Intense Femtosecond Laser Pulses Research Articles

Collimated quasi-monochromatic beams of accelerated electrons in the interaction of a weak-contrast intense femtosecond laser pulse with a metal foil

AbstractWe demonstrated experimentally the formation of monoenergetic beams of accelerated electrons by focusing femtosecond laser radiation with an intensity of $2\times 1{0}^{17} ~\mathrm{W} / {\mathrm{cm} }^{2} $ onto the edge of an aluminum foil. The electrons had energy distributions peaking in the 0.2–0.8 MeV range with energy spread less than 20%. The acceleration mechanism related to the generation of a plasma wave as a result of self-modulation instability of a laser pulse in a dense plasma formed by a prepulse (arriving 12 ns before the main pulse) is considered. One-dimensional and two-dimensional Particle in Cell (PIC) simulations of the laser–plasma interaction showed that effective excitation of a plasma wave as well as trapping and acceleration of an electron beam with an energy on the order of 1 MeV may occur in the presence of sharp gradients in plasma density and in the temporal shape of the pulse.

Computational modeling of laser-plasma interactions: Pulse self-modulation and energy transfer between intersecting laser pulses

The nonlinear interaction of intense femtosecond laser pulses with a self-induced plasma channel in air and the energy transfer between two intersecting laser pulses were simulated using the finite-difference time-domain particle-in-cell method. Implementation of a simple numerical code enabled modeling of various phenomena, including pulse self-modulation in the spatiotemporal and spectral domains, conical emission, and energy transfer between two intersecting laser beams. The mechanism for energy transfer was found to be related to a plasma waveguide array induced by Moiré patterns of the interfering electric fields. The simulation results provide a persuasive replication and explanation of previous experimental results, when carried out under comparable physical conditions, and lead to prediction of others. This approach allows us to further examine the effect of the laser and plasma parameters on the simulation results and to investigate the underlying physics.

Analysis of strained surface layers of ZnO single crystals after irradiation with intense femtosecond laser pulses

Structural modifications of ZnO single crystals that were created by the irradiation with femtosecond laser pulses at fluences far above the ablation threshold were investigated with micro-Raman spectroscopy. After light-matter interaction on the femtosecond time scale, rapid cooling and the pronounced thermal expansion anisotropy of ZnO are likely to cause residual strains of up to 1.8% and also result in the formation of surface cracks. This process relaxes the strain only partially and a strained surface layer remains. Our findings demonstrate the significant role of thermoelastic effects for the irradiation of solids with intense femtosecond laser pulses.

Enhancing the energy of terahertz radiation from plasma produced by intense femtosecond laser pulses

Terahertz (THz) radiation from atomic clusters illuminated by intense femtosecond laser pulses is investigated. By studying the angular distribution, polarization properties and energy dependence of THz waves, we aim to obtain a proper understanding of the mechanism of THz generation. The properties of THz waves measured in this study differ from those predicted by previously proposed mechanisms. To interpret these properties qualitatively, we propose that the radiation is generated by time-varying quadrupoles, which are produced by the ponderomotive force of the laser pulse.

Electron- and phonon-coupling in femtosecond laser-induced desorption of CO from Ru(0001)

We studied femtosecond laser-induced desorption of CO from Ru(0001) using intense near-infrared and visible femtosecond laser pulses. We find a pronounced wavelength dependence with a factor 3–4 higher desorption yield at comparable fluence when desorption is induced via 400nm light, compared to 800nm and attribute this difference to the difference in penetration depth of the incident light. All our data can be described using empirical friction-modeling to determine the desorption mechanism with the same mechanism for both wavelengths. We find that both hot electrons and phonons contribute to the desorption process.

Effect of an electric field on air filament decay at the trail of an intense femtosecond laser pulse

Air plasma density decay in a filament produced by an intense femtosecond laser pulse in an external electric field was investigated experimentally and theoretically. It was demonstrated by means of the terahertz scattering technique that the rate of plasma decay decreases with increasing electric field. At the electric field of 7 kV/cm the lifetime of plasma with the density above 10(16) cm(-3) was prolonged from 0.5 ns to 1 ns. Numerical simulation of electron density decay and electron temperature evolution was performed, taking into consideration dissociative and three-body electron-ion recombination as well as formation of complex positive ions. The simulation showed that under the electric field the electron temperature evolves nonmonotonically and passes through a minimum due to varying contribution of electron-ion collisions to electron heating in the field. The rate of three-body electron recombination with O(2)(+) ions of 2×10(-19)(300/T(e))(9/2) cm(6)/s was found from the experimental measurements at electron temperatures in the 0.25-0.4 eV range and electron densities in the 10(15)-10(17) cm(-3) range.

High-order harmonic generation from graphene: Strong attosecond pulses with arbitrary polarization

We explore high-order harmonic generation (HHG) from a graphene sheet exposed to intense femtosecond laser pulses based on the Lewenstein model. It is demonstrated that the HHG cutoff frequency increases with graphene size up to the classical limit for distant diatomic systems. In contrast to two-center systems, the cutoff frequency remains constant with increasing power of the harmonics as the graphene diameter extends beyond maximal electron excursion. It is shown that the extended nature of the graphene sheet allows for strong HHG signals at maximum cutoff for linearly as well as circularly polarized laser pulses, the latter opening for generation of strong circularly polarized attosecond pulses.

High harmonic generation in $\textbf{H}_{{2}}^{{+}} $ and HD + by intense femtosecond laser pulses: A wave packet approach with nonadiabatic interaction in HD +

We have theoretically investigated the high harmonic generation (HHG) spectra of \(\textrm{H}_2^+ \) and HD + using a time-dependent wave packet approach for the nuclear motion with pulsed lasers of peak intensities (I0) of 3.5 ×1014 and 4.5 ×1014 W/cm2, wavelengths (λL) of 800 and 1064 nm, and pulse durations (T) of 40 and 50 fs, for initial vibrational levels v0 = 0 and 1. We have argued that for these conditions the harmonic generation due to the transitions in the electronic continuum by tunnelling or multiphoton ionization will not be important. Thus, the characteristic features of HHG spectra in our model arise only due to the nuclear motions on the two lowest field-coupled electronic states between which both interelectronic and intraelectronic (due to intrinsic dipole moments, for HD + ) radiative transitions can take place. For HD + , the effect of nonadiabatic (NA) interaction between the two lowest Born–Oppenheimer (BO) electronic states has been taken into account and comparison has been made with the HHG spectra of HD + obtained in the BO approximation. Even harmonics and a second plateau in the HHG spectra of HD + with the NA interaction and hyper-Raman lines in the spectra of both \(\textrm{H}_2^+ \) and HD + for v0 = 1 have been observed for higher value of I0 or λL. Our calculations indicate reasonable efficiencies of harmonic generation even without involving the electronic continuum.

Interaction of electromagnetic fields and atomic clusters

In the framework of real-time real-space time-dependent density functional theory complemented with Ehrenfest molecular dynamics we have studied the response of nanostructures to intense femtosecond laser pulses. Examples of applications include laser desorption of hydrogen from graphene and Coulomb explosion of hydrocarbon molecules.

Nonlinear regime of the excitation of a surface electromagnetic wave on the silicon surface by an intense femtosecond laser pulse

A surface electromagnetic wave has been excited on an atomically smooth silicon surface by an intense infrared femtosecond laser pulse as a result of its self-diffraction on a microscale short-lived optical inhomogeneity of the excitation region rather than on the roughness of its surface relief. This wave has been visualized in the form of the pattern of its interference with the same incident infrared ultrashort pulse, which corresponds to the instantaneous surface dielectric constant grating (reflection), as well as the resulting surface relief grating, using time-resolved far-field optical reflection microscopy.

Energetic proton generation from intense Coulomb explosion of large-size ethane clusters

An experimental investigation is performed on the interaction of intense femtosecond laser pulses at the intensity of 6 × 1017 W/cm2 (55 fs, 160 mJ at 800 nm) with ethane cluster (C2H6)N jets prepared under the backing pressure of 30 bars at room temperature (298 K). The experiment results indicate the generation of energetic protons, whose average and maximum kinetic energies are 12.2 and 138.1 keV, respectively, by Coulomb explosion of (C2H6)N clusters. (C2H6)N clusters of 5 nm in radius are generated in the experiment, which are 1.7 times larger than that of (CH4)N clusters prepared in the same conditions. Empirical estimation suggests that (C2H6)N clusters with radius of about 9.6 nm can be prepared at 80-bars backing pressure at 308 K. While (C2H6)N clusters of so large size are irradiated by sufficiently intense laser pulses, the average energy of protons will be increased up to 50 keV. It is inferred that such large-size deuterated ethane clusters (C2D6)N will favor more efficient neutron generation due to the significant increase of the D-D nuclear reaction cross section in laser-driven cluster nuclear fusion.

Energy splitting of CdSe quantum dots induced by intense femtosecond laser excitation

Microscopic photoluminescence (PL) spectra of self-assembled CdSe quantum dots (QDs) grown by molecular beam epitaxy were investigated under excitation of intense femtosecond laser. Two samples with different QD sizes were fabricated. One had a single layer of larger CdSe QDs while the other had three layers of smaller QDs. The second harmonic radiation at 420 nm obtained from a mode-locked tunable Ti-Sapphire laser was used as the excitation source. The laser power density was in the order of kW cm−2 and the peak power density was in the order of GW cm−2 for the 150 fs laser pulse with a repetition rate of 78 MHz. The intense femtosecond laser pulses generated strong surface acoustic waves and modulated energy bands of electrons and holes of CdSe QDs. Increasing of the laser power resulted in the PL peak of the CdSe QDs splitting into four peaks for both QD samples: two peaks shifted to a lower energy side and the other two shifted to a higher energy side. The strong strain fields led to the mixing of heavy-hole state and light-hole state in the quantum dots. The strain fields further modulated the energy bands of electrons and holes and produced splitting of both electron–heavy hole (e-hh) transition and electron–light hole (e-lh) transition. For the sample with a single layer of smaller QDs, the energy splitting for both e-hh and e-lh transitions reached 23.5 meV at a peak power density of 0.32 GW cm−2. For the sample with three layers of larger QDs, the energy splitting was 19.9 meV for e-hh transition and 17.9 meV for e-lh transition at a peak power of 1.1 GW cm−2.

Nonsequential and Sequential Fragmentation ofCO23+in Intense Laser Fields

We experimentally studied the three-body fragmentation dynamics of CO(2) initiated by intense femtosecond laser pulses. Sequential and nonsequential fragmentations were precisely separated and identified for CO(2)(3+) to break up into O(+) + C(+) + O(+) ions. With accurate measurements of three-dimensional momentum vectors of the correlated atomic ions and calculations of the high-level ab initio potential of CO(2)(3+), we reconstructed the geometric structure of CO(2)(3+) before fragmentation, which turns out to be very close to that of the neutral CO(2) molecule before laser irradiation. Our study indicated that Coulomb explosion is a promising approach for imaging geometric structures of polyatomic molecules if the fragmentation dynamics can be clearly clarified and the appropriate dissociation potential is provided for multiply charged molecular ions.

A modified rate equation for the propagation of a femtosecond laser pulse in field-ionizing medium

A recombination rate of electron-ion in the strong-field atomic process is phenomenologically introduced into the ionization rate equation, and therefrom an ionization and recombination rates equation (IRRE) is obtained. By using the extended IRRE, the propagation equation of an intense femtosecond laser pulse in the gaseous medium is re-derived. Some new physical behaviors and characteristics caused by the introduced recombination rate are discussed in detail.

Field-free molecular alignment for probing collisional relaxation dynamics

We report the experimental study of field-free molecular alignment in CO${}_{2}$ gas mixtures induced by intense femtosecond laser pulses in the presence of collisional processes. We demonstrate that the alignment signals exhibit specific features due to nontrivial collisional propensity rules that tend to preserve the orientation of the rotational angular momentum of the molecules. The analysis is performed with a quantum approach based on the modeling of rotational $J$- and $M$-dependent state-to-state transfer rates. The present work paves the way for strong-field spectroscopy of collisional dynamics.

Quantum control of multi-photon dissociation of HCl+ with intense femtosecond laser pulses

The multi-photon dissociation of HCl(+) through three channels HCl(+)→H(1s|(2)S)+Cl(+)((3)P), H(+)+Cl((2)P(0)), and H((2)S)+Cl(+)((1)D) steered by intense femtosecond laser pulses are investigated theoretically using the quantum wave packet dynamics. The numerical calculations are performed in two cases without and with the coupling between the excited states. The results show that the dissociation is sensitive to the duration τ, peak intensity I(0), and the resonance of driving laser fields. In the case without the coupling, the effect of the permanent dipole moments on the dissociations dominates for τ < 15 fs, while with the increase of τ, the dissociation dynamics is mainly dominated by the transition dipole moment. In the case with the coupling, the above-threshold dissociation process is complex, and the non-resonant (λ = 400 nm) and resonant (λ = 800 and 1200 nm) laser fields lead to different variation of the branching ratios. The angle-resolved energy distribution is also discussed in detail.

Investigation of tunable coherent XUV light source by high harmonics generation using intense femtosecond laser pulses in Ne

The buleshift properties of high-harmonic generation with phase matching of different filling pressures and laser intensities are experimentally investigated in this paper. The laser buleshift generated in gas cell, with about 0.13 nm in plateau and 0.07 nm in cutoff is obtained. The buleshift is influenced by simply changing the filling pressure and laser intensity, namely, by varying the density of free electrons in gas cell.

Ultrashort pulse induced nanogratings

When intense femtosecond laser pulses are focused into a glass substrate, self-organized periodic nanostructures, so-called nanogratings, are generated in a certain parameter regime. To clarify the ultimate structure of the nanogratings we employed focused ion beam (FIB) milling and small angle X-ray scattering (SAXS). The results considerably show that voids are the primary constituents and their number increases with ongoing exposure to laser pulses. Potential applications will be highlighted.

Origin of energetic carbon ions with different charge states in ultrashort laser-thin foil interactions

High charge state carbon ions are observed from the rear surface of thin foil irradiated by intense femtosecond laser pulse at intensities up to 6.4×1018 W/cm2. The origin of the ions is studied by analyzing the basic ionization process occurring at the rear surface. It is shown that the normally dominant ionization process is field ionization by barrier suppression for charge states less than He-like (C4+), while collisional ionization is significant for C5+ and C6+.

Excitation and dissociation of molecules by femtosecond IR laser radiation in the gas phase and on dielectric surfaces

This paper presents an overview of early studies and new experimental data on the effect of near-IR () and mid-IR () intense femtosecond () laser pulses on polyatomic molecules in the gas phase and on the surface of substrates. We examine the vibrational dynamics of nine molecules containing a chromophore group, which are initiated by resonance femtosecond IR laser radiation at a wavelength of , and report measured characteristic times of intramolecular vibrational redistribution. The characteristic time of molecules containing a single group lies in the range and that of the and molecules lies in the nanosecond range (, respectively). Carbon structures have been observed for the first time to result from the decomposition of molecules on the surface of metal fluorides under the effect of femtosecond IR laser radiation in the wavelength range with no gas-phase decomposition of the molecules.