Enhanced soft X-ray emission from carbon nanofibers irradiated with ultra-short laser pulses

A comparative experimental study of the X-ray emission in the water-window spectral region has been performed using carbon nanofibers (CNFs) of different sizes and graphite plate targets, irradiated with ultra-short (Ti:sapphire) laser pulses. More than an order of magnitude enhancement in the X-ray yield is observed from CNFs of 60-nm diameter with respect to graphite targets. The X-ray emission from CNFs of 160-nm diameter was also high. The integrated X-ray yield of these carbon-based targets scales with the laser intensity (I _L) as I_L ^{~ 1.3-1.4}I_{\mathrm{L}}^{\sim 1.3-1.4} in the intensity range of 4×10¹⁶–4×10¹⁷ W/cm². The effect of the laser pulse duration on the X-ray emission from the CNFs was also studied by varying the pulse duration from 45 fs up to 3 ps at a constant fluence of 2×10⁴ J/cm². The optimum laser pulse duration for maximum X-ray emission increases with the diameter of the CNFs used. The results are explained from physical considerations of heating and hydrodynamic expansion of the CNF plasma in which resonance field enhancement takes place while passing through two times the critical density. The results add to the efforts towards achieving an efficient low-cost water-window X-ray source for microscopy.     

Thermonuclear fusion in the irradiation of large clusters of deuterium iodide with a field of a superatomic femtosecond laser pulse

A theory of thermonuclear fusion caused by the irradiation of deuterium-iodide clusters with the field of a superatomic femtosecond laser pulse is developed. It is based on considering the process in which the sequential above-barrier multiple internal ionization of atomic ions within a cluster is accompanied by external field ionization. The theory is illustrated by taking the example of a cluster that is formed by 10⁶ molecules of deuterium iodide and which is irradiated with a laser pulse of duration 50 fs and intensity 2×10¹⁸ W/cm² at the peak. This case is dominated by I²⁶⁺ atomic ions. The yield of neutrons from thermonuclear fusion in a deuteron-deuteron collision upon the passage of a laser pulse is calculated. The result is 10⁵ neutrons per laser pulse. The mean kinetic energy of deuterons is estimated at 50 keV. Owing to induced inverse bremsstrahlung in scattering on multiply charged atomic ions, the electron temperature increases up to 28 keV. The role of the Mie resonance in the heating of the electron component is discussed.     

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.     

Picosecond laser ablation of thin copper films

The ablation process of thin copper films on fused silica by picosecond laser pulses is investigated. The ablation area is characterized using optical and scanning electron microscopy. The single-shot ablation threshold fluence for 40 ps laser pulses at 1053 nm has been determinated toF _thres = 172 mJ/cm². The ablation rate per pulse is measured as a function of intensity in the range of 5 × 10⁹ to 2 × 10¹¹ W/cm² and changes from 80 to 250 nm with increasing intensity. The experimental ablation rate per pulse is compared to heat-flow calculations based on the two-temperature model for ultrafast laser heating. Possible applications of picosecond laser radiation for microstructuring of different materials are discussed.     

Mechanism of generation of high-intensity terahetrz radiation under the action of high-power laser pulsed on a target

The mechanism of generation of terahertz radiation upon irradiation of a target by short (∼ 0.1 ns) high-intensity laser pulses (I ∼ 10¹⁸−10¹⁹ W cm⁻²) is investigated by numerical simulation using the relativistic electromagnetic PIC code. The interaction of such a pulse with the target, a plasma is formed on it. Electrons emitted from the plasma form a virtual cathode whose oscillations are determined not only by their self-field, but also by the field of ions of the plasma. Generation occurs in the terahertz frequency range with the efficiency thrice as high as in the absence of ions (i.e., with traditional reditron generation mechanism). The explanation for this effect is also given.     

Resonant coupling of small size-controlled lead clusters with an intense laser field

We exposed small size-controlled lead clusters with a few hundreds of atoms to laser pulses with peak intensities up to 10¹⁵ W cm^-2 and durations between 60 fs to 2.5 ps. We measured kinetic energies and ionic charge of fragments as a function of the laser intensity and pulse duration. Highly charged Pbⁿ⁺ ions up to n = 26 have been detected presenting kinetic energies up to 15 keV. For comparison with our experimental results, we have performed simulations of the laser coupling with a cluster-sized lead nanoplasma using a qualitative model that was initially proposed by Ditmire and co-workers at LLNL for the case of rare gas clusters. From these simulations we conclude that two mechanisms are responsible for the explosion dynamics of small lead clusters. As already observed for large rare gas clusters (n = 10⁶), fragments with charge states below +10 are driven by Coulomb forces, whereas the higher charged fragments are accelerated by hydrodynamic forces. The latter mechanism is a direct consequence of the strong laser heating of the electron cloud in the nanoplasma arising from a plasmon-like resonance occurring at n _e = 3n _c. In order to obtain an optimized laser-nanoplasma coupling, our results suggest that the plasma resonance should occur at the peak intensity of the laser pulse. Due to inertial effects, even for such small-sized clusters, the observed optimum pulse duration is in the order of 1 ps which is in good agreement with our theoretical results. Received 18 March 2002 Published online 19 July 2002     

Entropy generation rate during laser pulse heating: Effect of laser pulse parameters on entropy generation rate

Laser pulse heating of solid surface and entropy generation during the heating process are considered. Time exponentially decaying pulse is accommodated in the analysis and the laser pulse parameter (β₁/β₂) resulting in minimum entropy generation rate is computed. Analytical solutions for temperature rise are presented and volumetric entropy generation rate is formulated. Two laser pulses resulting in low volumetric entropy generation rate are examined in detail and volumetric entropy generation rate is associated with the laser pulse parameter (β₁/β₂). It is found that volumetric entropy generation rate attains high values in the early heating period due to large (1/T²). Moreover, the laser pulse with high-peak intensity results in lower volumetric entropy generation rate than that corresponding to the low-intensity laser pulse with the same energy content.     

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.     

Biocompatible film deposition by using Nd:YAG pulsed laser

A Nd:YAG laser operating at the fundamental wavelength (1064 nm) and at the second harmonic (532 nm), with 9 ns pulse duration, 100–900 mJ pulse energy, and 30 Hz repetition rate mode, was employed to ablate in vacuum (10^?6 mbar) biomaterial targets and to deposit thin films on substrate backings. Titanium target was ablated at the fundamental frequency and deposited on near-Si substrates. The ablation yield increases with the laser fluence and at 40 J/cm ² the ablation yield for titanium is 1.2×10¹⁶ atoms/pulse. Thin film of titanium was deposited on silicon substrates placed at different distance and angles with respect to the target and analysed with different surface techniques (optical microscopy, scanning electron spectrosopy (SEM), and surface profile).

Hydroxyapatite (HA) target was ablated to the second harmonic and thin films were deposited on Ti and Si substrates. The ablation yield at a laser fluence of 10 J/cm ² is about 5×10¹⁴ HA molecules/pulse. Thin film of HA, deposited on silicon substrates placed at different distance and angles with respect to the target, was analysed with different surface techniques (optical microscopy, SEM, and Raman spectroscopy).

Metallic films show high uniformity and absence of grains, whereas the bio-ceramic film shows a large grain size distribution. Both films found special application in the field of biomaterial coverage.     

Neutron yield upon the irradiation of thin TiD<Subscript>2</Subscript> foils with a superintense laser pulse

The yield of neutrons from the thermonuclear-fusion reaction D(d, n)³He induced in a thin skin layer by the interaction of a high-intensity laser pulse of picosecond duration with thin TiD₂ foils is calculated. A multiple ionization of titanium atoms at the leading edge of the laser pulse is considered. The heating of free electrons proceeds via induced inverse bremsstrahlung in elastic electron scattering on multiply charged titanium ions. The electron temperature is calculated. It proves to be about 10 keV at the laser-pulse intensity of 5×10¹⁸ W/cm² at the peak. The neutron yield is estimated at 10⁴ per laser pulse. These results are in qualitative agreement with experimental data.     

氩团簇高信噪比13—23nm软x射线辐射谱实验观察

用150fs的掺钛蓝宝石激光系统, 在功率密度约为5×10¹⁵ W/cm²时 激励氩(Ar)团簇，利用具有空间分辨能力的平场光栅谱仪观察到13—23nm波段Ar的软x射线谱，并观察到Ar的11阶离子谱线.在较宽的激光脉宽和较低的激光功率密度情况下，通过激励Ar团簇，获得 了Ar的高阶电离度的实验结果，且谱线的信噪比明显好于光场感应电离的情况，说明团簇的 形成大幅度地提高了激光能量的吸收效率. 关键词： Ar团簇 超短强激光 软x射线辐射     

Muon Motion Induced by a Superintense Laser in Muon‐Catalyzed Fusion

Considering the mixture after muon‐catalyzed fusion (μ CF) reaction as overdense plasma, we study muon motion in the plasma produced by a superintense linearly polarized femtosecond laser pulse. Muon drift along the propagation of laser radiation remains after the end of the laser pulse. At the peak laser intensity of 10²¹W/cm², muon goes from the skin layer into field‐free matter at short time which is much less than the pulse duration, before the laser pulse reaches its maximum. Besides, the influence of the laser on other particles in the plasma is less. Hence, this work can avoid muon sticking to alpha (α) effectively and reduce muon‐loss probability in μ CF. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhanced production of fast multi-charged ions from plasmas formed at cleaned surface by femtosecond laser pulse

We present atomic, energy, and charge spectra of ions accelerated at the front surface of a silicon target irradiated by a high-contrast femtosecond laser pulse with an intensity of 3×10¹⁶ W/cm², which is delayed with respect to a cleaning nanosecond laser pulse of 3-J/cm² energy density. A tremendous increase in the number of fast silicon ions and a significant growth of their maximum charge in the case of the cleaned target from 5+ to 12+ have been observed. The main specific features of the atomic, energy, and charge spectra have been analyzed by means of one-dimensional hydrodynamic transient-ionization modeling. It is shown that fast highly charged silicon ions emerge from the hot plasma layer with a density a few times less than the solid one, and their charge distribution is not deteriorated during plasma expansion.This revised version was published online in August 2005 with a corrected cover date.     

Formation of amorphous carbon on melting of microcrystalline graphite by picosecond laser pulses

Observations of microcrystalline graphite subjected to picosecond laser pulses reveal the formation of a liquid phase with a subsequent transition to a uniform amorphous state of a surface layer upon solidification. This phenomenon is observed on a definite type of graphite and with the radiation incident on a plane parallel to the sixfold symmetry axis, and only for certain parameters of the laser pulse. A structural analysis of the amorphous phase is performed by electron microscopy and Raman scattering spectroscopy. A periodic structure with a period of the order of the wavelength of the heating pulse is formed in the heating region. The “rulings” of this periodic structure are oriented in the direction of polarization of the heating pulse. A study of the reflection kinetics of the probe laser pulse showed that the characteristic existence time of the liquid phase and of the solidification process is ∼10⁻¹⁰ s. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 10, 661–665 (25 November 1997)     

Surface heating of deuterium clusters by the field of a superintense ultrashort laser pulse for implementing the nuclear reaction d+d → 3He+n

A new mechanism for heating the electron component of plasmas formed upon the application of a superintense ultrashort laser pulse to atomic clusters is proposed. Clusters considered here consist of deuterium atoms. Upon the emission of a large number of electrons, an irradiated cluster, which acquires a positive charge, explodes (Coulomb explosion). Deuterons that are ejected as the result of this possess high kinetic energies, so that collisions between them can result in ³He formation accompanied by neutron emission. The new mechanism of the heating of the electron plasma from clusters is based on the conjecture that, when an ionization electron is reflected from the inner surface of the cluster ion in the presence of a laser field, it predominantly absorbs (rather than emits) laser photons.     

Charge composition of a cluster plasma upon irradiation of large atomic clusters by the field of a superatomic femtosecond laser pulse

A theory is developed for calculating the charge composition of a cluster plasma produced upon irradiation of large atomic clusters by the field of a superatomic femtosecond laser pulse. The theory is based on the overbarrier process of a successive multiple internal ionization of atomic ions inside a cluster accompanied by the external field ionization. Collision ionization is also taken into account in the calculations. The theory is illustrated by the example of a cluster consisting of 10⁶ xenon atoms irradiated by a 50-fs laser pulse with a peak intensity of 2×10¹⁸ W/cm². In this case, the Xe²⁶⁺ ions dominate. The amounts of atomic xenon ions with multiplicity up to 31 are calculated.     

Using IR laser radiation for backside etching of fused silica

Laser-induced backside etching of fused silica with gallium as highly absorbing liquid is demonstrated using pulsed infrared laser radiation. The influences of the laser fluence, the pulse number, and the pulse length on the etch rate and the etched surface topography were studied and the results are compared with these of excimer laser etching. The high reflectivity of the fused silica-gallium interface at IR wavelengths results in the measured high threshold fluences for etching of about 3 J/cm² and 7 J/cm² for 18 ns and 73 ns pulses, respectively. For both pulse lengths the etch rate rises almost linearly with laser fluence and reaches a value of 350 and 300 nm/pulse at a laser fluence of about 12 and 28 J/cm², respectively. The etching process is almost free from incubation processes because etching with the first laser pulse and a constant etch rate were observed. The etched surfaces are well-defined with clear edges and a Gaussian-curved, smooth bottom. A roughness of about 1.5 nm rms was measured by AFM at an etch depth of 0.95 μm. The normalization of the etch rates with respect to the reflectivity and the pulse length results in similar etch rates and threshold fluence for the different pulse widths and wavelengths. It is concluded that etching is a thermal process including the laser heating, the materials melting, and the materials etching by mechanical forces. The backside etching of fused silica with IR-Nd:YAG laser can be a promising approach for the industrial usage of the backside etching of a wide range of materials. PACS 81.65.C; 81.05.J; 79.20.D; 61.80.B; 42.55.L     

Thermonuclear fusion in a strong laser field

Thermonuclear fusion induced by the irradiation of solid deuterated cluster targets and foils with fields of strong femtosecond and picosecond laser pulses is discussed. The thermonuclear-fusion process D(d, n)³He in a collision of two deuterons at an energy of 50 to 100 keV in a deuterium cluster target irradiated with a strong laser pulse is discussed. A theory of thermonuclear fusion proceeding upon the irradiation of clusters formed by deuterium iodide (DI) molecules with the field of a superintense femtosecond laser pulse is developed. This theory is based on an above-barrier process in which the sequential multiple inner ionization of atomic ions within a cluster is accompanied by field-induced outer ionization. The yield of neutrons from thermonuclear fusion in a deuteron-deuteron collision after the completion of a laser pulse is calculated. The yield of neutrons is determined for the thermonuclear-fusion reaction proceeding in the interaction of an intense picosecond laser pulse with thin TiD₂ foils. A multiple ionization of titanium atoms at the front edge of the laser pulse is considered. The heating of free electron occurs in induced inverse bremsstrahlung in the process of electron scattering on multiply charged titanium ions. The yield of alpha particles in the thermonuclear-fusion reaction involving protons and ¹¹B nuclei that is induced in microdrops by a strong laser field is determined. Experimental data on laser-induced thermonuclear fusion are discussed.     

Pulse regime in formation of fractal fibers

The pulse regime of vaporization of a bulk metal located in a buffer gas is analyzed as a method of generation of metal atoms under the action of a plasma torch or a laser beam. Subsequently these atoms are transformed into solid nanoclusters, fractal aggregates and then into fractal fibers if the growth process proceeds in an external electric field. We are guided by metals in which transitions between s and d-electrons of their atoms are possible, since these metals are used as catalysts and filters in interaction with gas flows. The resistance of metal fractal structures to a gas flow is evaluated that allows one to find optimal parameters of a fractal structure for gas flow propagation through it. The thermal regime of interaction between a plasma pulse or a laser beam and a metal surface is analyzed. It is shown that the basic energy from an external source is consumed on a bulk metal heating, and the efficiency of atom evaporation from the metal surface, that is the ratio of energy fluxes for vaporization and heating, is 10^–3–10^–4 for transient metals under consideration. A typical energy flux (~10⁶ W/cm²), a typical surface temperature (~3000 K), and a typical pulse duration (~1 μs) provide a sufficient amount of evaporated atoms to generate fractal fibers such that each molecule of a gas flow collides with the skeleton of fractal fibers many times.