Laser-induced surface modifications, structure formation, and ablation of organic polymers
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In this paper we present recent results on laser-induce surface modifications and surface patterning by ablation. Different types of structure formation are discussed. The modeling of UV-laser ablation in nonstationary regimes is studied. Numerical calculations on the ablation rate are compared with experimental data.Keywords:
Laser Ablation
Structure formation
Surface structure
It is important to develop a practical technique for monitoring the ablation interaction at the fiber tip during contact laser ablation, because ablation interaction monitoring may offer useful information about ablation qualities and tissue types (normal aorta, fatty atheroumatous plaque, calcified plaque). In the case of pulsed Ho:YAG laser (/spl lambda/=2.1 /spl mu/m) ablation, the ablation bubble is generated at the fiber tip. Also the ablation bubble is indispensable for effective ablation, however excessive ablation bubble formation readily induces damage to the surrounding tissue. Therefore, real-time monitoring of the ablation bubble at the fiber tip is necessary. We have previously developed the fiber optic probe method to monitor the ablation bubbles. The backscattered light of the probe laser from the ablation area was monitored through the same fiber of which the Ho:YAG laser pulse was delivered. We measured the decay time of the backscattered light waveform. The theoretical explanation showed that the calculated bubble collapse time approximately coincided with the decay time of waveform. Since the ablation bubble collapse is strongly affected by mechanical properties of its adjacent tissues, the decay time suggested their surrounding tissue types. We demonstrated the possibility of tissue characterization by this probe method. In order to know the characteristic of the observed backscattered light to understand our probe method capability for tissue characterization, we performed comparative measurements for the ablation bubble by means of time-resolved photography and the probe method.< >
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Femtosecond-laser ablation rates for a broad range of metals have been investigated and general trends are discussed. A newly developed technique allows the accurate measurement of the ablation rates at all times during the formation of holes.
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In this work, a theoretical model based on molecular dynamics (MD) simulations and experimentation results of the femptosecond laser (FS) ablation, are applied for the description of ultrashort laser ablation of metals, with emphasis being given to the understanding of the material removal. The ablation of Fe with the use of 100 fs laser pulses, at a wavelength of 775 nm, is studied. Computational and experimental results have revealed that within the investigated laser fluences range (0.01 to 30 J/cm²), four different ablation areas accompanied by different ablation mechanisms, are distinguished. The threshold of the actual ablation has been determined to be at 0.1 J/cm². The ablation depth as a function of the laser fluence and the ablation threshold value have been evaluated and compared with the experimental data available.
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Despite extensive research work, accurate prediction of the ablation behavior in the high energy nanosecond laser ablation process is still lacking, which may differ significantly depending on the laser parameters, surrounding medium, and target material characteristics. In this paper, nanosecond laser ablation of aluminum in air and water is investigated through a self-contained hydrodynamic model under different laser fluences involving no phase explosion and phase explosion. The ablation depths and profiles are predicted and validated against the literature data and experiments. In case of nanosecond laser ablation of aluminum in water, deeper crater depths are found in all the conditions studied in this work, which may be attributed to the combination effects of laser ablation and shock compression. The analysis of the shock compression in air and water indicates that the shock compression is mainly responsible for this enhancement of ablation in water.
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Femtosecond laser ablation of stainless steel was studied. Two ablation regimes were identified. Analytical model of metal ablation volume calculation is proposed and confronted to experimental results. For desired microtexturing, the fluence and number of pulses are essential.
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Femtosecond(fs) pulse laser ablation of silicon targets in air and in vacuum is investigated using a timeresolved shadowgraphic method. The observed dynamic process of the fs laser ablation of silicon in air is significantly different from that in vacuum. Similar to the ablation of metallic targets,while the shock wave front and a series of nearly concentric and semicircular stripes,as well as the contact front,are clearly identifiable in the process of ablation under 1×10 5 Pa,these phenomena are no longer observed when the ablation takes place in vacuum. Although the ambient air around the target strongly affects the evolution of the ablation plume,the three rounds of material ejection clearly observed in the shadowgraphs of fs laser ablation in standard air can also be distinguished in the process of ablation in vacuum. It is proven that the three rounds of material ejection are caused by different ablation mechanisms.
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Laser ablation of Al, Ni, Fe, and Cu was investigated for pulse durations of 0.1, 1 and 5 ps at fluences up to 100 J.cm-2. The ablation depth per pulse was measured and the ablation thresholds were estimated. The increase of the pulse duration results in a decrease of the ablation rate. The presence of a molten phase is clearly expressed at fluences above 10 J.cm-2. Molecular Dynamics simulation was used to model the laser ablation process in the case of Fe, Al and Ni.
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A selective photothermolysis model has been widely used to explain skin tissue ablation by nanosecond laser pulses. Nevertheless, fundamental questions regarding the mechanism underlying the ablation process remain to be answered. We have investigated the surface ablation of fresh porcine skin tissue with 8 ns pulses at 1064 nm and its dependence on spot size. Histology analysis of the ablated tissue samples has been conducted. From these preliminary results we have plotted the ablation depth per pulse as a function of laser fluence at different spot sizes.
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Aluminum nanoparticles were synthesized by pulsed laser ablation of Al targets in ethanol for 5-15 minutes using the 1064 and 533 nm wavelengths of a Nd:YAG laser with energies of 280-320 mJ per pulse. It has been found that higher wavelength leads to significantly higher ablation efficiency, and finer spherical nanoparticles are also synthesized. Besides, it was obvious that higher ablation time resulted in higher ablated mass, while lower ablation rate was observed. Finer nanoparticles, moreover, are synthesized in higher ablation times.
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