Efficient water-window X-ray pulse generation from femtosecond-laser-produced plasma by using a carbon nanotube target

We adopt a multiwalled carbon nanotube target to increase the efficiency of water-window and [_]K X-ray pulse conversion from femtosecond-laser-produced plasma. The diameter of the carbon nanotubes is around 30 nm and the length is about 12-m. The X-ray fluence enhancement in the water-window region is sevenfold compared with a conventional carbon plate target. Further enhancement can be expected by optimizing the size of the carbon nanotubes. Soft X-ray pulse duration is 26 ps. It is also found that the [_]K X-ray line emission from the Si substrate of the carbon nanotube target was enhanced. This result indicates that by covering various solid materials with carbon nanotubes, enhanced short [_]K X-ray pulses with the corresponding wavelength can be obtained. These results show that carbon nanotubes are very attractive as a target for femtosecond laser-produced-plasma X-ray sources in single-shot X-ray microscopy and time-resolved X-ray diffraction. PACS 52.50.Jm; 52.38.-r; 52.38.Ph; 68.37.Yz; 78.67.-n     

Efficient high-repetition-rate fs-laser based X-ray source

The generation of [_Fe]K line radiation has been studied on a compact, ultrashort, laser based hard X-ray source, particularly suited for high-repetition-rate laser systems (1 to >10 kHz). Sub-millijoule, 25 fs Ti:sapphire laser pulses are applied to generate hard X-ray radiation on a commercially available ferric audio cassette tape target. Spectroscopic investigations reveal strong iron K emission in addition to a weak bremsstrahlung continuum. Under single exposure and single pulse conditions the X-ray yield is low. Energy conversion efficiencies of about _K=2×10^-7 are observed. Prepulse and multiple shot exposure techniques, both successfully used with high energy/low repetition rate systems, are investigated for these low energy/high repetition rate conditions. Depending on the applied pulse energy and intensity, the X-ray yield can be enhanced by more than three orders of magnitude. Photon fluxes exceeding 2×10⁹Kphotons/s are generated with the presented tape target yielding conversion efficiencies of nearly _K=10^-5. PACS 42.65.-k; 52.38.Ph; 52.50.Jm     

Experimental observation of soft X-rays and suprathermal ions from rare-gas cryogenic solid targets

We investigated characteristics of the soft X-ray emission in the wavelength range of 6–16 nm using various rare-gas cryogenic solid targets irradiated with a nanosecond laser pulse. Suprathermal ions having a maximum velocity of the order of 10⁷ cm/s were detected for three rare-gas targets (Ar, Kr, and Xe). The spatial distributions of the soft X-rays and the suprathermal ions were measured for Xe cryogenic solid targets and found to vary as cos² and cos⁵, respectively, where is the angle to the target normal. By integrating the observed cos² spatial distribution, the conversion efficiency of the soft X-ray emission in the wavelength range from 6 to 16 nm was evaluated to be 0.2%/pulse. PACS 52.38.ph; 52.50.Jm     

High-repetition-rate hard X-ray generation with sub-millijoule femtosecond laser pulses

Hard X-ray radiation is generated at 1-kHz repetition rate on metal targets using 30-fs sub-millijoule laser pulses. Spinning-disc targets of medium-Z (Ti, Cr, Fe, Ni, Cu, Zn and Mo) and high-Z (Ta) metals are investigated. For medium-Z targets, characteristic K-shell emission (K and K) is observed in the 4–20 keV energy range in addition to a broadband bremsstrahlung background. Whereas in former studies similar results have been obtained by applying laser pulses in the tens-to-hundreds-of-millijoules range, we observe characteristic X-ray generation even at applied pulse energies as low as 100 J. The well-defined emission wavelength, the high intensity and the high brightness of this radiation makes this source a promising tool for time-resolved experiments with high-repetition-rate (10 kHz) small-scale table-top laser systems . PACS 42.65.-k; 52.38.Ph; 52.50.Jm     

Comparison of simulated X-ray conversion efficiency of laser produced iron plasma with experimental results

X-ray resonance lines between 11 Å and 17 Å emitted from iron plasmas created by a modest KrF laser have been simulated by modifying the atomic and hydrodynamic code EHYBRID. Free–free and free–bound emission from the Si-, Al-, Mg-, Na-, Ne- and F-like ions is calculated in the simulation. In the original experiments, a KrF laser (249 nm wavelength) with focused irradiances between 1×10¹² W/cm² and 1×10¹⁵ W/cm² was focused on iron targets. The laser pulse duration was varied between 10 ps and 20 ns. We have calculated X-ray conversion efficiencies to be, for example, 0.5% over 2 sr for 2×10¹³ W/cm² and 20 ns pulse duration, in good agreement with experimental measurements. The simulation of X-ray emission is also presented for an experiment where a train of eight 7 ps KrF laser pulses is incident onto an iron target. PACS 52.50.Jm; 52.38.Ph; 52.65.Kj; 52.30.Ex; 32.30.Rj     

The dependence of the Fe <Emphasis Type="Italic">K</Emphasis><Subscript><Emphasis Type="BoldItalic">α</Emphasis></Subscript> yield on the chirp of the femtosecond exciting laser pulse

The hard X-ray yield generated with femtosecond laser pulses is studied for differently chirped irradiating laser pulses. The radiation of a Ti:sapphire CPA laser system (29 fs, 750 μJ, 1 kHz) is focused onto an iron containing solid state target producing incoherent hard X-ray radiation, Bremsstrahlung as well as target-specific K_α and K_β lines. The hard X-ray yield has been optimized by introducing negative and positive group delay dispersion (GDD) and third order dispersion (TOD) to the femtosecond laser pulse. The K_α yield could be enhanced by a factor of 1.7 and reached 1.9×10⁸ Fe K_α photons/s in 4π with the laser pulse positively chirped, and 1.5×10⁸ Fe K_α photons/s with the pulse negatively chirped. When the pulse energy is lowered to about 400 μJ the yield maximum at negative chirp vanishes and only the maximum at positive chirp remains. We explain this behavior with an increased electron temperature caused by the induced GDD and TOD in the pulse. PACS 42.65.Re; 52.38.Ph; 52.50.Jm     

Ultra-short efficient laser-driven hard X-ray source operated at a kHz repetition rate

Ultra-short bursts of hard X-ray radiation are generated by interaction of high-contrast femtosecond laser pulses with a jet of liquid Ga. The X-ray emission shows a strong dependence on the angle of incidence and polarization of the laser beam, consistent with the processes of resonant absorption and vacuum heating. As much as 60% of the total X-ray emission consists of [_Ga]K radiation (9.22–9.25 keV) with a photon flux of 6×10⁹ photons/(ssrad). Using this novel X-ray source, static diffraction patterns from a GaAs(111) crystal were recorded with an acquisition time of 2 s. PACS 52.38.Ph; 52.59.Px; 52.50.Jm     

Time of flight of dual-laser ablation carbon plasmas by optical emission spectroscopy

Plasma emission induced by dual pulse laser ablation on graphite target was analyzed by time resolved optical spectroscopy. Experiments were carried out at a pressure of 10^-5 Torr using two Nd:YAG lasers emitting at 1064 and 532 nm synchronized at up to 100 s delay. Up to fivefold intensification of the C II emission lines was observed, when the delay between lasers was set in the 500–1000 ns range. From the time of flight measurements, the ion energy distribution could be controlled by changing the pulse delay between lasers. PACS 52.38.Mf; 78.47.-p; 52.70.-m     

Controlling coherent and incoherent plasma emissions: the role of electron plasma waves

We present results of our two-pulse time resolved measurements of second harmonic and hard X-ray generation in the interaction of an intense (10¹⁶ Wcm^-2, 100 fs, 800 nm) laser with a preplasma generated on a solid surface. The time-resolved study as a function of both plasma scale length and laser polarization brings out interesting features of electron plasma wave dynamics. The harmonic and X-ray emission show contrary behavior and we interpret the results in terms of Resonance Absorption and Wave-Breaking mechanisms. Simple optimization of the scale length of the preplasma and the polarization parameters of the main pulse results in significant enhancements, up to a factor of 100 for X-rays and 10 for the second harmonic respectively. These results can help us understand the governing mechanisms for higher harmonic generation and for fast particle generation, aiding the development of more efficient sources. PACS 52.35.Fp; 42.65.Ky; 52.38.Ph     

Time-resolved analysis of the X-ray emission of femtosecond-laser-produced plasmas in the 1.5-keV range

Recent experimental results on ion beams produced in high-intensity laser–solid interactions indicate the presence of very intense electric fields in the target. This suggests the possibility of efficiently heating a solid material by means of the fast electrons created during the laser–solid interactions and trapped in the target, rather than by the laser photons themselves. We tested this mechanism by irradiating very small cubic aluminum targets with the LULI 100-TW, 300-fs laser at 1.06-m wavelength. X-ray spectra were measured with an ultra-fast streak camera, coupled to a conical Bragg crystal, providing spectra in the 1.5-keV range with high temporal and spectral resolution. The results indicate the creation of a hot plasma, but a very low coupling between the rapid electrons and the solid. A tentative explanation, in agreement with other experimental results and with preliminary particle-in-cell (PIC) simulations, points out the fatal role of the laser prepulse. PACS 52.50.Jm; 52.38.Ph; 52.38.Kd     

Pulsed laser deposition of hard coatings in atmospheric air

A new laser plasma technique for non-vacuum deposition of thin films has been proposed and experimentally realized. It is based on the fact that the plasma plume, which occurs under ablation of a target in air by high-intensity short laser pulses, can penetrate through a dense gas environment without significant cooling at the distance of about 1 mm. The technique has been applied to deposit diamond-like carbon (DLC) coatings on stainless steel substrates using four different values of pulse duration: 10 ns, 300 ps, 5 ps and 130 fs. Optimization of different experimental parameters including distance between the target and the substrate, laser intensity and gases (He, Ar, N₂, compressed air) blown in the deposition zone, has been performed. The deposition rate in the experiments was estimated as 2–5×10^-4 nm/(cm²pulse) for the pulse energy of 1–4 mJ. The deposited amorphous carbon films with thickness of several hundred nanometers have shown high average nanohardness (10–25 GPa depending on the irradiation conditions) and good adhesion to substrates (60 MPa). According to X-ray electron spectroscopy analysis the films consist of both sp²- and sp³-bonded carbon and contain 3–7% of free oxygen in bulk. The mechanisms of DLC non-vacuum laser deposition are discussed. To demonstrate the large potential of this technique, the first results on deposition of titanium nitride using ablation of titanium in air with nitrogen jet assistance are also presented. PACS 52.38.Mf; 81.15.Fg; 81.05.Uw     

Observation of new laser transitions and saturation effects in optically pumped NO

We report investigations of an NO laser employing specially profiled magnetic fields of up to 3.4T, and F₂ pump laser intensities as great as 20 MW cm^–2. We have observed laser oscillation at 226 nm on a rotational branch of the B'-X/it(3–11) band of NO for the first time, in addition to the previously reported oscillation at 218 nm on the B'-X/it(3–10) band. We have also observed visible laser emission on a rotational branch of the B ²-B ² II(3–1) band of NO. Saturation of the NO laser pulse energy with pump intensity has been observed, the total NO laser pulse energy having been increased to 490 J. The possibility of increasing the NO laser pulse energy towards 1 mJ per transition is discussed.     

The effect of mid-infrared and far-infrared emission,generated at NH3 excitation by intense radiation of a TEA CO2 laser,on ammonia absorption

The effect of Mid-InfraRed (MIR) ( 12 m) and Far-InfraRed (FIR) ( 100 m) emission from excited ammonia on the absorption of intense radiation of a TEA CO₂ laser has been studied experimentally under collisional and collisionless excitation conditions with ammonia pressures from 0.5 to 0.03 Torr. The energy of MIR and FIR emission was studied as a function of NH₃ pressure and laser energy fluence. Particular emphasis was given to the kinetics of MIR and FIR emission generation at different NH₃ pressures and to the measurement of the time delay of re-emitted pulses relative to the exciting CO₂ laser pulse. It has been found that the re-emission in the MIR range is highly collisional in nature. The intensity of MIR emission drops sharply (asp ³) with decreasing NH₃ pressure and its delay time relative to the exciting laser pulse increases. At the same time, re-emission in the FIR range (in the case of resonant excitation of NH3 at the 9R (30) line of CO₂ laser) is observed during an exciting pulse up top < 0.03 Torr. When binding the rotational sub-levels of a molecule with transitions, FIR emission acts as rotational relaxation and thus leads to an increase in NH₃ IR absorption even at collisionless excitation.     

Ultrafast electron dynamics of material surfaces under the action of femtosecond laser pulses

Electric collector investigations of the singleand multi-shot femtosecond laser ablation of optical-quality surfaces of different materials, including aluminum, copper, titanium, silicon, and graphite, show that the emission of erosion plasma is significantly lower than the energy density of laser ablation of these materials and replaces the dominant electron emission at lower energy densities. I–V characteristics and cumulative dependences of the collector signal are studied in the emission mode. The observed dependences of the electron and plasma emission signals on the laser pulse energy density are discussed.     

Detailed study of the effect of a short prepulse on soft X-ray spectra generated by a high-intensity KrF* laser pulse

The effect of a short prepulse (0.5 ps) on soft X-ray spectra from a plasma generated by a high intensity KrF* laser pulse (main pulse: 0.5 ps, intensity I _main=5.3×10¹⁵ W/cm²) on flat targets of Al and Cu has been studied in detail. The spectra have been measured as a function of the pulse separation t between the two pulses and the prepulse intensity I _pre. It was found that both the overall emission and the line emission increased with t (at constant I _pre) and with I _pre (at constant t). In particular, lines in the shorter wavelength region had higher intensity. The influence of the prepulse on the line emission of specific transitions in the Al spectra was investigated systematicly. An explanation for the observed effects is given.     

Selective metallization of polyimide by laser-induced plasma-assisted ablation (LIPAA)

We report micromachining of polyimide (PI) by laser-induced plasma-assisted ablation (LIPAA) using a fundamental wavelength of a commercial Q-switched Nd:YAG laser (1064 nm). It is found that an Au film on a glass target is effective for the LIPAA process of PI. The ablation rate reaches several tens of nanometers per pulse. After the LIPAA process, selective metallization of PI with excellent electrical properties is performed by successive electroless Cu plating. The Cu line width of 40 m, which agrees with the line width of regions ablated by the LIPAA process, is achieved using an encapsulated film. PACS 42.62.-b; 52.38.-r; 85.40.Ls     

Coupling of focused laser pulse to surfaces of transparent materials studied by time-resolved imaging technique

Short and intense laser pulse can process the surface and the inside of transparent materials by focusing the pulse at the desired position. Here we report the interaction of fundamental radiation (1064 nm) of the Q-switched Nd:YAG laser to the surface of PMMA as observed by an imaging system with nanosecond time resolution. The system used fundamental radiation of a Q-switched Nd:YAG laser as a processing laser and second harmonic radiation (532 nm) of another Nd:YAG laser as illuminating light. We observed shock waves which propagate into the material and into the atmosphere by shadowgraph and photoelastic method. Surface roughness of a sample is expected to affect the coupling of light and transparent materials for both normal and focused laser light. Our results have revealed the effects visually. For roughness larger than 0.6 m, all energy is absorbed at the surface, while the larger part of the energy is absorbed inside the material as the surface becomes smoother. PACS 52.38.MF; 79.20.DS; 87.63.Lk     

Textured thin films grown at room temperature by laser ablation

Thin films of SrFe₁₂O₁₉, BaFe₁₂O₁₉, Pb_0.76La_{0.16 0.08}Zr_0.53Ti_0.47O₃and Sr_0.3Ba_0.7Nb₂O₆ were grown on monocrystalline silicon substrates by pulsed laser deposition using a 20-ns Nd:YAG laser (1064 nm). The deposited thin films were analyzed by X-ray diffraction in the grazing incidence configuration. The analysis showed evidence of textured growth even though the films were grown at room temperature. Emission spectroscopy was used to establish the time of flight of the species within the plasma plume. Velocities of the order of 10⁶ cm/s were obtained. The high kinetic energy of the species is thought to be responsible for the film texture, as it is released in the substrate–film system, favoring a preferential growth. For all the ablated ceramics, singly ionized species were shown to expand at higher velocities than neutrals. For ions, no consistency in the mass–speed relation was obtained, suggesting both the presence of electric fields during the plasma formation and an evaporation of the target that depends on the vapor pressure of the elements. In this way species that are firstly evaporated will be attracted strongly by fast electrons, allowing heavy ions to acquire higher velocities than lighter ones. PACS 81.15.Fg; 52.38.Mf; 68.55.Jk; 52.38.Kd; 52.70.Kz     

Sound Generation by Breakdown Plasma Formations Initiated by Solid Aerosol Particles upon Exposure to Laser Radiation

The results of experimental investigations into the characteristics of acoustic signals generated by breakdown plasma formations initiated by a solid aerosol particle or an ensemble of monodisperse aerosol particles upon exposure to laser radiation with a wavelength of 1.06 m are presented as functions of the materials and sizes of particles and of the acting laser energy density. It is demonstrated that the acoustic signal amplitude and duration depend linearly on the effective sizes of particles initiating plasma formations with the proportionality coefficient depending on the particle material. The coefficient of laser energy conversion into the acoustic energy is estimated. The acoustic pulse shape is satisfactorily approximated by a blast N-wave for particles of micron sizes. A comparison of the obtained results with the data available from the literature indicates their good agreement.     

Tuning characteristics of a high pressure CO2 laser pumped CH3F Raman laser

Conclusion We described a CH₃F Raman laser pumped by a two stage 20 atmospheres CO₂ laser. The emission spectrum of the CH₃F laser at 11 Torr extends from 23 cm^–1 to 45 cm^–1 when the CO₂ laser is scanned over the 9R emission branch at a fixed pump power of 180 mJ. The emission spectrum shows a strong structure with large parts where the FIR energy decreases to zero. This fact makes the use of such a laser for spectroscopic scanning experiments in the FIR difficult. The laser is, however, very suited for working at fixed but adjustable FIR frequencies. The pulse energy in the maxima of the emission characteristics at a pump energy of 180 mJ exceeds 300 J, which corresponds a photon conversion coefficient of more than 6%.