Expansion of matter heated by an ultrashort laser pulse

Recent experiments have utilizied high-power subpicosecond laser pulses to effect the ultrafast heating of a condensed material to temperatures far above the critical temperature. Using optical diagnostics it was established that a complicated density profile with sharp gradients, differing substantially from an ordinary rarefaction wave, forms in the expanding heated matter. The present letter is devoted to the analysis of the expansion of matter under the conditions of the experiments reported by D. von der Linde, K. Sokolowski-Tinten, and J. Bialkowski, Appl. Surf. Science 109/110, 1 (1996); K. Sokolowski-Tinten, J. Bialkowski, A. Cavalleri et al., Proc. Soc. Photo-Opt. Instum. Eng. 3343, 46 (1998); and, K. Sokolowski-Tinten, J. Bialkowski, A. Cavalleri et al., Phys. Rev. Lett. 81, 224 (1998). It is shown that if the unloading adiabat passes through the two-phase region, a thin liquid shell filled with low-density two-phase matter forms in the expanding material. The shell moves with a constant velocity. The velocity in the two-phase material is a linear function of the coordinate (flow with uniform deformation), and the density is independent of the coordinate and decreases with time as t ⁻¹. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 4, 284–289 (25 February 1999)     

Nonlinear Thomson scattering of an ultrashort laser pulse

The nonlinear scattering of an ultrashort laser pulse by free electrons is considered. The pulse is described in the “Mexican hat” wavelet basis. The equation of motion for a charged particle in the field of a plane electromagnetic wave has an exact solution allowing, together with the instant spectrum approximation, the calculation of the intensity of nonlinear Thomson scattering for a high-intensity laser pulse. The spectral distribution of scattered radiation for the entire pulse duration is found by integrating with respect to time. The maximum of the emission spectrum of a free electron calculated in 10¹⁹–10²¹ W/cm² fields lies in the UV spectral region between 3 and 12 eV. A part of the continuous spectrum achieves high photon energies. One percent of the scattered energy for the field intensity 10²⁰ W/cm² is concentrated in the range ħω > 2.7 × 10² eV, for a field intensity of 10²¹ W/cm² in the range ħΩ > 7.9 × 10² eV, and for an intensity of 10²² W/cm² in the range ħΩ > 2.45 × 10⁵ eV. These results allow us to estimate nonlinear scattering as a source of hard X-rays.     

Monotonically chirped pulse evolution in an ultrashort pulse thulium-doped fiber laser

We report on monotonically positively chirped pulse operation of a hybridly mode-locked thulium fiber laser. Dispersion management was realized with a small-core, high-NA fiber providing normal dispersion in the 2 μm wavelength region. The laser delivered pulses with 0.7 nJ energy at the 1927 nm center wavelength and sub-500-fs pulse duration after compression.     

Confined micro-explosion induced by ultrashort laser pulse at SiO2/Si interface

Ultrashort laser pulses tightly focused inside a transparent material present an example of laser interaction with matter where all the laser-affected material remains inside the bulk, thus the mass is conserved. In this paper, we present the case where the high intensity of a laser pulse is above the threshold for optical breakdown, and the material is ionised in the focal area. We consider in detail a special case where a micro-explosion is formed at the boundary of a silicon surface buried under a 10-micron-thick oxidised layer, providing the opportunity to affect the silicon crystal by a strong shock wave and creating new material phases from the plasma state. We summarise the main conclusions on ultrafast laser-induced material modifications in confined geometry and discuss the prospects of confined micro-explosion for forming new silicon phases.     

Lattice deformation from interaction with electrons heated by an ultrashort laser pulse

We propose a model describing the destruction of metals under ultrashort intense laser pulses when heated electrons affect the lattice through the direct electron-phonon interaction. The metal consists of hot electrons and a cool lattice. The lattice deformation is estimated immediately after the laser pulse up to the electron temperature relaxation time. The hot electrons are described with help of the Boltzmann and heat conduction equations. We use an equation of motion for the lattice displacements with the electron force included. Estimates of the lattice deformation show that the ablation regime can be achieved. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 195–199 (10 August 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Generation of an attosecond X-ray pulse in a thin film irradiated by an ultrashort ultrarelativistic laser pulse

The possibility of generating an attosecond x-ray pulse in a thin solid-density plasma layer irradiated by a femtosecond laser pulse of ultrarelativistic intensity has been demonstrated in numerical simulation. Changes in the plasma layer parameters during the light pulse result in the generation of a wide, partly continuous radiation spectrum in the layer. The separation of limited parts in the reflected or transmitted light spectrum makes it possible to obtain isolated short electromagnetic pulses with an intensity reaching 1% of the exciting light intensity.     

Nuclear fusion induced by a super-intense ultrashort laser pulse in a deuterated glass aerogel

A SiO₂ aerogel with absorbed deuterium is proposed as a target for the fusion reaction d + d → He³ + n induced by a superintense ultrashort laser pulse. The multiple inner ionization of oxygen and silicon atoms in the aerogel skeleton occurs in the superintense laser field. All the formed free electrons are heated and removed from the aerogel skeleton by the laser field at the front edge of the laser pulse. The subsequent Coulomb explosion of the deuterated charged aerogel skeleton propels the deuterium ions up to kinetic energies of ten keV and higher. The neutron yield is estimated at up to 10⁵ neutrons per laser pulse for ～200–500 ps if the peak intensity is 10¹⁸ W/cm² and the pulse duration is 35 fs.     

超短激光脉冲

概述了超短激光脉冲技术的发展进程、研究进展及其在液态分子研究方面、超快现象等领域的应用。     

Diffraction of an ultrashort pulse by an aperture

The scalar problem of diffraction of an infinitely short pulse by a plane screen is solved within Kirchhoff’s approximation. The response of an infinitely small aperture is calculated, and the explicit solution is found for the case of a circular aperture.     

Čerenkov radiation excited by an ultrashort laser pulse with oblique amplitude front

We develop the theory of generation of terahertz ?erenkov radiation in a homogeneous nonlinear optical crystal if the amplitude and phase fronts of the excitation pulse do not coincide. It is shown that in the case of the nonlinear optical interaction, the use of laser pulses with oblique amplitude front makes it possible to increase the efficiency of generation of terahertz radiation. In contrast to the case where the amplitude and wave fronts coincide, radiation propagates in a strictly determined direction specified by the amplitude-front plane. It is established that the maximum generation efficiency is reached if the angle of inclination of the amplitude front coincides with that of the terahertz-radiation wave vector.     

Simulation of the expansion of a crystal heated by an ultrashort laser pulse

Molecular dynamics (MD) simulation of expansion of quickly heated Lennard-Jones (LJ) crystalline thin film is performed. The heating time is assumed to be much shorter than the characteristic expansion time. Such situation occurs when a femtosecond laser pulse is absorbed by a crystal. We established that plastic rarefaction wave is formed and propagates from the vacuum boundary into solid.     

Electron detachment from negative hydrogen and lithium ions by an ultrashort laser pulse

A model of electron detachment from negative ions is based on the approximation of instantaneous perturbation. A simple analytical representation for the ground state of the weakly bound electron is chosen to reproduce the radial distribution of electron density at any distance from the center of the ion. The probabilities of electron detachment from negative hydrogen and lithium ions are derived as functions of the transferred momentum of the laser field. The contribution of the inner shells of ions to the electron detachment is estimated. The momentum distribution of electrons in the final state is obtained. The effect of a pulse train on the total probability of detachment is considered for different pulse polarities. The conditions for the partial electron return to the initial state are revealed.     

Plasma heating by an ultrashort light pulse

Nonstationary absorption of the energy of a short electromagnetic-radiation pulse in a superdense semibounded plasma is considered. An approach is proposed for finding the electron distribution function, wherein the region of the skin layer (in the anomalous skin-effect regime) is described as a boundary condition for the distribution function over the entire plasma volume. A self-similar electron-distribution function is obtained and is used to analyze the dynamics of the electromagnetic-field absorption, of the heat transfer, and of the bremsstrahlung of the heated plasma.Quantum Radiophysics Division, Lebedev Physics Institute. Translated from Preprint No. 22 of the Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow, 1990.     

Experimental generation of high-contrast Talbot images with an ultrashort laser pulse

A femtosecond Ti:sapphire laser oscillator emitting pulses with 800 nm central wavelength, 10.9 fs pulse width, and 75 MHz repetition rate, combined with a dispersion-compensated diffractive system, was used to implement a large-area, high-contrast, broadband optical interference technique based on the Talbot effect. Chromatic artifacts associated with the huge spectrum of the optical source (approximately 150 nm) are compensated for with an air-separated hybrid diffractive-refractive lens doublet. The spatial resolution of the chromatically compensated Talbot images under femtosecond illumination is nearly identical to that achieved under continuous wave monochromatic illumination. Furthermore, the temporal width of the signal at the Talbot planes is limited by the group-delay-dispersion coefficient which is shown to be small. High-contrast one-dimensional periodic structures of 96.1 μm spacing generated by Talbot diffractometry are experimentally demonstrated.     

Hawking radiation from ultrashort laser pulse filaments

Event horizons of astrophysical black holes and gravitational analogues have been predicted to excite the quantum vacuum and give rise to the emission of quanta, known as Hawking radiation. We experimentally create such a gravitational analogue using ultrashort laser pulse filaments and our measurements demonstrate a spontaneous emission of photons that confirms theoretical predictions.     

Diffraction of an ultrashort pulse by a slit

An analysis of a specific feature of the interaction of an ultrashort pulse with a slit is made. Analytical expressions for a monochromatic wave in the form of an ultrashort pulse diffracted by an infinite slit are obtained.     

On self-focusing of an ultrashort intense relativistic laser pulse in a plasma

The development of the spatiotemporal (filamentation) instability of a laser pulse upon excitation of a plasma wave is studied numerically and analytically. It is shown that first, as in a medium with inertialless cubic nonlinearity, the filamentation of radiation occurs and then filaments are attracted to each other. The following evolution differs weakly from the evolution of a smoothed wave beam in a medium with inertial nonlinear response.     

Anomalous transmission of an ultrashort ionizing laser pulse through a thin foil

The formation of a highly anisotropic photoelectron velocity distribution as a result of the interaction of a powerful ultrashort laser pulse with a thin foil is found to yield a large skin-layer depth and an anomalous increase of the transmission coefficient. The physical reason for the effect is the influence of the incident wave magnetic field, through the Lorenz force, on the electron kinetics in the skin layer.