In the extreme ultraviolet (EUV) regime, the photon energies are above the threshold for photo-ionization in all solid materials. We examine the absorption coefficients for inverse bremsstrahlung and photo-ionization and the contributions of these processes to EUV absorption under different ionization models. The Saha- Boltzmann ionization model is considered with and without continuum lowering, as well as a novel rate equation model including the photo-ionization transitions in population calculations. We show that photo-ionization plays a significant role early in EUV ablation of solids, but inverse bremsstrahlung absorption dominates once temperatures in the ablated plasma exceed more than a few electron volts.
A capillary laser with output in the extreme ultra-violet at wavelength 46.9 nm is used to ablate solid targets of parylene- N (CH), PMMA, aluminum and gold. We summarize results obtained using different focusing optics: a Fresnel zone plate, an off-axis spherical multi-layer mirror and on-axis multi-layer and gold mirrors. The Fresnel zone plate has a small aperture and focuses a small fraction of the laser energy to a small diameter (< 1 μm) with peak intensities 6 x 109Wcm-2. The off-axis spherical multi-layer mirror allows for a measurement of the transmission of the laser through thin targets, but the off-axis geometry produced an aberrated focus. The on-axis multi-layer mirror allows focusing to intensities of approximately 5 x 1010 Wcm-2 with a cylindrically symmetric focus.
Compact extreme ultraviolet (EUV) laser sources can be used for laboratory-scale ablation experiments at intensities of 1 × 1011 Wcm−2 and higher. The depths of ablation achieved using focused laser output at 46.9 nm to irradiate solid targets of aluminum, gold, and copper have been modeled. Two simple models are considered; an adaptation of an ultra-short pulse model, and an ablation velocity model. We show that the attenuation length of the material plays an important role in the physics of the ablation. A more detailed one-dimensional model including absorption by inverse bremsstrahlung absorption and photo-ionization, corrected to include electron degeneracy effects, is used to evaluate the opacity of the ablation plasma and subsequent ablation depths.
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