Comment on “Nonlinear Compton scattering in ultrashort laser pulses”

The generation and detection of scalar or pseudoscalar high-frequency gravitational waves in some dielectrics, excited by ultrashort intense laser pulses are proposed. Hidden photons and axion particles are elemental excitations of such gravitational waves. The properties of longitudinal electromagnetic waves in dielectric media have been described from the point of view of photon-hidden photon conversion processes in these crystals. Scalar bound photonic states in dielectric media are analyzed. The experimental schemes for photon-hidden photon conversion with taking part in longitudinal electromagnetic excitations in media have been proposed. Hertz gravitational laboratory experiments with the illumination of the dielectric media by intense ultrashort laser pulses and the “Light Sching Trough Wall” scheme using are analyzed.

Enhanced water window x-ray emission (23–44 Å) from carbon clusters, formed in situ using a pre-pulse, irradiated by intense (I > 1017 W/cm2) ultra-short laser pulse, is demonstrated. An order of magnitude x-ray enhancement over planar graphite target is observed in carbon clusters, formed by a sub-ns pre-pulse, interacting with intense main pulse after a delay. The effect of the delay and the duration of the main pulse is studied for optimizing the x-ray emission in the water window region. This x-ray source has added advantages of being an efficient, high repetition rate, and low debris x-ray source.

Relativistically induced transparency regime has been already achieved experimentally by the expansion of ionized solid target during laser-target interaction for relatively longer laser pulses (about 500 fs), but not for ultrashort pulses (about 30 fs), where the expansion is not significant, and, thus, too high laser intensity (close to 10 23 W/cm 2 ) is required. However, when ultrashort intense laser pulse at higher harmonic frequency irradiates a thin solid foil, the target may become relativistically transparent for significantly lower laser pulse intensity compared with the irradiation at fundamental laser frequency. The relativistically induced transparency results in an enhanced heating of hot electrons as well as increased maximum energy of accelerated ions and their numbers. When fundamental laser frequency is converted to the third harmonics, we observed the increase of maximum energy of accelerated protons and numbers of high energy protons by factor of 2 in our two-dimensional particle-in-cell simulations. Lengthening of the laser pulse due to frequency conversion should not decrease the energy and numbers of accelerated protons. The proposed scheme enables to accelerate protons to energies over 250 MeV by ultrashort laser pulse of maximum intensity below 10 22 W/cm 2 irradiating 200 nm thick solid foil.

The simulations of three-dimensional particle dynamics show that when irradiated by an ultrashort intense laser pulse, the deuterated methane cluster expands and the majority of deuterons overrun the more slowly expanding carbon ions, resulting in the creation of two separated subclusters. The enhanced deuteron kinetic energy and a narrow peak around the energy maximum in the deuteron energy distribution make a considerable contribution to the efficiency of nuclear fusion compared with the case of homonuclear deuterium clusters. With the intense laser irradiation, the nuclear fusion yield increases with the increase of the cluster size, so that deuterated heteronuclear clusters with larger sizes are required to achieve a greater neutron yield.

Preface. SILAP 2000: an overview. Part I: Double Ionization of Complex Atoms. Energy distribution of two-electron ionization of helium in an intense field R. Lafon, et al. Double ionization in strong fields: ion momenta and correlated electron momenta A. Staudte, et al. Mechanism of the non sequential double ionization of helium D.G. Lappas, et al. Electron momentum distributions for double ionization in the strong field limit S.P. Goreslavski, S.V. Popruzhenko. S-matrix theory of 'recoil-ion' momentum distribution for double ionization in femtosecond laser fields F.H.M. Faisal, A. Becker. Non-sequential double ionization: a minimal correlation approach R. Kopold, W. Becker. Non-sequential double ionization of atoms in strong fields K. Sacha, B. Eckhardt. Double-electron ionization of two-electron system in strong laser field D.V. Tikhonova. Calculation of double ionization of helium H.G. Muller. The two-electron response in laser driven helium L.R. Moore, et al. The helium atom in strong and short laser pulses: multielectron H. Bachau, R. Hasbani. Dynamics of a two-electron system driven by an ultrashort and intense laser pulse B. Piraux, G. Lagmago-Kamta. Angular distributions for double ionization by an ultrashort, intense laser pulse: the case of Li- G. Lagmago-Kamta, A.F. Starace. Two-and-three electron atoms in strong laser fields P. Lambropoulos, et al. Part II: Ionization and Dissociation of Molecules. Tunnelling ionization and the Franck-Condon principle J.H. Posthumus, et al. Dissociative ionization of few-electron molecules in intense laser fields D. Dundas, et al. One-photon breakup of H+2 in a strong DC field Z. Mulyukov, R. Shakeshaft. Part III: Interaction of Clusters with Very Intense Femtosecond Laser Pulses. Nuclear fusion in gases of deuterium clusters and hot electron generation in droplet sprays under irradiation with an intense femtosecond laser T. Ditmire, et al. The absorption of energy by large atomic clusters from superintense laser pulses V.P. Krainov, M.B. Smirnov. Part IV: Production of Very Intense Femtosecond Laser Pulses. A semi-classical model for high-harmonic generation D.B. Milosevic. Beyond the simple maris model for high harmonic generation M. Brewezyk, K. Rzazewski. Anisotropy induced polarization effects in harmonic generation by an absorptive medium B. Borca, et al. About a new method of high harmonic amplification E.A. Nersesov, et al. X-ray generation via stimulated recombination of electrons and Bohr's correspondence principle A. Jaron, et al. Part V: Stabilization and Relativistic Effects in Super Strong Fields. Interference stabilization: LAMBDA- and V-schemes, dynamics of ionization, initial coherent population of Rydberg levels and quantum phase control of the ionization yield M.V. Fedorov, N.P. Poluektov. Computer experiments on atomic stabilization in a strong laser field A.M. Popov, et al. Quasistationary stabilization of the decay of a weakly-bound level and its breakdown in a strong laser field N.L. Manakov, et al. The strong field limit of atomic stabilization in ultrashort pulses M. Dorr, et al. 3-D numerical calculations of laser atom interactions -- subrelativistic and weakly relativistic regime A. Scrinzi, et al. Momentum space description of hydrogen atom interacting with a low frequency strong laser field A. de Bohan, et al. Relativistic effects in the atomic res

Control over the breakage of a certain chemical bond in a molecule by an ultrashort laser pulse has been considered for decades. With the availability of intense non-resonant laser fields it became possible to pre-determine femtosecond to picosecond molecular bond breakage dynamics by controlled distortions of the electronic molecular system on sub-femtosecond time scales using field-sensitive processes such as strong-field ionization or excitation. So far, all successful demonstrations in this area considered only fragmentation reactions, where only one bond is broken and the molecule is split into merely two moieties. Here, using ethylene (C2H4) as an example, we experimentally investigate whether complex fragmentation reactions that involve the breakage of more than one chemical bond can be influenced by parameters of an ultrashort intense laser pulse. We show that the dynamics of removing three electrons by strong-field ionization determines the ratio of fragmentation of the molecular trication into two respectively three moieties. We observe a relative increase of two-body fragmentations with the laser pulse duration by almost an order of magnitude. Supported by quantum chemical simulations we explain our experimental results by the interplay between the dynamics of electron removal and nuclear motion.

The surfaces of 63 extracted premolar teeth were processed with intense ultrashort laser pulses (λ = 795nm; pulse duration, 120fs; repetition rate, 1kHz) to produce cross patterns with different pitches (s) in the micrometer range in order to evaluate the influence of such microstructures on the shear bond strengths of orthodontic brackets to enamel. The samples were classified in nine groups corresponding to the control group (raw samples) and eight different laser-processed groups (cross patterns with s increasing from 15 to 180μm). Brackets were luted with Transbond(TM) XT adhesive resin to all the samples; after 72h, they all were submitted to strength test in a universal testing machine. Additionally, a third of the samples underwent morphological analysis of the debonded surface by means of scanning electron microscope microscopy and an analysis of the failure mode based on the adhesive remnant index. The results showed that enamel microstructuring with ultrashort laser pulses remarkably increase the bond strength of brackets. Dense cross patterns (s < 90μm) produce the highest increase of bond strengths as compared to control group whereas light ones (s > 90μm) give rise to smaller improvements of the bond strength. A strong correlation of this behavior with the predominant failure mode in both scenarios was found. So far, the best compromise between suitable adhesive efficiency, processing time minimization, and enamel surface preservation suggests the performance of cross patterns with pitches in the order of 90μm.

The dynamical properties of copper metal are obtained on a picosecond time scale using 100fs laser pulse at 1015Wcm−2—an intensity regime relevant to femtosecond micromachining. The dissipation mechanisms and scaling laws spanning a wide temperature range are obtained from femtosecond pump–probe reflectivity. We observe obliteration of the crystalline structure in copper within 400fs due to lattice disorder caused by the intense laser pulse. The electrical resistivity is obtained by studying the probe reflectivity evolution from 0to30ps. The “resistivity saturation” effect in an unexplored regime intermediate to hot plasma and cold solid is studied in detail. The temperature evolution and thermal conductivity values are also obtained.

Biological hard tissues, such as those found in bone and teeth, are complex tissues that build a strong mineral structure over an organic matrix framework. The laser-matter interaction for bone hard tissues holds great interest to laser surgery and laser dentistry; the use of short/ultrashort pulses, in particular, shows interesting behaviors not seen in continuous wave lasers. High laser energy densities in ultrashort pulses can be focused on a small irradiated surface (spot diameter is 10-50 &#956;m) leading to rapid temperature rise and thermal ablation of the bone tissue. Ultrashort pulses, specifically those in the picosecond and femtosecond ranges, impose several challenges in modeling bone tissue response. In the present paper we perform time-dependent thermal simulations of short and ultrashort pulse laser-bone interactions in singlepulse and multipulse (set of ultrashort pulses) modes of laser heating. A comparative analysis for both radiation modes is discussed for laser heating of different types of the solid bone on the nanosecond, picosecond and femtosecond time scales. It is shown that ultrashort laser pulses with high energy densities can ablate bone tissue without heating tissues bordering the ablation creator. This reaction is particularly desirable as heat accumulation and thermal damage are the main factors affecting tissue regrowth rates, and thus patient recovery times.

Es wurde die Ionisationsdynamik der Atome Ar und Ne als auch die der Moleküle N2 und O2 in intensiven ultrakurzen Laserpulsen mittels hochdifferentieller Elektron-Ion-Koinzidenz-Impulsspektroskopie untersucht. Das Hauptziel der Arbeit ist, den detaillierten Ionisationsmechanismus der nichtsequentiellen Doppelionisation (NSDI) und den Einfluss der atomaren und molekularen Strukturen auf die Impulskorrelation der Photoelektronen nach der NSDI zu verstehen. Die experimentellen Ergebnisse für Ne lassen sich gut durch den Mechanismus der instantanen rückstreuinduzierten Elektronenstoßionisation erklären, während für Ar die Annahme eines zusätzlichen Ionisationskanals notwendig ist. Das wahrscheinliche Szenario für diesen Mechanismus ist die instantane Elektronenstoßanregung des einfachgeladenen Ions mit folgender Ionisierung durch das elektrische Feld der Lichtwelle. Mit abnehmender Lichtintensität wird der Beitrag dieses Ionisationsmechanismus zu der Impulskorrelation der Photoelektronen größer. Die detaillierte Analyse der Elektronimpulsverteilungen für Ar und Ne deutet auf einen wesentlichen Einfluss des elektrischen Feldes der Lichtwelle zum Zeitpunkt des Elektronenstoßprozesses der NSDI hin. Für N2 und O2 wurden sehr unterschiedliche Elektron-Elektron-Impulskorrelationen gefunden. Beide Moleküle zeigen neue Charakteristiken, die für Atome nicht gefunden wurden. Die Rolle der zwei Kerne zeigt sich in dem Vergleich des N2-Moleküls und des Ar-Atoms, beide mit ähnlichen Ionisiationspotentialen, bei gleicher Lichtintensität. Um die Relevanz der Elektronenstoßionisation für die Moleküle N2 and O2 zu testen, wurde eine semiklassische Methode eingesetzt, die auf dem Rückstreumodel basiert. Die Modellrechnung berücksichtigt die Symmetrie des mit einem Valenzelektron besetzten molekularen Anfangszustands. Die Rechnung reproduziert qualitativ die wesentlichen Ergebnisse des Experiments. Daraus lassen sich zwei wichtige Schlüsse ziehen. Erstens, instantane Elektronenstoßionisation scheint der allgemeine Mechanismus der NSDI zu sein, sowohl für Atome als auch für Moleküle. Zweitens, die molekulare Struktur hat entscheidenden Einfluss auf die Impulskorrelation der Photoelektronen nach der NSDI. Es wurde herausgefunden, dass die bindende Symmetrie des molekularen Anfangszustandes den inelastischen Elektronenrückstoss stark modifiziert, während die antibindende Symmetrie hauptsächlich das Elektronentunneln in der erste Stufe der NSDI beeinflusst. In beiden Fällen kann der Vorgang als Interferenzeffekt der beiden beteiligten Elektronen interpretiert werden: sowohl zum Zeitpunkt des Rückstoßes des emittierten Elektronenwellenpakets mit dem gebundenen Elektron als auch im Endzustand der beiden Photoelektronen.

The experiments and simulations of fast particles production by an ultra-short laser pulse at intensity les 1.1times1019 W/cm2 interacting with a foil target of different materials and thickness were performed. The angular distributions of the ejected fast particles were detected in experiments and calculated

It is well established that upon ionization of sulphur hexafluoride, the SF6(+) ion is never observed in mass spectra. Recent work with ultrashort intense laser pulses has offered indications that when strong optical field are used, the resulting "bond hardening" can induce changes in the potential energy surfaces of molecular cations such that molecular ions that are normally unstable may, indeed, become metastable enough to enable their detection by mass spectrometry. Do intense, ultrashort laser pulses permit formation of SF6(+)? We have utilized intense pulses of 5 fs, 11 fs, and 22 fs to explore this possibility. Our results are negative: no evidence is discovered for SF6(+). However, multiply charged sulphur and fluorine ions from highly charged SF6(q+) ions are observed that enable us to resolve the controversy regarding the kinetic energy release accompanying formation of F(+) fragment ions. Quantum chemical computations of field-distorted potential energy curves of SF6 and its molecular ion enable us to rationalize our non-observation of SF6(+). Our findings have implications for high harmonic generation from SF6 in the few-cycle regime.

\n Direct laser writing (DLW) techniques with ultrashort laser pulses have been extensively used for materials processing. The nonlinear nature of the interaction between ultrashort pulses and matter confers high spatial localization to these techniques and allows the fabrication of compact devices in a single step with resolution in the micro/nanoscale. In this context, we can highlight the fabrication of microstructures in transparent materials, like polymers and glasses, since they allow for the realization of three-dimensional devices. For instance, high-quality direct laser-written waveguide circuits have been fabricated and applied to different fields, such as telecommunications, quantum optics, sensors, etc. As photonic circuits often guide high light intensity, the characterization of the nonlinear optical properties of these structures becomes essential to direct their technological application. In this work, we realized the third-order nonlinear optical characterization of fs-laser written waveguides, which were fabricated in special glasses: Corning&amp;reg; Gorilla&amp;reg; and Eagle XG&amp;reg; Glass. The nonlinear characterization was performed through the Dispersive-scan (D-scan) technique, the temporal analog of Z-scan that can be applied to waveguides. The results obtained for the Gorilla&amp;reg; Glass waveguides indicate that the fs-laser writing process is responsible for reducing the magnitude of the nonlinear refractive index, when compared to the bulk material. This effect depends on the writing parameters, and the reduction is stronger when higher energy pulses are employed. Raman spectroscopy measurements revealed that this reduction could be related to the generation of non-bridging oxygen hole centers (NBOHCs) upon fs-laser irradiation. The characterization of Eagle XG&amp;reg; Glass waveguides, fabricated in different writing regimes (repetitive and cumulative), confirms the reduction of the nonlinear refractive index in the repetitive regime. However, it also revealed that little to no modification occurs when the cumulative regime is used, meaning that heat accumulation can be responsible for erasing NBOHCs and restoring the optical nonlinearities. Our results bring new light to what is currently known about the light-matter interaction when ultrashort pulses are involved. Furthermore, they can help tailor the application of fs-laser written glass waveguides, according to the desire for high or low optical nonlinearities.\n

Spectral blueshifting and spatial defocusing of intense laser pulses in dense gases

Among the various ionization techniques used in mass spectrometry, resonant multiphoton ionization (REMPI) is the method of choice if the analysis demands high temporal resolution together with the ability to detect several preselected compounds simulta neously in a complex mixture or matrix. However, REMPI can only fulfill these demands if the excited molecular state(s) involved in the multiphoton ionization process are not significantly depleted by fast energy relaxation or by fragmentation processes during the duration of the ionizing laser pulse. Up to now, only lasers with pulse lengths in the nanosecond range were available for analytical applications of laser mass spectrometry. It turns out that, under these conditions, no useful mass spectra can be obtained for a broad variety of compounds such as organometallic compounds, biomolecules, chemical warfare agents and explosives. A possibility of extending the advantageous features of REMPI to these important groups of substances in particular, and to quickly relaxing molecules in general, is the application of ultrashort but intense laser pulses, which complete the ionization process so quickly that relaxation processes cannot become important. The mass spectra obtained in this way are characterized by an intense molecular ion signal accompanied by structure-specific fragments. Using selected examples of environmentally relevant substances, the influence of laser pulse duration on the features of the laser mass spectra is discussed. Furthermore, it is demonstrated that, even under the high laser intensities used in this experiment, non-resonant processes do not play a significant role and selective ionization is still po ssible.