically increasing function of time. We also studied the dependence of scattering probability on pulse duration at the long-time limit. It was shown that, at the long-pulse limit, the scattering probability is a linear function of pulse duration, while in the opposite case, it is a function with maximum. The position of this maximum is determined by the detuning of the scattering frequency from the carrier frequency of the pulse. [ABSTRACT FROM AUTHOR]

The paper is devoted to the investigation of the absorption of ultrashort laser pulses on atoms in plasmas, accounting for the different broadening mechanisms of atomic resonant transitions. The analysis is made in terms of the absorption probability during the entire interaction between the laser pulse and atom. Attention is mainly given to dependence of probability upon the pulse duration and the carrier frequency of the pulse. The results are presented via dimensionless parameters and functions describing the effect of finite pulse duration on atomic spectra for different broadening mechanisms, namely Doppler, Voigt, Holtsmark and their combinations, as well as the Stark line broadening of Rydberg atomic lines. [ABSTRACT FROM AUTHOR]