Francois Y. Genin

Comparison of structure and properties of femtosecond and nanosecond laser-structured silicon

We compare the optical properties, chemical composition, and crystallinity of silicon microstructures formed in the presence of SF6 by femtosecond laser irradiation and by nanosecond laser irradiation. In spite of very different morphology and crystallinity, the optical properties and chemical composition of the two types of microstructures are very similar. The structures formed with femtosecond (fs) pulses are covered with a disordered nanocrystalline surface layer less than 1 μm thick, while those formed with nanosecond (ns) pulses have very little disorder. Both ns-laser-formed and fs-laser-formed structures absorb near-infrared (1.1–2.5 μm) radiation strongly and have roughly 0.5% sulfur impurities.

Role of light intensification by cracks in optical breakdown on surfaces

The intensity distribution of an initially plane light wave incident on planar and conical surface cracks is calculated numerically by using a wave propagation computer code. The results show that light intensity enhancements caused by interference of internal reflections at the crack and the surface are very sensitive to the light polarization, the beam angle of incidence, and the crack geometry (e.g., crack width and orientation with the surface). The light intensity enhancement factor (LIEF) can locally reach 2 orders of magnitude for conical cracks of ideal shape. The electric field direction relative to the crack surfaces determines the light intensity profile around the crack. For normal-incidence illumination on the output surface, total internal reflection at the crack and the surface can occur and leads to higher LIEFs. For identical geometry and illumination conditions, a crack located on the entrance surface of an optic generates electric field enhancements that are weaker than those on the exit surface. As cracks on polished surfaces are randomly oriented, the probability for large intensity enhancements to occur is high. The model is able to predict quantitatively the magnitude of surface laser-induced damage threshold drop and damage propagation enhancement in dielectric materials that are due to cracks.

Extrapolation of damage test data to predict performance of large-area NIF optics at 355 nm

For the aggressive fluence requirements of the NIF laser, some level of laser-induced damage to the large 351 nm final optics is inevitable. Planning and utilization of NIF therefore requires reliable prediction of the functional degradation of the final optics. Laser damage test are typically carried out with Gaussian beams on relatively small test ares. The test yield a damage probability vs. energy fluence relation. These damage probabilities are shown to depend on both the beam fluence distribution and the size of area tested. Thus, some analysis is necessary in order to use these test results to determine expected damage levels for large aperture optics. We present a statistical approach which interprets the damage probability in terms of an underlying intrinsic surface density of damaging defects. This allows extrapolation of test results to different sized areas and different beam shapes. The defect density is found to vary as a power of the fluence.

Energy deposition at front and rear surfaces during picosecond laser interaction with fused silica

The difference between front-surface and rear-surface energy deposition of a 35 ps laser pulse (λ=1064 nm) in fused silica was investigated using shadowgraphic and laser-deflection techniques. Shock waves were observed in the glass and in air. Shock waves generated in air at the front surface are stronger than at the rear surface. Less than 35% of the energy incident on the surface drives the air shock waves at the rear surface. Up to 90% of the incident energy drives the air shock waves at the front surface. Laser-plasma interaction is responsible for this difference and for limiting the amount of energy deposited inside the sample during front-surface ablation. Energy deposition at the rear surface is mostly limited by self-focusing inside the material.

Morphologies of laser-induced damage in hafnia-silica multilayer mirror and polarizer coatings

Hafnia-silica multilayer mirrors and polarizers were deposited by e-beam evaporation onto BK7 glass substrates. The mirrors and polarizers were coated for operation at a wavelength of 1053 nm at 45 degrees and at Brewsters angle (56 degrees), respectively. They were tested with a single 3-ns laser pulse. The morphology of the laser-induced damage was characterized by optical and scanning electron microscopy. Four distinct damage morphologies were found: pits, flat bottom pits, scalds, and delaminates. The pits and flat bottom pits (less than 30 micrometer in diameter) were detected at lower fluences (as low as 5 J/cm2). The pits seemed to result from ejection of nodular defects by causing local enhancement of the electric field. Scalds and delaminates could be observed at higher fluences (above 13 J/cm2) and seemed to result from the formation of plasmas on the surface. These damage types often originated at pits and were typically less than 300 micrometer in diameter; their size increased almost linearly with fluence. Finally, the effects of the damage on the characteristics of the beam (reflectivity degradation and phase modulations) were measured.

Magnetorheological finishing for imprinting continuous-phase plate structures onto optical surfaces

Magnetorheological finishing (MRF) techniques have been developed to manufacture continuous phase plates (CPPs) and custom phase corrective structures on polished fused silica surfaces. These phase structures are important for laser applications requiring precise manipulation and control of beam-shape, energy distribution, and wavefront profile. The MRF’s unique deterministic-sub-aperture polishing characteristics make it possible to imprint complex topographical information onto optical surfaces at spatial scale-lengths approaching 1 mm. In this study, we present the results of experiments and model calculations that explore imprinting two-dimensional sinusoidal structures. Results show how the MRF removal function impacts and limits imprint fidelity and what must be done to arrive at a high quality surface. We also present several examples of this imprinting technology for fabrication of phase correction plates and CPPs for use at high fluences.

Effect of SiO2 overcoat thickness on laser damage morphology of HfO2/SiO2 Brewster's angle polarizers at 1064 nm

HfO2/SiO2 Brewsters angle polarizers are being developed at Lawrence Livermore National Laboratory for the National Ignition Facility. Damage threshold studies using a 3-ns pulse length 1064-nm laser have revealed a number of different damage morphologies such as nodular ejection pits, plasma scalds, and overcoat delaminations. Of these laser damage morphologies, delaminations have the most negative impact on fusion laser performance. By increasing the thickness of the SiO2 overcoat, the delamination morphology is eliminated without significantly modifying the spectral characteristics of the coating. A model of the thermal mechanical response of the overcoats is presented for various SIO2 overcoat thicknesses. The overcoat thickness influences the electric-field profile resulting in different thermal gradients between the outer SiO2 and HfO2 layers. This modeling effort attempts to understand the relationship between the thermal stress distribution in the overcoat and the occurrence of delamination.

Laser-induced damage of fused silica at 355 nm initiated at scratches

Scratches of measured width were produced on the surface of a IV grade fused silica window using a diamond tip. Two scratch morphologies were observed: plastic and brittle. The scratches were irradiated with a 355 nm laser pulse. The laser-induced damage threshold (LIDT) of the unscratched output surface was 15 J/cm2 at 3-ns. The LIDT of the scratched surface as a function of scratch width was then measured for both input and output surface scratches. Input surface scratches of width smaller than 10 micrometers did not influence the LIDT of the silica window. On the output surface, 7

Laser resistant coatings

mUm wide scratches lowered the LIDT by a factor of two. For larger scratches, the LIDT reached an asymptotic value of 5 J/cm2 on both input and output surface. Possible reasons for this LIDT drop could be electric field enhancement in the cracks below the scratch, the presence of contamination particles in the scratch, or the weakening of the material because of existing mechanical flaws.

Electric-field enhancement by nodular defects in multilayer coatings irradiated at normal and 45° incidence

Interest in laser resistant coatings started with Maiman’s invention of the ruby laser (Maiman 1960). It soon became apparent that the existing quality of optical coatings was insufficient to withstand the high-photon flux of a laser source (Glass and Guenther 1973). In 1965 the Office of Naval Research contracted with Baush & Lomb to study the damage thresholds of dielectric films and multilayer coatings to overcome the problems of absorption and coating defects (Turner 1972). In 1969, a symposium on Laser-Induced Damage in Optical Materials was formed and two years later, thin films became one of the four primary research topics. This symposium has been the premier repository of documented research in laser resistant optical thin films. Although the subject of laser damage in optical materials is a rather narrow field of research, there are some excellent books on the subject (Kozlowski 1995; Wood 1986).