Hardnes and resistivity of amorphous carbon thin films deposited by laser ablation

Amorphous carbon thin films were deposited by laser ablation of a graphite target, using the fundamental line of a 5 ns Nd:YAG laser. Deposition was carried out as a function of the plasma parameters (mean kinetic ion energy and plasma density), determined by means of a planar probe. In the selected working regimes the optical emission from the plasma is mainly due to atomic species, namely C⁺ (426.5 nm); however, there is also emission from other atomic species and molecular carbon. The hardness and resistivity could be varied in the range between 10 and 25 GPa, and 10⁸ and 10¹¹ Ω cm, respectively. The maximum values were obtained at a 200 eV ion energy and 6×10¹³ cm⁻³ plasma density, where the maximum quantity of C–C sp³ bonds was formed, as confirmed by Raman spectroscopy.     

Temperature and density spectroscopic measurements in different laser-generated plasmas

A pulsed Nd:Yag laser, at intensities of the order of 10¹⁰ W/cm², is employed to irradiate different thick metallic targets (Ti, Fe, Ag, and Ni) placed in vacuum. The obtained non-equilibrium plasmas are investigated with various analytical techniques. An electrostatic ion energy analyzer and different ion collectors are employed to monitor in situ the ions ejected from the plasma and to determine the core plasma temperature, the ion energy distributions and the ion angular emission. An optical spectrometer is employed to analyze the plasma corona emitted light vs. wavelength and to identify the emitted characteristic lines. The optical spectroscopy permitted to evaluate the electron temperatures and densities. Results show strong temperature and density gradients occurring in the laser-generated plasma plume.     

Study of the plasmas produced during the deposition of TiC/SiC thin films in a hybrid magnetron-laser system

Time- and spatially-resolved optical emission spectroscopy was performed to characterize the plasma produced in a hybrid magnetron-sputtering-laser deposition system, which is used for TiC or SiC thin films preparation. A graphite target was ablated by a KrF excimer laser (λ=248 nm,τ=20 ns) and either Ti or Si targets were used for DC magnetron sputtering in argon ambient. Spectra were measured in the range 250–850 nm. The evolution of the spectra with varying magnetron powers (0–100 W) and argon pressures (0.3–10 Pa) was studied. Spectra of the plasmas produced by a) the magnetron alone, b) the ablation laser alone, and c) the magnetron and the ablation laser together, were recorded. Spectra (a) were dominated by Ar atoms and Ar⁺ ions. Emission lines of Ti and Si were detected, when Ti target and Si target was used, respectively. Spectra (b) revealed emission of C, C⁺, C₂, Ar, Ar⁺. Spectra (c) showed presence of all previously mentioned species and further of Ti⁺ ions emission was detected. The research was supported by Grant Agency of the Czech Republic No. 202/06/02161, GA ASCR project number A1010110/01 and Institutional Research Plan AV CR No. AV0Z 10100522.     

Experimental and computational study of the passage of an electron beam through the curvilinear element of the Drakon stellarator system in a tenuous plasma

Results are presented from the first stage of studies on the passage of an electron beam with energy 100–500 eV in a magnetic field of 300–700 Oe through the curvilinear solenoid of the KRéL unit, the latter being a prototype of the closing segment of the Drakon stellarator system, in the plasma-beam discharge regime. The ion density at the end of the curvilinear part of the chamber, n _i ≈8×10⁸–10¹⁰ cm⁻³, the electron temperature T _e ≈4–15 eV, and the positions at which the beam hits the target for different distances from it to the electron source are determined experimentally. The motion of the electron beam is computationally modeled with allowance for the space charge created by the beam and the secondary plasma. From a comparison of the experimentally measured trajectories and trajectories calculated for different values of the space charge, we have obtained an estimate for the unneutralized ion density of the order of 5×10⁷ cm⁻³. Zh. Tekh. Fiz. 69, 22–26 (February 1999)     

Plasma plume induced during laser ablation of graphite

The plasma plume induced during ArF laser ablation of a graphite target is studied. Velocities of the plasma expansion front are determined by the optical time of flight method. Mass center velocities of the emitting atoms and ions are constant and amount to 1.7×10⁴ and 3.8×10⁴ m s⁻¹, respectively. Higher velocities of ions result probably from their acceleration in electrostatic field created by electron emission prior to ion emission. The emission spectroscopy of the plasma plume is used to determine the electron densities and temperatures at various distances from the target. The electron density is determined from the Stark broadening of the Ca II and Ca I lines. It reaches a maximum of ∼9.5×10²³ m⁻³ 30 ns from the beginning of the laser pulse at the distance of 1.2 mm from the target and next decreases to ∼1.2×10²² m⁻³ at the distance of 7.6 mm from the target. The electron temperature is determined from the ratio of intensities of ionic and atomic lines. Close to the target the electron temperature of ∼30 kK is found but it decreases quickly to 11.5 kK 4 mm from the target.     

Optical absorption in early stage low temperature laser produced plasma

We describe a new technique to measure the UV/visible absorption spectrum of the ablated material during the laser pulse. The technique utilizes the continuum emission from one laser produced plasma as a light source to measure the absorption properties of a second laser produced plasma which is formed on a semi-transparent target with an array of 40 μm holes. A 6 ns, 1064 nm laser was used to ablate a Ag target and the plasma absorption was measured in the range 450–625 nm for a laser fluence of 1 J cm⁻². The total absorption cross-section is (0.5–1.5)×10⁻¹⁷ cm² in the range 450–540 nm. By comparing the measured absorption with a calculation using the plasma spectroscopy code FLYCHK it can be concluded that, in the wavelength region examined here, the absorption is mainly due to bound-bound transitions.     

Effect of the ceramic grain size and concentration on the dynamical mechanical and dielectric behavior of poly(vinilidene fluoride)/Pb(Zr<Subscript>0.53</Subscript>Ti<Subscript>0.47</Subscript>)O<Subscript>3</Subscript> composites

Laser-induced backside wet and dry etching (LIBWE and LIBDE) methods were developed for micromachining of transparent materials. Comparison of these techniques is helpful in understanding the etching mechanism but was not realized due to complications in setting up comparable experimental conditions. In our comparative investigations we used a solid tin film for dry and molten tin droplets for wet etching of fused-silica plates. A tin–fused-silica interface was irradiated through the sample by a KrF excimer laser beam (λ=248 nm, FWHM=25 ns); the fluence was varied between 400 and 2100 mJ/cm². A significant difference between the etch depths of the two investigated methods was not found. The slopes of the lines fitted to the measured data (sl_LIBDE=0.111 nm/mJ cm⁻², sl_LIBDE=0.127 nm/mJ cm⁻²) were almost similar. Etching thresholds for LIBDE and LIBWE were approximately 650 and 520 mJ/cm², respectively. To compare the dependence of etch rates on the pulse number, target areas were irradiated at different laser fluences and pulse numbers. With increasing pulse number a linear rise of depth was found for wet etching while for dry etching the etch depth increase was nonlinear. Secondary ion mass spectroscopic investigations proved that this can be due to the reconstruction of a new thinner tin-containing surface layer after the first pulse.     

Formation of excited states of atoms and ions in laser plasma from CuInS2

Emission spectra and the energy distribution of the excited-state population density of atoms and ions in erosion laser plasma from CuInS₂ with various crystal-structure orderings are analyzed. It is shown that increased ordering of the target crystal structure causes the excited-state energies of indium atoms generated in the laser erosion plume to increase and that sulfur atoms always emit only in transitions from highly excited states. The ratio of relative ion concentrations in the laser plasma plume is Cu⁺/In⁺/S⁺ = 0.3/0.08/2, which corresponds neither to the atomic ratio of Cu/In/S (1/1/2) in the target nor to the ratio of ionization energies. The results are explained by recombination processes for ions and by the atomization specifics of the CuInS₂ target exposed to long-wavelength radiation. The atomization consists essentially of dissociative processes expressed by CuInS₂ → CuInS + S and CuInS₂ → Cu + InS + S. The electron temperature of polycrystal (single-crystal) plasma at a distance of 1 mm from the target is 0.3 eV (0.4 eV) for atoms and 1.3 eV (2.7 eV) for ions and varies negligibly for plasma up to a distance of 7 mm from the target. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 2, pp. 217–223, March–April, 2008.     

Fe and Fe+2%Si targets as ion sources via UV laser ablation plasma

In the last years the ion component of a laser-produced plasma has been considered and studied as an object to provide high-density ion sources, which can be applied in many fields such as laser-induced implantation. In this work a KrF laser beam of 10⁸ W/cm² irradiance was focused onto single-crystalline Fe and single-crystalline Fe with 2% of Si targets and the characteristics of both free expanding laser-produced plasmas were compared. The time-of-flight (TOF) method was applied to determine the ion charge yield at various laser fluences and the ion angular spread. The analyses of TOF spectra showed a synergetic effect of the silicon admixture in target material on the Fe ions production. Besides, this admixture was also responsible of the increasing of the plasma temperature which corresponds in turn to the increasing of the average kinetic energy of the particles as well as of the more collimated ion distribution.     

Characterization of compact bright soft X-ray source based on picosecond laser plasma

Radiation emission of silicon and aluminum plasmas produced by 40-ps laser pulses with peak intensity above 10¹⁴ W/cm² was studied. High-resolution soft X-rayspectra of H-like and He-like ions were analyzed to determine plasma parameters. We compared the line shape of resonance transitions and their intensity ratios to corresponding dielectronic satellites and the intensities of the intercombination lines of He-like ions with the results of model calculations. Such comparisons gave average values of the electron number density N_e=(1-1.9)×10²¹ cm^-3 and the electron temperature T_e=460–560 eV for Si plasmas and about 560 eV for Al plasmas produced by the first and the second laser harmonics. The plasma size is about 100 μm. According to our estimations, more than 10¹² photons were produced within the resonance line spectral width and in the solid angle 2π during the total decay period. PACS 41.50.+h; 52.25.Os; 52.50.Jm     

纳秒激光诱导紫铜黄铜等离子体特征参数的对比研究

基于1064 nm Nd:YAG激光器,对比研究了紫铜和黄铜等离子的特征参数。洛仑兹函数拟合Cu I 324.75 nm得到紫铜和黄铜等离子体的电子密度分别是3.61017 cm-3和3.31017 cm-3。为了减小谱线自发辐射跃迁几率不确定性和测量误差带来的计算误差,采用改进型迭代玻耳兹曼算法精确求解紫铜等离子体和黄铜等离子体的电子温度分别是6316 K和6051 K,分析表明,两种等离子体特征参数的差异主要是由于黄铜中的锌元素的电离能（9.39 eV）大于铜元素的电离能（7.72 eV）而造成的。实验数据证实激光诱导的紫铜和黄铜等离子体满足局部热力学平衡模型和光学薄模型。     

Vibrational wave packet dynamics in the silver tetramer probed by NeNePo femtosecond pump-probe spectroscopy

We present results on the ultrafast dynamics of mass-selected neutral Ag₄ clusters using NeNePo (negative ion - neutral - positive ion) femtosecond pump-probe spectroscopy. One-color pump-probe spectra of the Ag₄ ^-/Ag₄/Ag₄ ⁺ system measured at 385 nm and an internal cluster temperature of 20 K display a complex beat structure over more than 60 ps. The oscillatory structure is attributed to vibrational wave packet dynamics in an excited “dark" state of neutral Ag₄. A dominant 740 fs wave packet period as well as wave packet dephasing and rephasing are observed in the spectra. Fourier analysis of the spectra yields a group of frequencies centered around 45 cm^-1 and an anharmonicity χ _e2νχ _eχ _e of 2.65 cm^-1 for the active vibrational mode. Received 30 November 2000     

Optical emission spectroscopy investigation of sputtering discharge used for SiOxNy thin films deposition and correlation with the film composition

The r.f. discharge of sputtering silicon target using argon-oxygen-nitrogen plasma was investigated by optical emission spectroscopy. Electronic temperature (T_e) and emission line intensity were measured for different plasma parameters: pressure (from 0.3 to 0.7 Pa), power density (0.6-5.7 W cm⁻²) and gas composition. At high oxygen concentration in the plasma, both T_e and the target self-bias voltage (V_b) steeply decrease. Such behaviour traduces the target poisoning phenomenon. In order to control the deposition process, emission line intensity of different species present in the plasma were compared to the ArI (λ = 696.54 nm) line intensity and then correlated to the film composition analysed by Rutherford Backscattering Spectroscopy.     

Spatial characteristics of Kα radiation from weakly relativistic laser plasmas

The spatial dependence of K _α emission generated from laser-produced hot electrons is investigated experimentally and theoretically. In addition, the conversion efficiency of K _α production as a function of laser intensity is measured and compared with modeling results. We use the terawatt Ti:sapphire laser at MPQ and vary the peak intensity from 10¹⁵ to 10¹⁸ W/cm² with a pulse duration of 200 fs. A solid Cu target is placed at various positions in the laser focus, which allows one to vary the intensity but keep the total energy on the target constant. When the target is near best focus, the FWHM of the K _α emission, measured using a knife-edge, is considerably larger than the FWHM of the laser intensity. In measuring the efficiency of K _α production using the fundamental wavelength of the laser, a clear maximum of K _α emission is observed at a position away from best focus, where the peak intensity is down by more than an order of magnitude from the value at best focus. When the second harmonic of the laser is used, the K _α emission is peaked near best focus. The K _α emission from layer targets is used to obtain an estimate of the temperature of the hot electrons. Modeling of K _α production, using a Monte Carlo electron/photon transport code, shows the relationship between incident electron energy and the emitted K _α emission. Efficient K _α generation from the low-intensity wings of the laser pulse contributes to the large spot size of the K _α emission. The lower electron temperatures that are expected for the second harmonic explain the differences in the location of maximum K _α emission for the two wavelengths. We discuss the use of K _α emission in photoionizing inner-shell electrons with the goal of achieving X-ray lasing at short wavelengths. Received: 6 April 1999 / Revised version: 31 May 1999 / Published online: 11 August 1999     

Electron temperature (T e) measurements by Thomson scattering system

Thomson scattering technique based on high power laser has already proved its superoirity in measuring the electron temperature (T _e and density (n _e) in fusion plasma devices like tokamaks. The method is a direct and unambiguous one, widely used for the localised and simultaneous measurements of the above parameters. In Thomson scattering experiment, the light scattered by the plasma electrons is used for the measurements. The plasma electron temperature is measured from the Doppler shifted scattered spectrum and density from the total scattered intensity. A single point Thomson scattering system involving a Q-switched ruby laser and PMTs as the detector is deployed in ADITYA tokamak to give the plasma electron parameters. The system is capable of providing the parameters T _e from 30 eV to 1 keV and n _e from 5 × 10¹²cm⁻³−5 × 10¹³cm⁻³. The system is also able to give the parameter profile from the plasma center (Z=0 cm) to a vertical position of Z=+22 cm to Z=−14 cm, with a spatial resolution of 1 cm on shot to shot basis. This paper discusses the initial measurements of the plasma temperature from ADITYA.     

Resonant charging of Xe clusters in helium nanodroplets under intense laser fields

We theoretically investigate the impact of multiple plasmon resonances on the charging of Xe clusters embedded in He nanodroplets under intense pump-probe laser excitation (τ = 25 fs, I ₀ = 2.5 × 10¹⁴   W/cm², λ = 800 nm). Our molecular dynamics simulations on Xe₃₀₉He_10 000 and comparison to results for free Xe₃₀₉ give clear evidence for selective resonance heating in the He shell and the Xe cluster, but no corresponding double hump feature in the final Xe charge spectra is found. Though the presence of the He shell substantially increases the maximum charge states, the pump-probe dynamics of the Xe spectra from the embedded system is similar to that of the free species. In strong contrast to that, the predicted electron spectra do show well-separated and pronounced features from highly efficient plasmon assisted electron acceleration for both resonances in the embedded clusters. A detailed analysis of the underlying ionization and recombination dynamics is presented and explains the apparent disaccord between the resonance features in the ion and electron spectra.     

High density plasmas for recombination X-ray lasers

The scaling of recombination XUV lasers to shorter wavelengths requires laser plasmas produced at initial electron densities close to solid. With pump laser pulses longer than a few tens of picoseconds the hydrodynamic motion of the plasma during the interaction makes this difficult to achieve. In contrast, when picosecond laser pulses are used the laser energy is absorbed close to solid density since the plasma expansion is insignificant during the laser pulse. This results in hot near solid density plasmas which are needed for hydrogenic recombination X-ray lasers operating in the water window. Experimental observations have shown that a fully ionized aluminium plasma with a temperature of about 400 eV and a density well above 10²³ cm^–3 is produced when an aluminium target is irradiated with a single 3.5 ps high power KrF laser pulse.     

Stark broadening of the hydrogen Paschen <Emphasis Type="Bold">γ</Emphasis> transition at electron densities of the order of cm

Stark broadening measurements and calculations of the Paschen γ spectral line of hydrogen ( λ = 1.094[: MU :] m) are reported. Investigations have been performed at plasma electron densities between 1.4×10 ¹⁵ cm ^-3 and 3.7×10 ¹⁵ cm ^-3 . As the light source a wall-stabilized arc operated in a helium-hydrogen gas mixture at atmospheric pressure has been applied. The radiation of the plasma emitted from nearly homogeneous plasma layers in end-on direction, was measured with the use of a grating spectrometer equipped with a charge coupled device (CCD) detector. The radiance calibration was carried out against light outputs originating from a tungsten strip radiation standard. The measured FWHM are compared with results of our calculations based on computer simulation techniques (full computer simulation method -- FCSM). Our broadening data are also compared with results of other theoretical approaches (MM-method, quasi-static approximation) and with experimental data obtained at electron densities about one order of magnitude larger than ours. Received 21 January 2003 Published online 24 April 2003 RID="a" ID="a"e-mail: wujec@uni.opole.pl     

Laser-induced cavities and solitons in overcritical hydrogen plasma

A picosecond CO₂ laser was used successfully in a number of experiments exploring advanced methods of particle acceleration [1]. Proton acceleration from gas-jet plasma exemplifies another advantage of employing the increase in laser wavelength from the optical to the mid-IR region. Recent theoretical- and experimental-studies of ion acceleration from laser-generated plasma point to better ways to control the ion beam’s energy when plasma approaches the critical density. Studying this regime with solid-state lasers is problematic due to the dearth of plasma sources at the critical electron density ∼10²¹ cm⁻³, corresponding to laser wavelength λ = 1 μm. CO₂ laser offers a solution. The CO₂ laser’s 10 μm wavelength shifts the critical plasma density to 10¹⁹ cm⁻³, a value attainable with gas jets. Capitalizing on this approach, we focused a circular polarized 1-TW CO₂ laser beam onto a hydrogen gas jet and observed a monoenergetic proton beam in the 1–2 MeV range. Simultaneously, we optically probed the laser/plasma interaction region with visible light, revealing holes bored by radiation pressure, as well as quasi-stationary soliton-like plasma formations. Our findings from 2D PIC simulations agree with experimental results and aid in their interpretation.     

Dynamics of ions produced by laser ablation of ceramic Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Al at 193 nm

We study the dynamics of ions produced upon ablation of Al and ceramic Al₂O₃ targets using nanosecond laser pulses at 193 nm (6.4 eV) as a function of the laser fluence from threshold up to 12 J cm⁻². An electrical (Langmuir) probe located at 40 mm from the target surface has been used for determining the ion yield and calculating the kinetic energy distributions. The results for both targets show the existence of a significant amount of ions having kinetic energies >200 eV (≈20% around threshold fluence), and kinetic energies are up to >1.5 keV. The results are related with the existence of direct photonionization processes caused by the photon energy of the laser being higher than the ionization potential of Al (5.98 eV). Comparison of the ion yield when ablating the two types of targets for fluences above threshold to data reported in the literature suggests that the magnitude of the yield and its threshold are parameters depending on the thermal properties of the target rather than on the laser wavelength. Around threshold, the different behavior of ion yield when ablating Al and Al₂O₃ targets suggests that the threshold for neutral aluminium and ion species in the case of ablation of the Al₂O₃ target must be similar.