P.A. Atanasov

Femtosecond laser ablation of nickel in vacuum

We present an experimental characterization and a theoretical analysis of ultrashort laser ablation of a nickel target, which highlights the more general and peculiar features of femtosecond (fs) laser ablation of metals. The study has been carried out by using visible (527 nm) laser pulses of ≈ 300 fs duration. The vacuum expansion dynamics of the ablated species has been investigated by using fast photography and optical emission spectroscopy, while the fs laser pulse–metal interaction has been studied theoretically by means of molecular dynamics simulations. Special attention has been given to the study of the dependence of ablation depth on laser fluence, which has been carried out by comparing the SEM analysis of micro-holes drilled into the nickel samples with the predictions of the theoretical model. The main outcomes of our investigation, which are very satisfactorily reproduced and accounted for by the theoretical model, are (i) the nonlinear dependence of the ablation yield on the laser fluence, and its reliance to the electron heat diffusion, in the process of redistribution of the absorbed energy, (ii) the splitting of the material blow-off into two main classes of species, atoms and nanoparticles, characterized by different expansion dynamics, and (iii) the different degrees of heating induced by the laser pulse at different depths into the material, which causes the simultaneous occurrence of various ablation mechanisms, eventually leading to atoms and nanoparticles ejection.

Experimental and theoretical investigations of femtosecond laser ablation of aluminum in vacuum

We used time-gated optical emission spectroscopy to investigate the characteristics of aluminum plumes and their vacuum expansion after femtosecond laser ablation at different fluences. The prominent feature is the presence of two main classes of species in the plume: very fast Al atoms and ions preceding the plume bulk essentially constituted of much slower Al nanoparticles expanding with a ten times smaller average velocity. Atomic force microscopy of deposited Al nanoparticles evidenced an average size of about 10nm with a pretty narrow size distribution. These results and the peculiar feature of nanoparticle formation during femtosecond laser irradiation of matter were very satisfactorily interpreted and reproduced by molecular-dynamics simulation of the process. Finally, the analysis of the dependence on laser fluence of the ablation process showed an initial logarithmic increase of ablation yield, up to about 500mJ∕cm2, followed by a sudden and very steep increase at higher fluences. According to ou...

Ablation of metals by ultrashort laser pulses

Ablation of Fe by ultrashort laser pulses with durations 0.1, 1, and 5 ps were investigated experimentally. The laser fluence varied from the ablation threshold up to 100 J cm−2. Above 1 J cm−2, the ablation rate depended on the laser pulse duration, with the shortest pulse producing the highest value. A change in the ablation rate as the laser fluence increased was also observed. These results were analysed using molecular dynamics simulations. We show that the change in the ablation rate is connected to an overheating of the material above the critical point, which results in a steep rise of the pressure developed. Furthermore, due to the electron heat diffusion, the overheated volume increases and involves material located deeper than the skin depth. An increase in the pulse duration results in a decrease in the degree of overheating.

Laser drilling of silicon nitride and alumina ceramics: A numerical and experimental study

Laser drilling of silicon nitride (Si3N4) and alumina (Al2O3) ceramics targets was studied theoretically and experimentally. A one-dimensional model based on the heat-transfer equation was developed in order to describe the process. It includes a change of the type of the equivalent heat source during the laser pulse. The drilling of Si3N4 and Al2O3 ceramic slabs was performed by using a TEM00 Q-switched 10 ns pulse Nd:YAG laser. When the absorption of the plasma formed during the drilling process was taken into account, the theoretical results obtained agreed well with the experimental ones.

Al doped ZnO thin films for gas sensor application

Highly textured pure and Al doped ZnO thin films have been produced by pulsed laser deposition for optical gas sensor application. The influence of the processing parameters such as substrate temperature and oxygen pressure applied during depositions, and dopant concentration on the structural, morphological, and optical properties of the films were investigated. All deposited films are textured along the (002) direction. The substrate temperature and the oxygen pressure have stronger influence on the film crystallinity compared to the presence of the dopants. The grain size of the films prepared from 2 wt% Al2O3 doped ZnO target is approximately the same as the one produced from pure ZnO tagret. The increase in the dopant concentration into the ZnO target (to 5 wt% Al2O3) leads to an increase of the in-plane grain size of the as-deposited films which is well known to increase the gas sensitivity. At the same time, the use of doped targets increase the droplets on the film surface which deteriorates the optical detection of the gas sensing effect. The increase of the dopant concentration reduces the film transmission in the visible range and the transmission cut-off edge is shifted to the shorter wavelengths. The films deposited from 2 wt% Al2O3 doped ZnO target at oxygen pressure of 0.05 mbar and 300°C substrate temperature have good mode properties which makes them good candidates for optical sensors.

An analysis of the dependence on photon energy of the process of nanoparticle generation by femtosecond laser ablation in a vacuum

The dependence on laser wavelength of the process of nanoparticle generation by ultrashort laser ablation of solid matter in a vacuum has been investigated both experimentally and theoretically. The study has been carried out for a Ni target by using laser pulses of ≈300 fs duration at two different laser wavelengths: in the visible (λ = 527 nm) and ultraviolet (λ = 263 nm), respectively. The size distribution of the nanoparticles, which is quite broad in the case of visible light, becomes significantly narrower and slightly shifts towards smaller sizes for ultraviolet light. Molecular dynamics simulations confirm the dependence of the process on the laser wavelength by showing that the laser photon energy affects the material relaxation and, thus, the nanoparticle generation process. This, in turn, indicates that the photon energy can be used as an effective parameter to control the nanoparticle size distribution in femtosecond laser ablation of solid matter.

Ultrafast laser ablation of gold thin film targets

Ultrafast laser ablation of a gold thin film is studied and compared with that of a bulk target, with particular emphasis given to the process of nanoparticles generation. The process is carried out in a condition where a single laser shot removes all the irradiated film spot. The experimental results evidence interesting differences and, in particular, a reduction of the nanoparticles size, and a narrowing of a factor two of their size distribution in the case of ablation of a thin film target, a feature which we relate to a more uniform heating of the target material. We thus show that ultrashort laser ablation of thin films provides a promising way of controlling plume features and nanoparticles size.

Structure and superconducting properties of YBa2Cu3O7−x films prepared by nitrogen laser evaporation and CO2 laser annealing in oxygen

Superconducting YBa2Cu3O7−x thin films were obtained under high vacuum (10−5 Torr) on substrates of polycrystalline Al2O3 sapphire, SrTiO3, and Si, having zero resistance at 81, 85, 87, and 79 K, respectively. A N2 laser of 3.5 J cm−2 energy density was used for the evaporation. The substrates were heated by a cw single‐mode CO2 laser and the annealing was performed by the same laser in O2 atmosphere. Local planar superconducting regions were obtained by focusing the radiation of the cw CO2 laser upon the films. The films were investigated by scanning electron microscope, x‐ray microanalysis, and x‐ray diffraction.

Nanostructuring of thin Au films by means of short UV laser pulses

The particle size distribution, morphology and optical properties of the Au nanoparticle (NP) structures for surface enhanced Raman signal (SERS) application are investigated in dependence on their preparation conditions. The structures are produced from relatively thin Au films (10–20 nm) sputtered on fused silica glass substrate and irradiated with several pulses (6 ns) of laser radiation at 266 nm and at fluencies in the range of 160–412 mJ/cm2. The SEM inspection reveals nearly homogeneously distributed, spherical gold particles. Their initial size distribution of the range of 20–60 nm broadens towards larger particle diameters with prolonged irradiation. This is accompanied by an increase in the uncovered surface of the glass substrate and no particle removal is observed. In the absorption profiles of the nanostructures, the broad peak centred at 546 nm is ascribed to resonant absorption of surface plasmons (SPR). The peak position, halfwidth and intensity depend on the shape, size and size distribution of the nanostructured particles in agreement with literature. From peak intensities of the Raman spectra recorded for Rhodamine 6G in the range of 300–1800 cm−1, the relative signal enhancement by factor between 20 and 603 for individual peaks is estimated. The results confirm that the obtained structures can be applied for SERS measurements and sensing.

Gold nanoparticles as nanoheaters and nanolenses in the processing of different substrate surfaces

We present results of our recent study on the heating process and near field localization arising when gold nanoparticles are irradiated by ultrashort laser pulses at wavelength of 800 nm. The system under consideration consists of Au nanoparticles with diameter of 40, 80, or 200 nm in vacuum or deposited on different substrates. Substrate materials with different dielectric properties are used in order to sense and visualize the nanoparticle heating and near field electromagnetic distribution. The theoretical analysis is based on the optical properties obtained by the Mie scattering theory. The absorption coefficients calculations are implemented in a two-temperature heat model for estimation of the nanoparticle temperature. The near field distribution in the vicinity of the particles is calculated by the finite difference time domain (FDTD) method. It is found that at even moderate laser fluences the temperature of the particle can reach a value sufficient for bubble formation in biological tissues. The analysis of the near field distribution shows that when the particle is deposited on a substrate surface, the dielectric properties of the substrate define the spatial distribution and the enhancement of the near field intensity. The observed localization and field enhancement may result in a precise modification of the substrate with a resolution defined only by the nanoparticle size. Such modifications are experimentally observed in different substrates.