H. Johansen

Laser damage of alkaline-earth fluorides at 248 nm and the influence of polishing grades

Abstract Damage behaviour and thresholds for single 248 nm/14 ns excimer laser pulses have been investigated for single crystals of CaF2 and BaF2 with (111) surface orientation. The probe beam deflection technique was applied as a sensitive tool for detecting the onset of single-shot damage. Below the plasma threshold, we observed one- and two-photon absorption for CaF2 and BaF2, respectively. When testing the influence of different polishing techniques, we found the lowest thresholds for conventional hard-polish. Advanced methods as ductile machining or chemical polishing lead to a distinct increase in damage threshold up to and even better than what is observed for cleaved surfaces. SEM investigations of irradiated areas show that damage preferably takes place at residual steps or other structural defects.

Laser-damage of cleaved and polished CaF2 at 248 nm

Abstract We investigated the surface damage of CaF 2 induced by irradiation with 248 nm 14 ns laser pulses. Experiments were carried out in single shot mode in air. The photoacoustic deflection technique was utilized for in situ damage detection and irradiated spots were inspected by scanning electron microscopy. Fracture along the natural cleavage planes was observed. Comparison of cleaved and polished samples in connection with modelling the thermoelastic response of the crystal during damage reveals that polishing causes enhanced surface absorption. Damage topography at the periphery of the irradiated spots was used to obtain a rough estimate of the temperature rise leading to fracture. Electron microscopy shows surface modifications of weakly irradiated spots where no ablation or cracking could be detected.

Laser damage of CaF2(111) surfaces at 248 nm

Abstract Single-shot laser damage of polished and cleaved CaF2(111) surfaces induced by(248 nm)(14 ns) pulses was studied by electron microscopy and probe beam deflection. It is shown that the onset of laser damage is fracturing rather than melting. The fragment thickness is found to increase with fluence which points to an enhanced near surface absorption of about 0.4 μm characteristic depth. Bending of the fragments with typical curvature radii around 100 μm indicates that within this fragment thickness a transition to plasticity has taken place.

Laser-stimulated desorption and damage at polished CaF2 surfaces irradiated with 532 nm laser light

Abstract Laser-stimulated emission of Ca neutrals from polished CaF 2 (111)-surfaces was studied with a quadrupole mass filter system under UHV conditions. For intensity dependent measurements performed at various spots of the crystal surface a threshold pulse energy could be determined where a sharp decrease in transmitted light intensity occurred. Only for a few measurements this threshold coincided with the ablation threshold (type 1 spots). In most cases, however, the transmission breakdown threshold was found to be distinctly lower than the ablation threshold (type 2 spots). For type 2 spots a weak emission of delayed calcium atoms was observed at the transmission breakdown threshold. Irradiated spots have been examined by optical and scanning electron microscopy. Damage phenomena observed included coloration, cracking along crystal axes, surface erosion and surface melting depending on incident laser intensity. Pronounced differences between type 1 and type 2 spots with respect to laser light entrance and exit surface damage morphology were found.

Influence of ion bombardment on the refractive index of laser pulse deposited oxide films

Abstract HfO2, ZrO2 and Y2O3 films for optical applications were prepared by ion assisted laser pulse deposition. The influence of oxygen ion bombardment of growing films on the refractive index was investigated in the ion energy range of 150–600 eV and in the ion current density range of 50–250 μA/cm2. Films deposited without additional ion bombardment or using low ion energy up to 150 eV or low ion current density up to 60 μA/cm2 possess a high bulk-like refractive index, whereas above these thresholds the refractive index steadily decreases. In the case of hafnia, for example, the highest refractive index, obtained for stoichiometrical films with low absorption, amounted to 2.15 at 600 nm wavelength. Due to relatively strong ion bombardment it decreased to 1.80. This behaviour is a result of ion induced modifications of microstructure. While films with high refractive index were of amorphous structure and had a high packing density with low porosity, increasing ion bombardment of the growing films led to increasing crystallization within the films and, finally, to polycrystalline films combined with increasing columnar film growth and rougher surfaces. Larger voids between the columns result in lower packing density and, therefore, lower refractive index. Using appropriate deposition parameters, oxide films with low absorption coefficients and high laser damage thresholds at 1.06 μm wavelength can be prepared.

Laser damage and ablation of differently prepared CaF2(111) surfaces

Ablation thresholds and damage behavior of cleaved and polished CaF2(111) surfaces produced by single shot irradiation with 248 nm/14 ns laser pulses have been investigated using the photoacoustic mirage technique and scanning electron microscopy. The standard polishing yields an ablation threshold of typically 20 J/cm2. When surfaces are polished chemo-mechanically the threshold is raised to 43 J/cm2. Polishing by diamond turning leads to intermediate values around 30 J/cm2. Cleaved surfaces possess no well-defined damage threshold. The damage topography of conventionally polished surfaces shows ablation of flakes across the laser heated area with cracks along the cleavage planes. In the case of chemo-mechanical polishing only a few cracks appear. Diamond turned surfaces show small optical absorption, but cracks and ablation of tiles. The origin of such different damage behavior is discussed.

Properties of pulsed laser deposited optical coatings

Hafnia and yttria films for optical applications were prepared by pulsed laser ablation with oxygen 1on bombardment of the growing films. The influence of the laser and ion beam parameters on the refractive index and microstrucmre of the films was investigated. The optical quality of the films with respect to absorption and laser damage was characterized at the Nd:YAG-laser wavelength by measuring the laterally resolved absorptivity and the laser damage thresholds. Both hafnia and yttria films prepared at low ion energy and intensity (150 eV, 50 /xA/cm 2) were amorphous and had a high bulk-like refractive index and packing density. At high ion energy and intensity (700 eV. 400 /xA/cm 2) the films became polycrystalline with high refractive index and packing density at relatively high growth rates above 10 to 20 nm/min or relatively low refractive index and packing density at low growth rates. Films with high laser damage threshold at 1.06 /xm wavelength could only be prepared with strong oxygen bombardment. Highly reflective multilayer systems of only one material with alternately high and low refractive index were prepared by varying the parameters of oxygen ion bombardment during deposition. Their reflectivity and laser damage threshold at 1.06/xm as well as their microstructure were investigated.

Laser direct writing of titanium silicide thin films

Abstract Investigations concerning the laser-induced chemical vapour deposition of TiSi 2 thin films from SiH 4 and TiCl 4 using a direct writing method are presented. A CO 2 -laser at λ=10.6μm and an argon ion laser in the VIS (488–527 nm, multiline mode) were used for the preferably thermal deposition on SiO 2 and c-Si substrates. The influence of the deposition parameters gas composition, laser power, scan velocity and laser wavelength on structural, geometric, and electrical properties is reported.

Charging phenomena in low-voltage electron microscopy of laser-fractured fluoride surfaces

Surfaces of fluoride crystals, fractured by a single excimer laser pulse and then covered by a thin conductive layer, are imaged by scanning electron microscopy in the low‐voltage secondary electron mode. As a result of charging, at lower primary electron energies a contrast enhancement can be obtained for surface fragments that are no longer tightly attached to the crystal. This differs from high‐energy (≳10 keV) imaging which only yields topographic contrasts and allows the analysis of the fractured structure by edge and shadowing effects. Even contrast inversion from positive to negative charging of an entire fragment can be achieved, depending on the primary electron energy. It is shown that this effect can be utilized to discriminate between fragments with a good mechanical contact to the bulk and partially detached ones by systematically studying the contrast as a function of electron energy and specimen inclination.

Influence of LN2 substrate cooling on optical properties of laser-pulse-deposited oxide films

Abstract In this paper we will present the results of investigations of the influence of substrate cooling by liquid nitrogen during film deposition on the optical properties of oxide films prepared by means of laser pulse deposition. A CO2-TEA laser at 10.6 μm wavelength (pulse duration 200 ns, repetition rate 40 Hz, laser fluence 5 × 107 W/cm2, monomode) and an excimer laser at 248 nm wavelength (pulse duration 20 ns, repetition rate 200 Hz, laser fluence 5 × 107 W/cm2) were used for the evaporation and, respectively, the ablation of sintered oxide targets. Moreover, the growing films were bombarded by oxygen ions of varying energy and current density. As will be shown, the variation of refractive index with ion bombardment changed only imperceptibly due to substrate cooling, i.e. films deposited at relatively low ion energy and current density possess a high bulk-like refractive index and films deposited at relatively high ion energy and current density possess a rather low refractive index. We could observe, however, that, in comparison with uncooled substrates, cooling of the substrates by liquid nitrogen results in a marked decrease in absorption together with a marked increase of the laser damage thresholds of the films at 1.06 μm wavelength. For example, the laser damage threshold of hafnia films increased by a factor of 1.5.