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.     

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\,\hbox{nm} wavelength, 9\,\hbox{ns} pulse width, 900\,\hbox{mJ} maximum pulse energy and a maximum power density of the order of 10^{10}\,\hbox{W}/\hbox{cm}^2 . The laser-target interaction produces a strong gold etching with a production of a plasma in front of the target. The plasma contains neutrals and ions having high charge state. Time-of-flight measurements are presented for the analysis of the ion production and ion velocity. A cylindrical electrostatic deflection ion analyzer permits to measure 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\,\rm{J/cm}^2 and 160\,\rm{J/cm}^2 , respectively. The maximum ion energy reaches about 2\,\hbox{keV} and 8\,\hbox{keV} at these low and at high laser fluence, 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.     

Silicon epitaxy by pulsed laser annealing of evaporated amorphous films

Epitaxial growth of amorphous silicon layers deposited on Si-(100) substrates at room-temperature has been achieved by irradiation with very short laser pulses, as was evidenced from Rutherford backscattering analysis in combination with the channeling effect technique.     

Non-equilibrium effects in profile spreading on stepped surfaces

We study non-equilibrium effects in spreading and collective diffusion of adatoms on stepped surfaces through Monte Carlo simulations of a lattice-gas model. The spreading density profiles are analyzed by the Boltzmann-Matano method to determine the temporal behavior of the effective collective diffusion coefficients. We find that the presence of steps induces considerable non-equilibrium effects in diffusion. For spreading along the steps, we find that these deviations can be explained by the slow approach of the different adparticle concentrations on terraces and at step edges towards equilibrium. For spreading across the steps, however, we find no such dependence, indicating the breakdown of the linear response theory at early times.     

激光散斑效应对激光雷达探测性能的影响

分析了激光散斑效应对激光雷达探测性能的影响. 给出了远场情况下接收物镜所采集激光散斑数的表达式. 在物镜所采集的散斑和光电子数均较大情况下，提出了光电子所遵守的分布. 分别导出了机载和星载激光雷达的探测概率. 分析表明激光雷达探测性能与所采集的散斑数密切相关，接收的散斑数仅与激光束腰和物镜的口径有关，随着接收散斑数的增加，散斑噪声的影响将减弱. 关键词： 激光雷达 激光散斑 探测概率     

Removal of submicron particles from nickel-phosphorus surfaces by pulsed laser irradiation

Pulsed laser cleaning was demonstrated to be an efficient way for removing submicron particles from the nickel-phosphorus (NiP) surface both experimentally and theoretically. Experimentally, it is found that using KrF excimer laser with a pulse width of 23 ns the cleaning threshold is about 20 mJ / cm² for removing quartz particles from the NiP surface and laser cleaning efficiency increases rapidly with increasing laser fluence. The theoretical analysis shows that the peak cleaning force (per unit area) is larger than the adhesion force (per unit area) for submicron quartz particles on the NiP surface when it is irradiated by excimer laser with a fluence above 10 mJ / cm². Therefore, it is possible to remove submicron quartz particles from NiP surfaces by laser irradiation. The difference between the cleaning force (per unit area) and the adhesion force (per unit area) increases with increasing laser fluence, leading to a higher cleaning efficency for quartz particles on the NiP surface.     

Temperature behavior of implanted and pulsed laser irradiated GaAs

Recent studies have shown that the Low-Power Pulsed-Laser Annealing (LPPLA) of ion-implanted GaAs specimens can be realized in a power-density window in which a complete structural reordering is guaranted. As the experimentally employed conditions allow us to describe the theoretical problem in an unidimensional space domain, we describe here a method to investigate the in-depth temperature behavior during the low-power pulsed-laser irradiation of ion-implanted semiconductors. The application of this method to GaAs specimens shows that the upper limit of the energy density window is connected with the exceeding of the critical temperature T _c below which the As evaporation rate is negligible.     

Formation of highly organised,periodic microstructures on steel surfaces upon pulsed laser irradiation

We present results on the growth of highly organised, reproducible, periodic microstructure arrays on a stainless steel substrate using multi-pulsed Nd:YAG (wavelength of 1064 nm, pulse duration of 7 ns, repetition rate of 25 kHz, beam quality factor of M ²∼1.5) laser irradiation in standard atmospheric environment (room temperature and normal pressure) with laser spot diameter of the target being ∼50 μm. The target surface was irradiated at laser fluence of ∼2.2 J/cm² and intensity of ∼0.31×10⁹ W/cm², resulting in the controllable generation of arrays of microstructures with average periods ranging from ∼30 to ∼70 μm, depending on the hatching overlap between the consecutive scans. The received tips of the structures were either below or at the level of the original substrate surface, depending on the experimental conditions. The peculiarity of our work is on the utilised approach for scanning the laser beam over the surface. A possible mechanism for the formation of the structures is proposed.     

Reactions of evaporated lithium films with silica surfaces

Reactions of thin films of clean lithium with freshly cleaved silica surfaces have been characterized by X-ray photoelectron spectroscopy (XPS). Reaction rates (near room temperature) are low enough that spectroscopic analysis is possible during the entire course of the reaction. The major product of the reaction is composed of oxidized lithium, a metasilicate (SiO^2-₃) species, and a unique form of reduced Si, possibly a lithium-rich alloy phase. For confined regions, where the lithium concentration can be made extremely high (as in these thin film depositions) the reaction with the active metal gives a product distribution not predicted by previous thermodynamic arguments.     

Imaging metal surfaces by pulsed laser stimulated field desorption of field adsorbed atoms

Atomic images of Pt and Mo surfaces have been obtained using pulsed laser stimulated field desorption of field adsorbed atoms. The resolution of the desorption images is slightly inferior to low temperature field ion images. The direct correspondence of the images produced by thermally desorbed field adsorbed atoms with field ion images supports previous calculations suggesting apex site field adsorption of image gas atoms on field emitter surfaces.     

Ordered step arrays on evaporated As (0001) vicinal surfaces

A computational procedure dealing with a one-dimensional epitaxial monolayer model was developed in part I. In this part it is extended and applied to the two-dimensional case, allowing for misfit along two perpendicular interfacial directions. The model employed differs slightly from that used by van der Merwe in that the overgrowth film is simulated by a plane of atoms linked to each other by elastic springs. This allows for an exact determination of the equilibrium boundary conditions. The results show (i) that the rectangular boundary edge is slightly deformed, lateral contractions occurring where the misfit dislocations intersect the boundary edge, (ii) that the dependence of stable structures on misfit is in good agreement with the analytical results of van der Merwe, (iii) that misfit dislocations are introduced alternately at the mutually perpendicular edges of a system having quadratic symmetry, (iv) that a segmented dependence of lowest energy on crystal size is obtained, one segment for each additional dislocation, (v) that a saw-toothed dependence of average strain on crystal size, in qualitative agreement with the experimental work of Vincent, results and (vi) that a fine structure in the energy curves results from discrete adatom peripheral growth.     

Non-equilibrium behavior at a liquid-gas critical point

Second-order phase transitions in a non-equilibrium liquid-gas model with reversible mode couplings, i.e., model H for binary-fluid critical dynamics, are studied using dynamic field theory and the renormalization group. The system is driven out of equilibrium either by considering different values for the noise strengths in the Langevin equations describing the evolution of the dynamic variables (effectively placing these at different temperatures), or more generally by allowing for anisotropic noise strengths, i.e., by constraining the dynamics to be at different temperatures in d _|| - and d _⊥-dimensional subspaces, respectively. In the first, isotropic case, we find one infrared-stable and one unstable renormalization group fixed point. At the stable fixed point, detailed balance is dynamically restored, with the two noise strengths becoming asymptotically equal. The ensuing critical behavior is that of the standard equilibrium model H. At the novel unstable fixed point, the temperature ratio for the dynamic variables is renormalized to infinity, resulting in an effective decoupling between the two modes. We compute the critical exponents at this new fixed point to one-loop order. For model H with spatially anisotropic noise, we observe a critical softening only in the d _⊥-dimensional sector in wave vector space with lower noise temperature. The ensuing effective two-temperature model H does not have any stable fixed point in any physical dimension, at least to one-loop order. We obtain formal expressions for the novel critical exponents in a double expansion about the upper critical dimension d _c = 4 - d _|| and with respect to d _|| , i.e., about the equilibrium theory. Received 4 April 2002 Published online 13 August 2002     

Dispersion-mode pulsed laser

A new self-consistency condition in pulsed lasers with strong intracavity dispersion imposes dispersion modes with specific cavity-length dependent pulse rates, utilizing pulse-train self-imaging properties of a temporal Talbot effect. We give an experimental demonstration of such a laser operation, using a long fiber cavity. We also demonstrate temporal Talbot imaging of a train of short pulses that propagate along large distances of dispersive fibers.     

Segregation of evaporated gallium in laser doped silicon

Silicon wafers of n-type covered with a thin evaporated layer of pure gallium have been exposed to Q-switched Nd : YAG laser pulses (32 ns; 280, 410, $\text{560}\text{MW}\text{cm}{}^2$). Rutherford backscattering measurements with a 3 MeV He⁺ beam show that a significant fraction of the gallium is incorporated in the silicon substrate. A drastic segregation accumulates the Ga atoms near the surface.     

Inverse pulsed laser deposition

Since the advent of pulsed laser deposition (PLD), several different target-substrate arrangements have been proposed. Besides the most common on-axis PLD, several off-axis geometries were studied, mainly to protect the substrate from the agglomerated species (clusters, droplets, particulates) of the plasma plume, which are detrimental to the homogeneity of films. Recently we introduced a novel geometry, termed inverse pulsed laser deposition (IPLD), in which the substrate is placed parallel to and slightly above the target plane. In this paper we summarize our results on this new geometry, and show how it can extend the perspectives of pulsed laser deposition, e.g., by improving the surface morphology of the films. Effects of ambient pressure are presented and exemplified on metallic and compound IPLD films, including Ti, CN _x , and Ti-oxides. AFM topographic images are used to prove that under optimized conditions IPLD is capable of growing compact and smooth films that are superior to PLD ones. A special—but easy-to-implement—IPLD arrangement is also introduced that considerably improves the homogeneity of IPLD films. In this geometry, the properties (e.g., deposition rate and roughness) of the films grown in the 1–25 Pa pressure domain are examined.     

Growth-induced magnetic anisotropy behavior in thin garnet films fabricated by pulsed laser deposition

The effect of target rotation rate on the magneto-optical properties and induced magnetic anisotropy of pulse laser deposition grown Bi:YIG garnets has been investigated. The rotation rate dependence of magnetic properties of thin films has been studied using magneto-optical Faraday rotation combined with magneto-optical Kerr measurement setup. Our results show that decreasing the target rotation rate can also increase the polar coercive field and hence magnetic anisotropy. Also the spectral answer of samples show the capability of use of them in one off optical ranges such as visible, near infrared and far infrared regions.     

Dual-wavelength injection-locked pulsed laser

An injection-locked pulsed Ti:sapphire laser oscillating at dual wavelengths is demonstrated for the first time to our knowledge. By use of two feedback loops, seeds of two independent master lasers are locked on specific longitudinal modes of a power oscillator, leading to a stable dual-wavelength oscillation over a long time scale. The two injection-locked pulsed outputs completely overlap in time, with spectral purities reaching a Fourier-transform limit. The dual-wavelength oscillation is controlled by the master lasers only, allowing for flexible selectivity of the two wavelengths and full controllability of the relative two-wavelength pulsed energies.     

Self-mode-locked pulsed monomode laser

We predict the existence of a new pulsed-laser operation regime, when the phases and polarizations of the two coupled cold-cavity eigenstates of a monomode solid-state laser are taken into account in the derivation of the Maxwell-Bloch equations. This monomode pulsed regime is experimentally observed, without any normal mode locking or Q switching occurring inside the cavity. We obtain close agreement between experiments and theory, even in the simple case of a Nd:YAG microchip laser, for which sech(2) pulses at nearly megahertz repetition rates are readily observed.