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A femtosecond pulse laser in the visible spectral region shows promise as a potentially new powerful corneal sculpting tool. It combines the clinical and technical advantages of visible wavelengths with the high ablation quality observed with nanosecond-pulse excimer lasers at 193 nm. A femtosecond and a nanosecond dye laser with pulse durations of 300 fs and 7 ns, and centre wavelengths at 615 nm and 600 nm, respectively, both focused to an area of the order of 105 cm2, have been applied to human corneal ablation. Nanosecond laser pulses caused substantial tissue disruption within a 30100 m range from the excision edge at all fluences above the ablation threshold of F th60 J cm2 (I th9 GW cm2). Completely different excisions are produced by the femtosecond-pulse laser: high quality ablations of the Bowman membrane and the stroma tissue characterised by damage zones of less than 0.5 m were observed at all fluences above ablation threshold of F th1 J cm2 or I th3 TW cm2 (3×1012 W cm2). The transparent cornea material can be forced to absorb ultrashort pulses of extremely high intensity. The fs laser generates its own absorption by a multiphoton absorption process.
In situ second harmonic generation measurements during the electrodeposition of Ni on n-Si(111)
(1993)
Textile, aluminium and polyethylene used as components in laser protection curtains were investigated with respect to their ablation behaviour. Employing 33-fs pulses (800 nm wavelength, 1 kHz repetition rate), ex situ geometrical measurements of the ablation cavities and in situ acoustic investigations with a microphone were performed to determine the ablation thresholds in the single- and multi-pulse cases. The acoustical method proved advantageous for complex surface morphologies and/or single laser pulse interactions. Incubation phenomena can be observed for all the materials studied. Technically relevant multi-pulse ablation thresholds are presented and are compared with the single-pulse (1-on-1) irradiation.
Ablation experiments employing Ti:sapphire laser pulses with durations from 30 to 340 fs (centre wavelength 800 nm, repetition rate 1 kHz) were performed in air. Absorbing filters (Schott BG18 and BG36) served as targets. The direct focusing technique was used under single- and multi-pulse irradiation conditions. Ablation threshold fluences were determined from a semi-logarithmic plot of the ablation crater diameter versus laser fluence. The threshold fluence decreases for a shorter pulse duration and an increasing number of pulses. The multi-pulse ablation threshold fluences are similar to those of undoped glass material (~1 J cm-2). That means that the multi-pulse ablation threshold is independent on the doping level of the filters. For more than 100 pulses per spot and all pulse durations applied, the threshold fluence is practically constant. This leads to technically relevant ablation threshold values.
We report measurements of the optical breakdown threshold and ablation depth in dielectrics with different band gaps for laser pulse durations ranging from 5 ps to 5 fs at a carrier wavelength of 780 nm. For tau <100 fs, the dominant channel for free electron generation is found to be either impact or multiphoton ionization (MPI) depending on the size of the band gap. The observed MPI rates are substantially lower than those predicted by the Keldysh theory. We demonstrate that sub-10-fs laser pulses open up the way to reversible nonperturbative nonlinear optics (at intensities greater than 1014 W/cm2 slightly below damage threshold) and to nanometer-precision laser ablation (slightly above threshold) in dielectric materials.
Laser cleaning of delicate biological composite materials such as ancient parchment manuscripts from the 15th and 16th century and printed paper from the 19th century is demonstrated with an ultraviolet excimer pulsed laser at 308 nm. Laser fluence levels must stay below the ablation and destruction threshold of the parchment or paper substrate, and have to surpass the threshold of the contaminant matter. Foreign layers to be removed must exhibit a higher optical density than the artifact substrates. Synthetic carbonaceous dirt modelled by water-soluble black crayons showed a characteristically weak featureless laser-induced plasma spectroscopy spectrum near the noise limit. It turned out that laser-induced plasma spectroscopy is of limited use in monitoring halting points (or etch-stops) because it relies on the destruction not only of the laterally inhomogenously distributed contaminant but also of pigment phases on a microscopically rough parchment substrate. Laser-induced fluorescence spectroscopy, however, promises to be a valuable non-destructive testing technique for etch-stop monitoring.
Curved substrates can be micro-structured by laser ablation, which is not possible with standard lithographic methods. The novel femtosecond-pulse laser technique allows the production of defined and reproducible micro-perforations of originally analyte-impermeable membranes. The trans-membrane analyte flux can be controlled both by the variation of the laser focus diameter resulting in different areas of single perforations, and the number of perforations in arrays on small membrane areas. This leads to a higher degree of variability as well as reproducibility of the diffusion qualities of sensor membranes, and marks the main innovation with this technique compared to the hand-made mechanical perforation by specially grinded needles used up to now. Touchless micro-perforation of small membrane areas with negligible heat damage of the structures adjacent to the perforation allows the application of analyte door membranes directly onto curved surfaces of miniaturized needle-sensors assigned for in vivo glucose monitoring, for the first time.
High-power lasers in industrial and R & D applications raise the general problem of reliability and degradation of optical components. A systematic study of nonlinear interaction of various transparent dielectric materials as e.g. glasses, fused silica, and polymers, with laser-pulses in the intensity range of up to 1013 W cm-2 is presented. On the other hand, femtosecond-pulse laser processing in the visible spectral range (300 fs; 620 nm, ~ 2 eV) allows precise microstructuring of transparent dielectrics without disruption of the remnant material. Damage and ablation threshold fluences occur above 1.2 J cm-2 at both silicate glasses and fused silica. Two different photon absorption mechanisms have been observed. The first occurs during the initial laser pulses in the incubation range. There, multiphoton absorption results in moderate energy volume densities. These are sufficient to generate morphological changes and optically active defect sites (colour centres) which provide a much higher absorptivity relevant for the second mechanism. It results in gasification without participation of melt.
A first investigation on the ablation of composite materials like carbon and silicon-carbide reinforced alumo- and borosilicate glasses by 300 fs laser pulses (620 nm) in comparison to experiments with 17 ns pulse excimer laser (308 nm) treatment is presented. In all composites, femtosecond laser scans with a fluence of less than 2 J cm-2 produce well defined cuts with smooth side walls in contrast to the nanosecond laser result where extremely incongruent ablation is observed. Visible lasers should not be applicable because SiC and the glasses are practically transparent. Visible subpicosecond pulses of high intensity in the TW cm-2 range allow multi-photon absorption accompanied by incubation phenomena. The morphology of the groove edges reveal the contrasting ablation thresholds and rates of the fibre and glass materials. The ablation thresholds of the transparent components, i.e. the glass matrices and SiC, are about one order of magnitude greater than that of carbon which is ~ 0.15 J cm-2. Incubation effects are important for the absorption mechanisms in the transparent materials. When a critical number of pulses has not been reached at the SiC-glass composites, only the glass is preferentially ablated, and the fibres remain intact. This is in contrast to the C-glass composites where the ablation behaviour is opposite.