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.     

Neutrals' temperature in laser-generated plasma at LNS

A Nd–YAG laser operating at 532 nm with a maximum intensity of 10¹⁰ W/cm² was used to ablate aluminium and tantalum targets placed in vacuum.

A mass quadrupole spectrometer (MQS) at high sensitivity, operating in the range of 1–300 amu, with a resolution better than 1 amu, was used to analyse the atomic emission produced by the laser ablation. The neutrals' emission produced by laser-generated plasma at INFN-LNS was investigated in terms of temperature, ablation yield, angular distribution and velocity.

The neutrals' detection through the MQS permitted to measure the mass energy distribution. Results demonstrate that the maximum temperatures of the neutral species are of the order of 100 eV. The angular emission of neutrals is peaked along the normal to the target surface, as it was observed for the ions; the ablation yield increases suddenly at low laser intensity and decreases at high laser intensity, owing to the higher ionization processes; the flow velocity follows the adiabatic expansion of the plasma in vacuum and it is of the order of 10⁴ m/s.

Measurements will be presented and discussed, according to the available models.     

Dynamics of femtosecond laser-produced plasma ions

We investigated the ion laser-produced plasma plume generated during ultrafast laser ablation of copper and silicon targets in high vacuum. The ablation plasma was induced by ≈50 fs, 800 nm Ti:Sa laser pulses irradiating the target surface at an angle of 45°. An ion probe was used to investigate the time-of-flight profiles of the emitted ions in a laser fluence range from the ablation threshold up to ≈10 J/cm². The angular distribution of the ion flux and average velocity of the produced ions were studied by moving the ion probe on a circle around the ablation spot. The angular distribution of the ion flux is well described by an adiabatic and isentropic model of expansion of a plume produced by laser ablation of solid targets. The angular distribution of the ion flux narrows as the laser pulse fluence increases. Moreover, the ion average velocity reaches values of several tens of km/s, evidencing the presence of ions with kinetic energy of several hundred eV. Finally, the ion flux energy is confined in a narrow angular region around the target normal.     

Laser-induced ablation and crater formation at high laser flux

Laser ablation and crater formation have been studied on a copper target using a 10 Hz Nd:YAG laser system delivering pulses up to 100 mJ in 40 ps with a flux on target F?≤?5000 J/cm². Crater dimensions were measured using optical microscope or scanning electron microscope. In order to understand the process of crater formation, we considered various theoretical models present in the literature and revised them taking into account the occurrence of plasma phenomena, which are important at the intensities used in this experiment. We also compared our experimental results with other results obtained at the PALS laboratory, using a 0.44 μm wavelength laser and much higher laser intensities. Finally, we explore the possibility of extending the information derived from laser-produced craters to other types of craters.     

Carbon nanocrystals produced by pulsed laser ablation of carbon

Plasma laser ablation experiments were performed irradiating glassy-carbon targets placed in vacuum through a pulsed Nd:YAG laser operating at the second harmonic (532 nm), 9 ns pulse width and 10⁹ W/cm² density power.

Thin films of ablated carbon were deposited on silicon oxide substrates placed at different distances and angles with respect to the target.

The analysis of the deposited material was carried out by using surface profiler, scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) and Raman spectroscopy.

Results show the evidence of carbon nanocrystals and nanostructures with dimension of the order of 100 nm deposited on the substrates together with a large amount of amorphous phase. The spectroscopic investigations and the SEM images indicate the formation of nanodiamond seeds as a nucleation process induced on the substrate surface. Nanostructures were investigated as a function of the laser intensity and angle distribution. Experimental results were compared with the literature data coming from nanodiamonds growth with different techniques.

Experiments performed at Instituto Nazionale di Fisica Nucleare-Laboratori Nazionali del Sud (INFN-LNS) of Catania (Italy) and data analysis conducted at Dipartimento di Fisica and DFMTA of the Università of Messina (Italy), CNR-ITIS of Messina and ST-Microelectronics of Catania will be presented and discussed.     

Guidelines for efficient direct ablation of dielectrics with single femtosecond pulses

We provide guidelines to femtosecond laser users to select ad hoc laser parameters, namely the fluence and pulse duration, in the context of the development of ablation processes at the surface of dielectrics using single femtosecond pulses. Our results and discussion are based on a comprehensive experimental and theoretical analysis of the energy deposition process at the surface of fused silica samples and of their postmortem ablation characteristics, in the range of intensities from 10¹³ to 10¹⁵ W/cm². We show experimentally and numerically that self-induced plasma transient properties at the pulse timescale dramatically determine the efficiency of energy deposition and affect the resulting ablation morphology. In practice, we determine that the precise measurement of two characteristic fluence values, namely the laser-induced ablation threshold F _th,LIAT and the fluence F _opt for maximum ablation efficiency, are only required to qualify the outcomes of laser ablation at the surface of a dielectric in an extended range of applied fluence.     

Studies of craters’ dimension for long-pulse laser ablation of metal targets at various experimental conditions

Long pulse laser shots of the PALS iodine laser in Prague have been used to obtain metal target ablation at various experimental conditions. Attention is paid mainly to the dependencies of the crater diameter on the position of minimum laser-focus spot with regard to the target surface, by using different laser wavelengths and laser energies. Not only a single one, but two minima, independently of the wavelength, of the target irradiation angle and of the target material, were recorded. Significant asymmetries, ascribed to the non-linear effects of intense laser beam with pre-formed plasma, were found, too. Estimations of ejected mass per laser pulse are reported and used to calculate the efficiency of laser-driven loading. Results on metal target ablation and crater formation at high intensities (from 2 × 10¹³ to 3 × 10¹⁶ W/cm²) are presented and compared. Crater depth, crater diameter and etching yield are reported versus the laser energy, in order to evaluate the ablation threshold fluence.     

Generation of a fast-ion beam upon the interaction of a multiterawatt picosecond laser pulse with a solid target

The parameters of fast particles generated upon the interaction of 10¹⁹ W/cm² laser pulses with solid targets are studied. The spatial and energy parameters of fast ions are investigated. It is found that approximately 1–3% of the laser energy is transformed to the energy of mega-and submegaelectronvolt ions at laser pulse intensities ≥10¹⁸ W/cm². It is shown experimentally that an ion beam is directed perpendicular to the target surface. The analytic and numerical simulations agree with experimental results and predict the propagation of fast electrons in the mirror direction with respect to the incident laser beam and of ions perpendicular to the target. The theoretical calculations are compared with the experimental output and spectra of fast electrons and ions.     

Energy analysis of protons emitted from Nd:YAg laser-generated plasmas

Hydrogenated targets have been irradiated in vacuum with the pulsed Nd:YAg laser at intensities of the order of 10¹⁰ W/cm². The laser-generated plasma, produced by the interaction with the solid, emits protons and other ions along the normal to the target surface. Ion collectors and ion energy analyzer were used to measure the current, the angular emission and the energy distributions of the emitted protons. Time-of-flight measurements, Coulomb–Boltzmann-distributions and the fits of experimental data were also used in order to evaluate the equivalent ion plasma temperature and the ion acceleration developed in the non-equilibrium-pulsed plasma.     

RBS analysis of ions implanted in light substrates exposed to hot plasmas laser-generated at PALS

Ge and Ta ion implantation of silicon and carbon substrates has been obtained at PALS Research Laboratory in Prague by using laser pulses of 400 ps duration, 438 nm wavelength, 10^14?16 W/cm² intensity. Substrates were exposed in vacuum at different distances from the target and at different angles with respect to the normal to the target surface. ‘On line’ measurements of ion energy were obtained with time-of-flight techniques by using an electrostatic deflector as ion energy analyzer. ‘Off line’ measurements of ion energy were obtained by Rutherford backscattering spectrometry (RBS) of 2.25 MeV He²⁺ beam at CEDAD Laboratory of Lecce University. The RBS spectra have given the depth profiles of the ion-implanted species and the implanted doses as a function of the laser intensity, angular position and target distance. A spectra deconvolution method based on the ion stopping power in the substrate matrix was applied in order to evidence the energy of the implanted ions. Measurements indicate that ions with energy ranging between 100 keV and 10 MeV and dose of the order of 10^14?16/cm² are implanted and that the process of ion implantation occurs mainly in substrates placed at little angles with respect to the normal to the target surface. Only a thin film deposition occurs for substrates placed at large angles with respect to the normal direction. Results indicate that the ion energies measured with the ‘on line’ and the ‘off line’ techniques are in good agreement.     

Non-equilibrium plasma production by pulsed laser ablation of gold

A gold target has been irradiated with a Q-switched Nd:Yag laser having 1064?nm wavelength, 9?ns pulse width, 900?mJ maximum pulse energy and a maximum power density of the order of 10¹⁰?W/cm². The laser–target interaction produces a strong gold etching with production of a plasma in front of the target. The plasma contains neutrals and ions having a high charge state. Time-of-flight (TOF) measurements are presented for the analysis of the ion production and ion velocity. A cylindrical electrostatic deflection ion analyzer permits measurement of the yield of the emitted ions, their charge state and their ion energy distribution. Measurements indicate that the ion charge state reaches 6⁺ and 10⁺ at a laser fluence of 100?J/cm² and 160?J/cm², respectively. The maximum ion energy reaches about 2?keV and 8?keV at these low and high laser fluences, respectively. Experimental ion energy distributions are given as a function of the ion charge state. Obtained results indicate that electrical fields, produced in the plume, along the normal to the plane of the target surface, exist in the unstable plasma. The electrical fields induce ion acceleration away from the target with a final velocity dependent on the ion charge state. The ion velocity distributions follow a “shifted Maxwellian distribution”, which the authors have corrected for the Coulomb interactions occurring inside the plasma.     

Hard X radiation and fast particles in laser plasma experiments at laser intensities of up to 5×1018 W/cm2 on the target surface

Results are presented from an investigation of the hard X-ray spectrum and the parameters of fast particles in experiments on the interaction of laser pulses with solid targets in the PROGRESS-P facility at laser intensities of up to 5×10¹⁸ W/cm² on the target surface. The maximum energy of fast electrons obtained from direct measurements is found to be 8–10 MeV.     

Deposition of nanoparticles during laser ablation of nanoparticle-containing targets

We analyze the morphology of ablated nanoparticles after their laser-induced deposition on various substrates. We show that, at moderate laser intensity of the 210 ps pulses on the surface of nanoparticle-containing materials (<5×10⁹ W?cm^?2), the deposited material remains approximately the same as the initial nanoparticles. We compare these deposited nanoparticles with the debris obtained by the laser ablation of bulk material of the same origin as nanoparticles at different intensities of laser radiation. The presence of nanoparticles in laser plumes allowed for analyzing high-order nonlinear optical properties of nanoparticles. The efficient high-order harmonic generation was achieved during propagation of femtosecond pulses through such plasmas.     

Characterization of laser-generated silicon plasma

A study of visible laser ablation of silicon, in vacuum, by using 3 ns Nd:YAG laser radiation is reported. Nanosecond pulsed ablation, at an intensity of the order of 10¹⁰ W/cm², produces high non-isotropic emission of neutrals and ionic species. Mass quadrupole spectrometry, coupled to electrostatic ion deflection, allows estimation of the energy distributions of the emitted species from plasma. Neutrals show typical Boltzmann-like distributions while ions show Coulomb-Boltzmann-shifted distributions depending on their charge state. Time-of-flight measurements were also performed by using an ion collector consisting of a collimated Faraday cup placed along the normal to the target surface. Surface profiles of the craters, created by the laser radiation absorption, permitted to study the ablation threshold and ablation yields of silicon in vacuum. The plasma fractional ionization, temperature and density were evaluated by the experimental data. A special regard is given to the ion acceleration process occurring inside the plasma due to the high electrical field generated at the non-equilibrium plasma conditions. The angular distribution of the neutral and ion species is discussed.     

Temperature measurements in plasmas generated by using lasers at different intensities

The temperature of laser-generated pulsed plasmas is an important property that depends on many parameters, such as the particle species and the time elapsed from the laser interaction with the matter and the surface characteristics.

Laser-generated plasmas with low intensity (<10¹⁰ W/cm²) at INFN-LNS of Catania and with high intensity (>10¹⁴ W/cm²) in PALS laboratory in Prague have been investigated in terms of temperatures relative to ions, electrons, and neutral species. Time-of-flight (ToF) measurements have been performed with an electrostatic ion energy analyzer (IEA) and with different Faraday cups, in order to measure the ion and electron average velocities. The IEA was also used to measure the ion energy, the ion charge state, and the ion energy distribution.

The Maxwell–Boltzmann function permitted to fit the experimental data and to extrapolate the ion temperature of the plasma core.

The velocity of the neutrals was measured with a special mass quadrupole spectrometer. The Nd:Yag laser operating at low intensity produced an ion temperature core of the order of 400 eV and a neutral temperature of the order of 100 eV for many ablated materials. The ToF of electrons indicates the presence of hot electron emission with an energy of ～1 keV.     

Protons’ generation by laser irradiation at 5 × 10 W/cm from silicon dielectric targets containing an excess of hydrogen

A study of silicon plasma generated in vacuum by 532 nm Nd:YAG laser at intensities of about 5 × 10⁹ W/cm² from dielectric targets containing a relatively huge quantity of hydrogen was presented.Time-of-flight technique was employed to measure the particles’ energy and the relative yield with respect to other ion species. Plasma-accelerated ions show Coulomb-Boltzmann-shifted distributions depending on their charge state.Mass quadrupole spectrometry allowed the estimation of the relative hydrogen amount inside the different samples considered: silicon (Si), silicon nitride (Si₃N₄) and hydrogenated annealed silicon (Si(H)) as a function of the ablation depth and irradiation time.Depth profiles of the laser craters permit to calculate the ablation yield at the used laser fluence. The plasma temperature and density was evaluated by the experimental data. A special regard is given to the protons’ generation process occurring inside the plasma, due to the possible influence of the hydrogen excess on the treated samples in comparison to the not-hydrogenated silicon ones.     

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.     

Synthesis of single-walled carbon nanotubes in an expanding vapor-gas flow produced by laser ablation of a graphite-catalyst mixture

Single-walled carbon nanotubes (SWCNTs) are synthesized by the ablation of a catalyst-containing carbon target with a cw CO₂ laser. Emphasis is on ablation conditions that are favorable to self-organized SWCNT synthesis. It is shown that the graphite target intensely evaporates with the formation of fractal-like tubes at the edge of the jet when the laser power density exceeds 10⁵ W/cm². Still more favorable conditions for carbon nanotube synthesis are set if the power density lies within 2×10⁴–5×10⁴ W/cm². Under these conditions, both individual SWCNTs and their bundles of diameter from 1.1 to 1.5 nm are produced, as shown by Raman scattering and electron microscopy studies. In this series of experiments, the maximal fraction of SWCNTs reaches 20%. A mechanism of SWCNT fast growth in the laser torch is suggested.     

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.     

Advanced targets,diagnostics and applications of laser-generated plasmas

High-intensity sub-nanosecond-pulsed lasers irradiating thin targets in vacuum permit generation of electrons and ion acceleration and high photon yield emission in non-equilibrium plasmas. At intensities higher than 10¹⁵?W/cm² thin foils can be irradiated in the target-normal sheath acceleration regime driving ion acceleration in the forward direction above 1?MeV per charge state. The distributions of emitted ions in terms of energy, charge state and angular emission are controlled by laser parameters, irradiation conditions, target geometry and composition. Advanced targets can be employed to increase the laser absorption in thin foils and to enhance the energy and the yield of the ion acceleration process. Semiconductor detectors, Thomson parabola spectrometer and streak camera can be employed as online plasma diagnostics to monitor the plasma parameters, shot by shot. Some applications in the field of the multiple ion implantation, hadrontherapy and nuclear physics are reported.