Alexander V. Bulgakov

Synthesis and laser processing of ZnO nanocrystalline thin films

Abstract We present the results of experiments on synthesis of ZnO nanoclusters by reactive pulsed laser deposition (PLD). The nanoclusters were formed and crystallised in the gas phase and deposited on SiO2 substrates. The nanostructured films were characterised by conventional photoluminescence (PL). The PL spectra consist of a narrow UV excitonic band and a broad visible band related to defects in the film. The film preparation conditions such as the substrate temperature, ambient gas nature and pressure, were optimised in order to increase the intensity of excitonic emission and prevent the formation of defects. A post-growth annealing by UV laser radiation improved the optical quality of the deposited films. The photoluminescence intensity was found to be dependent significantly on the laser fluence and on the number of shots per site. The nature of the defects responsible for the observed luminescence in a visible range is discussed.

Production of Gas Phase Zinc Oxide Nanoclusters by Pulsed Laser Ablation

We present experimental results on the photoluminescence (PL) of gas-suspended zinc oxide nanoclusters prepared during ablation of sintered ZnO targets by a pulsed ArF laser in the presence of oxygen ambient gas. The PL spectra in the UV spectral region correspond to the exciton recombination in the nanoclusters which are crystallized and cooled down to the temperature of the ambient gas in the ablation chamber. The time evolution of the spectra as well as their dependence on the ambient gas pressure are discussed.

Pulsed laser ablation of solids and critical phenomena

We consider the possible manifestations of critical phenomena under pulsed laser ablation (PLA). The mechanism of phase explosion under nanosecond laser ablation is considered and the possibility of estimating the critical temperature from PLA experiments is discussed. A model based on the Euler equations and generalized van der Waals equation is developed to describe rarefaction shock waves (RSW) in near-critical matter. For a near-surface slab of a gold target heated above the critical point and expanding freely in vacuum, the evolution of the RSW has been studied. The possibility of RSW formation in stellar matter is discussed.

Impacts of Ambient and Ablation Plasmas on Short- and Ultrashort-Pulse Laser Processing of Surfaces

In spite of the fact that more than five decades have passed since the invention of laser, some topics of laser-matter interaction still remain incompletely studied. One of such topics is plasma impact on the overall phenomenon of the interaction and its particular features, including influence of the laser-excited plasma re-radiation, back flux of energetic plasma species, and massive material redeposition, on the surface quality and processing efficiency. In this paper, we analyze different plasma aspects, which go beyond a simple consideration of the well-known effect of plasma shielding of laser radiation. The following effects are considered: ambient gas ionization above the target on material processing with formation of a “plasma pipe”; back heating of the target by both laser-driven ambient and ablation plasmas through conductive and radiative heat transfer; plasma chemical effects on surface processing including microstructure growth on liquid metals; complicated dynamics of the ablation plasma flow interacting with an ambient gas that can result in substantial redeposition of material around the ablation spot. Together with a review summarizing our main to-date achievements and outlining research directions, we present new results underlining importance of laser plasma dynamics and photoionization of the gas environment upon laser processing of materials.

Gas-dynamic acceleration of laser-ablation plumes: Hyperthermal particle energies under thermal vaporization

The expansion of a plume produced by low-fluence laser ablation of graphite in vacuum is investigated experimentally and by direct Monte Carlo simulations in an attempt to explain hyperthermal particle energies for thermally vaporized materials. We demonstrate that the translation energy of neutral particles, ∼2 times higher than classical expectations, is due to two effects, hydrodynamic plume acceleration into the forward direction and kinetic selection of fast particles in the on-axis region. Both effects depend on the collision number within the plume and on the particles internal degrees of freedom. The simulations allow ablation properties to be evaluated, such as ablation rate and surface temperature, based on time-of-flight measurements. Available experimental data on kinetic energies of various laser-produced particles are well described by the presented model.

Cluster Generation Under Pulsed Laser Ablation Of Compound Semiconductors

A comparative experimental study of pulsed laser ablation in vacuum of two binary semiconductors, zinc oxide and indium phosphide, has been performed using IR‐ and visible laser pulses with particular attention to cluster generation. Neutral and cationic ZnnOm and InnPm particles of various stoichiometry have been produced and investigated by time‐of‐flight mass spectrometry. At ZnO ablation, large cationic (n>9) and all neutral clusters are mainly stoichiometric in the ablation plume. In contrast, indium phosphide clusters are strongly indium‐rich with In4P being a magic cluster. Analysis of the plume composition upon laser exposure has revealed congruent vaporization of ZnO and a disproportionate loss of phosphorus by the irradiated InP surface. Plume expansion conditions under ZnO ablation are shown to be favorable for stoichiometric cluster formation. A delayed vaporization of phosphorus under InP ablation has been observed that results in generation of off‐stoichiometric clusters.

Numerical study of gas-phase cluster synthesis under ns laser ablation

A theoretical study of cluster formation in the plume produced by nanosecond laser pulses of moderate intensity in vacuum is undertaken on the basis of the Smoluchowski rate equations. Dynamics of the laser-induced plume expansion is described in a spherical approximation, taking into account ionization/recombination kinetics, the heat release to the plasma flow due to condensation and three-body recombination, and the vibrational relaxation of clusters. As a model system, carbon cluster formation in the plume during pulsed laser ablation of graphite has been studied. The Cn particles with n = 1-100 are considered with allowance for the isomers (chains, rings, fullerenes). The cluster collision cross sections are evaluated from the experimental data. Calculations with various initial compositions of the vaporized material (different ratios of atoms, dimers and trimers) have shown a strong impact of this factor on the expansion and ionization dynamics of the plume. The obtained results on the cluster size distribution and impact of the cluster internal degrees of freedom on the cluster growth process are analyzed in comparison with available experimental data.

Laser-induced transfer of nanoparticles for gas-phase analysis

An experimental study of laser-induced forward transfer of nanoparticles from a metal-coated glass substrate is presented. Nanoparticles are efficiently removed from the substrates due to transient blister formation. A combination of mass spectrometry, atomic force microscopy studies of the irradiated substrates, and theoretical considerations of temperature distributions and stress in the films during irradiation serves to provide insight into the mechanisms involved.

Blister-based-laser-induced-forward-transfer: A non-contact, dry laser-based transfer method for nanomaterials

We show that blister-based-laser-induced forward-transfer can be used to cleanly desorb and transfer nano- and micro-scale particles between substrates without exposing the particles to the laser radiation or to any chemical treatment that could damage the intrinsic electronic and optical properties of the materials. The technique uses laser pulses to induce the rapid formation of a blister on a thin metal layer deposited on glass via ablation at the metal/glass interface. Femtosecond laser pulses are advantageous for forming beams of molecules or small nanoparticles with well-defined velocity and narrow angular distributions. Both fs and ns laser pulses can be used to cleanly transfer larger nanoparticles including relatively fragile monolayer 2D transition metal dichalcogenide crystals and for direct transfer of nanoparticles from chemical vapour deposition growth substrates, although the mechanisms for inducing blister formation are different.

Influence of water environment on nanosecond laser-induced damage thresholds of noble metals and alloys

Pulsed Laser Ablation in Liquid (PLAL) is a flexible technique for synthesis of nanoparticles of various materials, in particular of noble metals [1]. Despite of the widespread use of this method, processes involved in PLAL are still poorly understood. The presence of the liquid makes the PLAL process much more complicated as compared to conventional ablation in vacuum or in an ambient gas. The poor current knowledge of the PLAL process can be illustrated by the example of the laser-induced damage thresholds (DTs) in liquid. The available data on the DTs under PLAL are rather contradictory and provide threshold laser fluences higher [2, 3], equal to [4], and lower [5] than the corresponding values in air. Various mechanisms are invoked to explain the differences. Thus, higher DTs under PLAL, observed in most experiments, are explained by conductive heat transfer to the liquid [3] or by vapor pressure and confinement effects [2] while an increase of the surface absorptivity in liquid or enhanced shockwave recoil pressure are assumed to be responsible for lower thresholds [5].