Pulsetrain-burst machining has been shown to have advantages over single-pulse laser processing of materials
and biological tissues. Ultrafast lasers are often able to drill holes in brittle and other difficult materials without
cracking or swelling the target material, as is sometimes the case for nanosecond-pulse ablation; further,
pulsetrain-bursts of ultrafast pulses are able to recondition the material during processing for instance, making
brittle materials more ductile and striking advantages can result. In the work we report, we have investigated
hole-drilling characteristics in metal and glass, using a Nd:glass pulsetrain-burst laser (1054 nm) delivering 1-10
ps pulses at 133 MHz, with trains 3-15 μs long. We show that as the beam propagates down the channel being
drilled, the beam loses transverse coherence, and that this affects the etch-rate and characteristics of channel shape:
as the original Gaussian beam travels into the channel, new boundary conditions are imposed on the
propagating beam principally the boundary conditions of a cylindrical channel, and also the effects of plasma
generated at the walls as the aluminum is ablated. As a result, the beam will decompose over the dispersive
waveguide modes, and this will affect the transverse coherence of the beam as it propagates, ultimately limiting
the maximum depth that laser-etching can reach.
To measure transverse beam coherence, we use a Youngs two-slit interference setup. By measuring the fringe
visibility for various slit separations, we can extract the transverse coherence as a function of displacement across
the beam. However, this requires many data runs for different slit separations. Our solution to this problem
is a novel approach to transverse coherence measurements: a modified Michelson interferometer. Flipping the
beam left-right on one arm, we can interfere the beam with its own mirror-image and characterise the transverse
coherence across the beam in a single shot.
Ultrafast-laser micromachining has promise as an approach to trimming and 'healing' small laser-produced damage sites in laser-system optics--a common experience in state-of-the-art high-power laser systems. More-conventional approaches currently include mechanical micromachining, chemical modification, and treatment using cw and long-pulse lasers. Laser-optics materials of interest include fused silica, multilayer dielectric stacks for anti-reflection coatings or high-reflectivity mirrors, and inorganic crystals such as KD*P, used for Pockels cells and frequency-doubling. We report on novel efforts using ultrafast-laser pulsetrain-burst processing (microsecond bursts at 133 MHz) to mitigate damage in fused silica, dielectric coatings, and KD*P crystals. We have established the characteristics of pulsetrain-burst micromachining in fused silica, multilayer mirrors, and KD*P, and determined the etch rates and morphology under different conditions of fluence-delivery. From all of these, we have begun to identify new means to optimize the laser-repair of optics defects and damage.
The emission spectra of laser produced plasmas of pure tin targets are dominated by recombination continuum emission throughout the entire EUV spectral region with intense structure due to line emission dominating the spectra in the 13 - 14 nm region. This feature arises from resonant 4p64dn - 4p54dn+1 + 4p64dn-14f emission lines that are generally concentrated in a narrow band, 5 - 10 eV wide, which overlaps considerably in adjacent ion stages to form an intense unresolved transition array (UTA). Such plasmas are optically thick; the strongest lines are attenuated and frequently appear in absorption. However, if tin comprises a few percent of a predominantly low-Z matrix, the recombination is suppressed and the plasmas can become optically thin to resonance radiation. Under these conditions, resonance line emission can dominate the spectra. The application of a collisional radiative (CR) model, combined with ab initio atomic structure calculations, allows one to estimate the laser plasma parameters that will optimize the UTA as efficient narrow bandwidth emitters of EUV radiation. The dependence on laser power density of both in-band emission and debris generation from pure tin targets is presented. The influence of a pre-pulse on the plasma output is also investigated.