Investigation of plume dynamics during picosecond laser ablation of H13 steel using high-speed digital holography

Ablation of H13 tool steel using pulse packets with repetition rates of 400 and 1000 kHz and pulse energies of 75 and 44μJ, respectively, is investigated. A drop in ablation efficiency (defined here as the depth per pulse or μm/μJ) is shown to occur when using pulse energies of E$_{pulse}$>44μJ, accompanied by a marked difference in crater morphology. A pulsed digital holographic system is applied to image the resulting plumes, showing a persistent plume in both cases. Holographic data are used to calculate the plume absorption and subsequently the fraction of pulse energy arriving at the surface after traversing the plume for different pulse arrival times. A significant proportion of the pulse energy is shown to be absorbed in the plume for E$_{pulse}$>44μJ for pulse arrival times corresponding to > 1 MHz pulse repetition rate, shifting the interaction to a vapour-dominated ablation regime, an energetically costlier ablation mechanism.This work was collaboratively carried out under EPSRC Grant Number EP/K030884/1, as part of the EPSRC Centre for Innovative Manufacturing in Laser-based Production Processes. One of the authors acknowledges his PhD studentship by the Federal Government of Nigeria (TETFUND) in conjunction with the Federal University of Petroleum Resources Effurun (FUPRE)

Influence of the pulse number and fluence of a nanosecond laser on the ablation rate of metals, semiconductors and dielectrics

We investigated the pulsed laser ablation of metallic (Al), semiconductor (Si), and wide bandgap dielectric (LiNbO3) targets in air at normal atmospheric conditions by using 4.5 ns pulses at 532 nm wavelength. We determined the dependence of the ablation rate on the pulse number and laser fluence. The number of consecutive laser pulses hitting the target on the same area was between 5 and 40, and the laser fluence was varied in the range of 10–250 J/cm2 by changing the irradiated area at the target surface. We find that the ablation rate of the three targets is approximately constant when the pulse number is smaller than 15. Further increase of the pulse number leads to a decrease of the ablation rate, the fastest decrease of the ablation rate with pulse number being observed for the dielectric target. The dependence of the ablation rate on the laser fluence indicates two different regimes. In the first regime, which is for values of the fluence smaller than the threshold value (~70 J/cm2 for Al, ~90 J/cm2 for Si, and ~180 J/cm2 for LiNbO3), the ablation rate increases approximately logarithmically with the fluence. In the second regime, characterized by values of the fluence greater than the threshold value, there is a steep increase of the ablation rate. This sudden jump of the ablation rate at the threshold fluence is due to the transition from normal vaporisation to phase explosion, and to the changes in the dimensionality of the plasma-plume hydrodynamics from one-dimensional to three-dimensional

Topography evolution of germanium thin films synthesized by pulsed laser deposition

Germanium thin films were deposited by Pulsed Laser Deposition (PLD) onto single crystal Ge (100) and Si (100) substrates with a native oxide film on the surface. The topography of the surface was investigated by Atomic Force Microscopy (AFM) to evaluate the scaling behavior of the surface roughness of amorphous and polycrystalline Ge films grown on substrates with different roughnesses. Roughness evolution was interpreted within the framework of stochastic rate equations for thin film growth. Here the Kardar-Parisi-Zhang equation was used to describe the smoothening process. Additionally, a roughening regime was observed in which 3-dimensional growth occurred. Diffusion of the deposited Ge adatoms controlled the growth of the amorphous Ge thin films. The growth of polycrystalline thin Ge films was dominated by diffusion processes only in the initial stage of the growth

Standardization and optimization of medicines according to dispersive structure in pharmaceutical technology

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