Intense Femtosecond Laser Pulses Research Articles

Attosecond lighthouses from plasma mirrors

The nonlinear interaction of an intense femtosecond laser pulse with matter can lead to the emission of a train of sub-laser-cycle--attosecond--bursts of short-wavelength radiation1, 2. Much effort has been devoted to producing isolated attosecond pulses, as these are better suited to real-time imaging of fundamental electronic processes3, 4, 5, 6. Successful methods developed so far rely on confining the nonlinear interaction to a single sub-cycle event7, 8, 9. Here, we demonstrate for the first time a simpler and more universal approach to this problem10, applied to nonlinear laser-plasma interactions. By rotating the instantaneous wavefront direction of an intense few-cycle laser field11, 12 as it interacts with a solid-density plasma, we separate the nonlinearly generated attosecond pulse train into multiple beams of isolated attosecond pulses propagating in different and controlled directions away from the plasma surface. This unique method produces a manifold of isolated attosecond pulses, ideally synchronized for initiating and probing ultrafast electron motion in matter.

Fragmentation of Organic Molecules by Intense Femtosecond Lasers

Results will be presented from recent experimental studies of intense femtosecond (fs) laser pulse interactions with small organic molecules. By electrostatically trapping the ionisation products to obtain high resolution mass spectra, we have characterised how fragment spectra change for a range of laser pulse parameters.

Ultrafast electron dynamics on the silicon surface excited by an intense femtosecond laser pulse

The electron dynamics on the silicon surface during the pump ultrashort infrared laser pulse is studied by time-resolved optical microscopy and electron-emission measurements. It is found that the optical response of the material under the conditions where a dense electron-hole plasma is formed is determined by the renormalization of the band spectrum of the material rather than by intraband transitions of photoexcited carriers. Nonlinear Auger recombination in the plasma enhanced by the plasma-induced renormalization of the band gap and accompanied by the generation of hot charge carriers stimulates intense prompt emission of such carriers from the surface of the photoexcited material, whose work function decreases owing to the large plasma-induced renormalization of the energies of higher conduction bands.

THz emission control by tuning density profiles of neutral gas targets during intense laser-gas interaction

Ionization currents generated from two-color or few-cycle intense femtosecond laser pulses interaction with neutral gas targets can emit strong THz waves. Here it is found that the initial non-uniformity of the gas density can significantly affect the ionization currents and subsequent THz emission both in amplitude and in spectrum. Density profile effects on the forward and backward emissions have been studied in details by particle-in-cell simulations, in which the field ionization module is included. Increasing the gas density gradient length, the emitted forward THz spectrum shifts from high to low frequency, and the spectrum width reduces, which offers a way to obtain a tunable THz emission source by laser-gas interaction.

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Modeling of material properties after ultrashort laser and XUV excitation

By means of first principles calculations, we studied the possibility of manipulating structural properties of different materials via excitation with intense femtosecond laser or extreme ultraviolet (XUV) pulses. For silicon and boron-nitride nanotubes, we performed ab initio molecular dynamics simulations using the code CHIVES, developed in our group, to describe their laser-induced structural dynamics. For both materials, we determined the damage thresholds. We also investigated the structural response of magnesium and copper to ultrashort XUV excitation. For this purpose, we performed frozen-phonon calculations based on all-electron density functional theory and allowed the possibility of core-hole excitation. We found that Cu undergoes bond hardening and Mg bond softening upon creation of core holes and hot electrons, where we defined the bond strength by the vibrational frequencies.

Structures of N2Ar, O2Ar, and O2Xe dimers studied by Coulomb explosion imaging

We use intense femtosecond laser pulses to multiply ionize and directly image the structures of N(2)Ar, O(2)Ar, and O(2)Xe dimers by coincidently measuring the momenta of the fragment ions. All these dimers are found to have an equilibrium T-shaped structure in which the bond of the diatomic molecule is perpendicular to the dimer axis. The equilibrium distance between the rare-gas atom and the center-of-mass of the diatomic molecule is estimated to be R(N2-Ar) ~ 3.86 Å, R(O2-Ar) ~ 3.65 Å, and R(O2-Xe) ~ 4.07 Å, respectively. For the T-shaped N(2)Ar dimer, both sequential and direct triple-ionization-induced three-body breakups are observed. In contrast to N(2)Ar dimer, other structures are found to coexist with the dominating T-shaped one for O(2)Ar and O(2)Xe.

Cavitation dynamics and directional microbubble ejection induced by intense femtosecond laser pulses in liquids

We study cavitation dynamics when focusing ring-shaped femtosecond laser beams in water. This focusing geometry reduces detrimental nonlinear beam distortions and enhances energy deposition within the medium, localized at the focal spot. We observe remarkable postcollapse dynamics of elongated cavitation bubbles with high-speed ejection of microbubbles out of the laser focal region. Bubbles are ejected along the laser axis in both directions (away and towards the laser). The initial shape of the cavitation bubble is also seen to either enhance or completely suppress jet formation during collapse. In the absence of jetting, microbubble ejection occurs orthogonal to the laser propagation axis.

Study of the plasma wave excited by intense femtosecond laser pulses in a dielectric capillary

Laser wakefield in a gas-filled capillary driven by a 1-TW femtosecond Ti:Sa laser pulse is studied experimentally by observing driving pulse spectrum modifications, which are caused by the combined action of the optical field ionization and the plasma density oscillations. Good agreement between the results of extensive numerical simulations and the experimental data allows us to estimate the accelerating gradients in the wake, which range from 5 to 10 MV/cm for typical experimental conditions.

Amorphous to crystalline phase transition in carbon induced by intense femtosecond x-ray free-electron laser pulses

We present the results of an experiment where amorphous carbon undergoes a phase transition induced by femtosecond 830 eV x-ray free-electron laser pulses. The phase transition threshold fluence is found to be 282 +/- 11 mJ/cm(2). Atomic force microscopy, photoelectron microscopy, and micro-Raman spectroscopy give experimental evidence for the phase transition in terms of a volume expansion, graphitization, and change of local order of the irradiated sample area. The interaction is modeled by an accurate time-dependent treatment of the ionization dynamics coupled to a two-temperature model. At the phase transition fluence threshold the free-electron density N-e is found to be at maximum 9 x 10(20) cm(-3) while the ion (atom) temperature is found to be 1050 K, e.g., above the crystallization activation temperature reported in the literature. This low ionization rate and high atom temperature suggest a thermally activated phase transition.

Simulation of femtosecond laser pulse propagation in air

We studied the effects of different peak powers and initial pulse shapes during femtosecond laser pulse propagation in air. The full 3D simulation result showed that there was a bundle of discontinuous filaments along intense femtosecond laser pulse propagation axis. Comparisons between pulses with different input peak powers and initial pulse shapes could help us to find ways for forming long and uniform plasma channel in air.

X‐ray emission from CsCl aqueous solutions when irradiated by intense femtosecond laser pulses and its application to time‐resolved XAFS measurement of I− in aqueous solution

This paper reviews X‐ray emission from electrolyte aqueous solutions when irradiated by intense femtosecond laser pulses in air. First, some examples of X‐ray emission spectra were shown when alkali halide aqueous solutions such as CsCl are used as samples. Dependence of X‐ray emission spectra on laser intensity, solute atomic numbers, and solute concentrations are discussed on the basis of multiphoton ionization, resonance absorption, and so on. Laser chirp effects on X‐ray emission were also discussed for understanding mechanisms occurring during a single laser pulse. X‐ray intensity increase up to 103–104 times with prepulse irradiation was also introduced, and the mechanism of X‐ray intensity increase was discussed from the viewpoint of laser ablation induced by the prepulse irradiation. Finally, preliminary results of time‐resolved X‐ray absorption fine spectroscopy using potassium iodide aqueous solution were presented. Copyright © 2012 John Wiley & Sons, Ltd.

Near-field optical microscopy of femtosecond-laser-reshaped silver nanoparticles in dielectric matrix

Samples containing single silver nanoparticles have been irradiated by intense femtosecond laser pulses to gain a persistent transformation of their shape to ellipsoidal forms. Irradiated and non-irradiated regions of these samples have been analyzed by microscope spectrometry as well as near-field scanning optical microscopy (NSOM) with several wavelengths and different linear polarizations. The results show the outstanding capability of NSOM technique to detect the individual shape of transformed metallic nanoparticles and to analyze their orientation and aspect ratio.

Plasma effects in harmonic emission in relativistic laser interactions with overdense plasma targets examined from single particle radiation models

Intense femtosecond laser pulses incident on supercritical plasmas can generate reflected high-order harmonics of the fundamental frequency that extend over the xuv spectral region, characterized by a power decay Pm ∼ m−p, where m is the harmonic number. It has been shown in earlier reports that plasma effects can modify the emission spectrum producing decay indexes p significantly weaker than the universal value 8/3 predicted by the similarity theory, these being within the range 4/3 ≤ p ≤ 8/3. In this paper we employ two single particle radiation models to demonstrate that plasma density effects substantially modify the emission radiated by a relativistic electron accelerated by the electric field of the laser pulse, and show that the harmonic power decay is commonly characterized by the indexes 4/3 and 5/3, with remarkable similitudes to those particle-in-cell results recently reported in ultrarelativistic laser-plasma interactions.

Dissociative double ionization of CO2induced by intense femtosecond laser pulses

We experimentally investigated dissociative double ionization of CO${}_{2}$ induced by intense femtosecond laser pulses. Three-dimensional momenta were precisely measured for correlated fragmental ions CO${}^{+}$ and O${}^{+}$. The dissociation dynamics of CO${}_{2}$${}^{2+}$ was theoretically simulated by using a Coulomb potential approximation. The ultrashort dissociation time (\ensuremath{\sim}400 fs) can well explain the observed anisotropic angular distribution of fragmental ions relative to the laser polarization.

Fingerprints of level depletion in the photoelectron spectra of small Na clusters in the ultraviolet domain

In the framework of time-dependent density functional theory (TDDFT), we study electronic emission from small Na clusters after irradiation by an intense femtosecond laser pulse. Photoelectron spectra (PES) are compared with level depletion, i.e., the electron loss at each single-electron level. Laser frequencies in the UV range (from 9 to 19 eV) are explored. We find a clear correlation of the peak areas in the photoelectron spectra with the state depletion. This thus allows experimental access to the ionization mechanism. This correlation, moreover, demonstrates that one can find a physical interpretation of the Kohn–Sham orbitals in PES, as in real-time TDDFT.

Application of the Z-scan technique to determine the optical Kerr coefficient and two-photon absorption coefficient of magnetite nanoparticles colloidal suspension

We investigate the occurrence of the optical Kerr effect and two-photon absorption when an oil-based magnetic Fe3O4 nanoparticles colloidal suspension is illuminated with high intensity femtosecond laser pulses. The frequency of the pulses is controlled and the Z-scan technique is employed in our measurements of the nonlinear optical Kerr coefficient (n2) and two-photon absorption coefficient (β). From these values it was possible to calculate the real and imaginary parts of the third-order susceptibility. We observed that increasing the pulse frequency, additional physical processes take place, increasing artificially the absolute values of n2 and β. The experimental conditions are discussed to assure the obtention of reliable values of these nonlinear optical parameters, which may be useful in all-optical switching and optical power limiting applications.

Optimizing High Harmonic Generation in Hollow Core Photonic Crystal Fiber as New Optical Waveguide Materials

We investigated the high harmonic generation (HHG) from the interaction of intense femtosecond laser pulse and rare gas in the hollow core photonic crystal fiber(HCPCF) as new optical waveguide materials. The grating-assisted phase matching (GAPM) of HHG in HCPCF is demonstrated to be experimentally feasible and is especially suitable for photon energies in the range of 100~1keV. The conversion efficiency is expected to be dramatically improved for the longer interaction length provided by the very low loss of HCPCF.

Electron and ion dynamics in graphene and graphane fragments subjected to high-intensity laser pulses

In the framework of real-time real-space time-dependent density functional theory complemented with Ehrenfest molecular dynamics we have studied the response of small graphene and graphane fragments to intense femtosecond laser pulses. In particular, we have investigated how the response changes with laser pulses of different frequency (near IR, visible, and UV). The results of our simulations show that graphene has a very high immediate (i.e., within laser-pulse duration) damage threshold. They also suggest that, similar to the case of other hydrogenated surfaces, it should be possible to selectively desorb hydrogens from graphane without destroying the underlying carbon structure provided that the laser pulse parameters are properly chosen.

Full-vectorial modeling of femtosecond pulses for laser inscription of photonic structures

During the last decade, microfabrication of photonic devices by means of intense femtosecond (fs) laser pulses has emerged as a novel technology. A common requirement for the production of these devices is that the refractive index modification pitch size should be smaller than the inscribing wavelength. This can be achieved by making use of the nonlinear propagation of intense fs laser pulses. Nonlinear propagation of intense fs laser pulses is an extremely complicated phenomenon featuring complex multiscale spatiotemporal dynamics of the laser pulses. We have utilized a principal approach based on finite difference time domain (FDTD) modeling of the full set of Maxwell's equations coupled to the conventional Drude model for generated plasma. Nonlinear effects are included, such as self-phase modulation and multiphoton absorption. Such an approach resolves most problems related to the inscription of subwavelength structures, when the paraxial approximation is not applicable to correctly describe the creation of and scattering on the structures. In a representative simulation of the inscription process, the signature of degenerate four wave mixing has been found. © 2012 Optical Society of America.