Mary L. Dawes

Interactions of wide band gap single crystals with 248 nm excimer laser radiation. III: The role of cleavage-induced defects in MgO

When single‐crystal, arc‐fused MgO is cleaved and exposed to single pulses of 248 nm, 3–10 J/cm2 excimer laser light (in vacuum), micron‐sized holes are produced in the irradiated area. Cleavage produces micron‐sized sites which exhibit highly localized absorption, resulting in decomposition, melting, and vaporization of the crystal at these sites. At fluences between 1 and 2 J/cm2, single laser pulses can produce localized, superficial melting and decomposition, leaving clusters of Mg‐rich droplets on the surface. The mass‐resolved charged and neutral particle emission produced by irradiation of the cleaved MgO surface with single laser pulses are examined. There are considerable variations in these emissions when single laser pulses strike different regions of the cleavage surface; these variations are strongly correlated with the production of conical holes in the irradiated region. A phenomenological model for the creation of these sites involving dislocation driven processes during cleavage and the r...

Interaction of wide band gap single crystals with 248 nm excimer laser irradiation. VII. Localized plasma formation on NaCl single crystal surfaces

Wide band gap insulators containing defects exposed to nanosecond pulses of UV laser radiation at fluences close to the damage threshold often display highly localized flashes of light. In this work, we show that flashes observed during irradiation of cleaved, single crystal NaCl at relatively low fluences are due to localized plume fluorescence. By comparing time-resolved optical images of this fluorescence with subsequent scanning electron microscope images of surface topography, we show that these flashes are often associated with micron-dimension surface and near-surface damage, typically associated with cleavage steps. With continued laser irradiation, plume fluorescence at previously damaged regions usually grows stronger from pulse to pulse. In some cases, weak plume fluorescence disappears after one laser pulse, and may or may not reappear with continued irradiation. We interpret these results in terms of localized laser absorption by deformation-induced defects generated during cleavage. Delibera...

Pulsed laser positive ion desorption from a model hydrated inorganic crystal (CaHPO4·2H2O) at 248 nm

Abstract We examine the laser-induced positive ion emission of a typical, wide band gap, hydrated inorganic single crystal, CaHPO 4 ·2H 2 O (brushite), at 248 nm (KrF excimer) in vacuum. As-grown brushite is quite resistant to laser damage and yields little ion emission at fluences below 1 J/cm 2 . In the presence of surface damage by abrasion or heating-induced transformations, intense laser-induced ion emission appears at lower fluences. The ion energy and fluence dependence are consistent with a defect mediated, multiple photon emission mechanism. In particular, the transport of hydrated Ca + to the surface, followed by adsorption at anion defects (and removal of water by evaporation), can provide an ideal environment for ion emission. The implications with regard to the UV laser ablation of hydrated environmental and biological minerals are discussed.

Consequences of Combining Laser Irradiation with Other Stimuli on Laser Desorption and Ablation from Wide Bandgap Insulators

When a transparent, wide bandgap insulator is exposed to sub-bandgap laser irradiation, defects at and near the surface often dominate the response in terms of particle emission and eventual ablation of material. We explored the consequences of applying a variety of stimuli that can generate defects on single crystal surfaces of inorganic ionic crystals. The stimuli include: electron beam irradiation, a second laser beam, mechanical treatment, and thermal treatment. We found that a common theme evolves where these stimuli generate sites for strong interactions of the probing laser beam, leading to a dramatic decrease in the laser intensities needed for ejection of ions, neutrals, and eventual plume formation.

Interaction of wide band gap single crystals with 248 nm excimer laser irradiation. VI. The influence of thermal pretreatment on laser desorption of positive ions from a water-containing ionic crystal (CaHPO4⋅2H2O)

Morphological changes during dehydration of single crystals containing lattice H2O (hydrates) can be dramatic, leading to surfaces that are rough, polycrystalline, and laden with defects. Previous studies have shown that ionic crystal surfaces containing point defects readily desorb energetic positive ions under UV excimer laser irradiation. In this work, we examine the consequences of exposing thermally pretreated single crystal brushite (CaHPO4⋅2H2O) to 30 ns pulses of 248 nm laser irradiation and show that the surface modifications strongly enhance the emission of Ca+ and other positive ions.

Structures obtained from resolidification of flame-melted single-crystal germanium

When single-crystal germanium is partially melted with a gas flame in air, resolidification produces novel structures which may be useful for nanoscale fabrication. These features are dominated by a single, sharp cone, up to 5 mm long with a typical aspect ratio of 1.6. If impurity concentrations are sufficiently high, one or more micron-sized, impurity-rich spheres are formed at the tip of the cone. The surface displays a distinctive morphology which depends strongly on the chemical environment. After slow resolidification in air, the cone body and spheres are coated with fractal-like aggregates of spherical oxide particles. More rapid resolidification in laser-melted Ge can produce sharp (100 nm radius of curvature), clean cones. The observed features are attributed to dynamic resolidification, impurity segregation, and diffusion-limited aggregation processes.

Onset of laser plume formation at 248 nm on cleaved-single crystal NaCl: evidence for highly localized emissions

When a wide bandgap insulator is exposed to UV laser radiation at fluences close to the damage threshold, many laser pulses may be required before the onset of damage. Typically, damage is accompanied by the formation of a highly localized, bright, fluorescent plume. In this work, we explore the onset of damage and plume formation by imaging the laser-induced fluorescence from cleaved, single crystal NaCl exposed to pulsed 248-nm laser radiation (KrF excimer). Subsequent observations of the resulting surface topography are made by scanning electron microscopy. The onset of plume fluorescence is accompanied by localized fracture, usually associated with cleavage steps but originating some microns beneath the surface. Strong laser interactions are typically confined to the fractured region, which grows from pulse to pulse. We interpret these results in terms of absorption by deformation- induced defects along cleavage steps. Deformed material, produced by pushing a small metal sphere against the surface, damages at especially low fluences, consist with this interpretation. Indirect evidence suggests that thermal effects are localized along dislocation cores, which serve as recombination centers for laser-induced excitations.

Investigations of laser desorption from modified surfaces of ionic single crystals

Wide band gap insulators irradiated with UV and/or IR pulsed lasers have been shown to yield neutrals, photoinduced electrons, and energetic positive ions suggesting occupied electronic defect states in the band gap. We explore the consequences of applying a variety of stimuli, which can generate defects on single crystal surfaces of inorganic ionic crystals. The stimuli include electron beam irradiation, a second laser beam, mechanical treatment, and thermal treatment. Our experiments on a wide variety of ionic crystals find that a common theme evolves where these stimuli generate sites that strongly interact with the probing laser beam. Such interactions lead to dramatic decrease in the laser intensities needed for ejection of ions, neutrals, as well as eventual plume formation, and result in increased vulnerability of the materials to the laser radiation.