Wolfgang Kautek

Laser ablation of dielectrics with pulse durations between 20 fs and 3 ps

Laser‐induced ablation has been extended down to a pulse duration of 20 fs generated by a Ti sapphire laser system at a wavelength of 780 nm. Barium aluminum borosilicate glass with an extremely high glass transformation temperature (∼600 °C) served as target material. The most significant observation was a substantial decrease of the ablation threshold fluence at pulse durations below 100 fs. All results indicate a dominant role of multiphoton absorption in addition to collisional ionization in this time domain.

Ultrashort pulse laser ablation of polycarbonate and polymethylmethacrylate

Abstract Ablation experiments with ultrashort laser pulses (pulse duration 150 fs, wavelength 800 nm) on polymers (PC, PMMA) relevant for biomedical technology have been performed in air. The lateral and vertical machining precision was evaluated by optical, atomic force and scanning electron microscopy. The ablation threshold reaches values in the range of 0.5–2.5 J/cm 2 and depends significantly on the number of laser pulses applied to the same spot. The hole diameters are influenced by the laser fluence and the number of laser pulses. The relation between the ablation threshold and the number of laser pulses applied to the same spot is described in accordance with an incubation model.

Photoactive thin film semiconducting iron pyrite prepared by sulfurization of iron oxides

Abstract Photoactive iron pyrite (FeS2) thin film layers have been synthesized by a simple method involving the reaction of Fe3O4 or Fe2O3 with elemental sulfur. The films were formed on a variety of different substrate materials by converting or sulfurizing iron oxide layers. The subsequent sulfur treatment of the oxide layers consisted of exposure of the films to gaseous sulfur in open or closed ampules at 350°C for 0.5–2 h. The morphology, composition and photoactivity of the films produced were checked using X-ray diffraction, X-ray photoelectron spectroscopy (ESCA), optical absorption, steady state and transient photoconductivity. The best films showed good crystallinity and purity with concurrent photoconductivity and photoelectrochemical response. The ability of this technique to produce photoactive material can be explained by interpretation of the Gibbs ternary phase diagram for the FeOS system, and may be related to the production of photoactive pyrite in nature. A discussion is made as to the future improvement of the solar cell response by proper optimization of geometric and configurational properties.

Femtosecond-pulse visible laser processing of transparent materials

Abstract 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.

On the damage behavior of dielectric films when illuminated with multiple femtosecond laser pulses

The physical effects reducing the damage threshold of dielectric films when exposed to multiple femtosecond pulses are investigated. The measured temperature increase of a Ta2O5 film scales exponentially with the pulse fluence. A polarized luminescence signal is observed that depends quadratically on the pulse fluence and is attributed to two-photon excitation of self-trapped excitons that form after band-to-band excitation. The damage fluence decreases with increasing pulse number, but is independent of the repetition rate from 1 Hz to 1 kHz at a constant pulse number. The repetition rate dependence of the breakdown threshold is also measured for TiO2, HfO2, Al2O3, and SiO2 films. A theoretical model is presented that explains these findings.

Femtosecond pulse laser ablation of metallic, semiconducting, ceramic, and biological materials

Production of holes and grooves of < 30 micrometers diameter with high aspect ratio value is a delicate task either for mechanical tools, or for conventional nanosecond pulse lasers like e.g. pulsed Nd:YAG or excimer lasers. They later tend to cause microcracks extending from an annular melting zone, or substantial disruption, respectively. Experimental results are presented which demonstrate that the development of intense ultrashort pulse laser systems (>> 1012 W cm-2, (tau) < 1 ps) opens up possibilities for materials processing by cold plasma generation and ablation of metals, semiconductors, ceramics, composites, and biological materials. A femtosecond and a nanosecond dye laser with pulse durations of 300 fs (< 200 (mu) J) and 7 ns (< 10 mJ), and center wavelengths at 612 and 600 nm, respectively, both focused on an area of the order of 10-5 cm2, have been applied either to absorbing substrates, like polycrystalline gold, silicon (111), aluminum nitride ceramics, or transparent materials, like synthetic and human dental hydroxyapatite composites, bone material, and human cornea transplants. The fs-laser generates its own absorption in transparent materials by a multiphoton absorption process, and thus forces the absorption of visible radiation. Because the time is too short (< ps) for significant transport of mass and energy, the beam interaction generally results in the formation of a thin plasma layer of approximately solid state density. Only after the end of the subpicosecond laser pulse, it expands rapidly away from the surface without any light absorption and further plasma heating. Therefore, energy transfer (heat and impulse) to the target material, and thermal and mechanical disruption are minimized. In contrast to heat- affected zones (HAZs) generated by conventional nanosecond pulse lasers of the order of 1 - 10 micrometers , HAZs of less than 0.02 micrometers were observed.

Femtosecond pulse laser processing of TiN on silicon

Ultrashort pulse laser microstructuring (pulse duration 130 fs, wavelength 800 nm, repetition rate 2 Hz) of titanium nitride (TiN) films on silicon substrates was performed in air using the direct focusing technique. The lateral and vertical precision of laser ablation was evaluated. The TiN ablation threshold changed with the number of pulses applied to the surface due to an incubation effect. An ablation depth per pulse below the penetration depth of light was observed. Columnar structures were formed in the silicon substrate after drilling through the TiN layer.

Anisotropic photocorrosion of n-type MoS2 MoSe2, and WSe2 single crystal surfaces: the role 0f cleavage steps, line and screw dislocations

The photocorrosive behavior of the semiconducting transition metal dichalcogenides in contact with aqueous electrolytes is extremely anisotropic. Enhanced reactivity was observed at cleavage steps and on either line or screw dislocation cores emerging at the surface exposing mainly (1011) faces lying in 〈1120〉 directions. Corrosion patterns, however, could never be observed on smooth (0001) faces. The corrosion products of the molybdenum compounds are soluble, whereas WSe2 formed WO3 precipitates on the surface.

Laser interaction with coated collagen and cellulose fibre composites: fundamentals of laser cleaning of ancient parchment manuscripts and paper

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.

Femtosecond laser irradiation of indium phosphide in air: Raman spectroscopic and atomic force microscopic investigations

Surface modification and ablation of crystalline indium phosphide was performed with single and double 130 fs pulses from a Ti:sapphire laser. The morphological features resulting from laser processing, have been investigated by means of micro Raman spectroscopy as well as by optical, atomic force and scanning electron microscopy. The studies indicate amorphous, ablated and recrystallized zones on the processed surface. In the single-pulse irradiation experiments, several different threshold fluences could be assigned to the processes of melting, ablation and polycrystalline resolidification. Residual stress has been detected within the irradiated areas. Double-pulse exposure experiments have been analyzed in order to clarify the effect of cumulative damage in the ablation process of indium phosphide.