Mark R. Kozlowski

Reactive evaporation of low-defect density hafnia

Motivation for this work includes observations at Lawrence Livermore National Laboratory of a correlation between laser damage thresholds and both the absorption and the nodular-defect density of coatings. Activated oxygen is used to increase the metal-oxidation kinetics at the coated surface during electron-beam deposition. A series of hafnia layers are made with various conditions: two µ-wave configuations, two sources (hafnium and hafnia), and two reactive oxygen pressures. Laser damage thresholds (1064-nm, 10-ns pulses), absorption (at 511 nm), and nodular-defect densities from these coatings are reported. The damage thresholds are observed to increase as the absorption of the coatings decreases. However, no significant increase in damage thresholds are observed with the coatings made from a low nodular-defect density source material (hafnium). Hafnia coatings can be made from hafnium sources that have lower nodular-defect densities, lower absorption, and damage thresholds thatare comparable with coatings made from a conventional hafnia source.

Growth of laser-initiated damage in fused silica at 351 nm

The effective lifetime of optics in the UV is limited both by laser induced damage and the subsequent growth of laser initiated damage sites. We have measured the growth rate of laser induced damage in fused silica in both air and vacuum. The data shows exponential growth in the lateral size of the damage site with shot number above threshold fluence. The concurrent growth in depth follows a linear dependence with shot number. The size of the initial damage influences the threshold for growth; the morphology of the initial site depends strongly on the initiating fluence. We have found only a weak dependence on pulse length for growth rate. Low fluence conditioning in air may delay the onset of growth. Most of the work has been on bare substrates but the presence of a sol-gel AR coating has no significant effect.

Investigation of processes leading to damage growth in optical materials for large-aperture lasers

Damage growth in optical materials used in large-aperture laser systems is an issue of great importance to determine component lifetime and therefore cost of operation. Small size damage sites tend to grow when exposed to subsequent high-power laser irradiation at 355 nm. An understanding of the photophysical processes associated with damage growth is important to devise mitigation techniques. We examine the role of laser-modified material and cracks formed in the crater of damage pits in the damage growth process using fused-silica and deuterated KDP samples. Experimental results indicate that both of the above-mentioned features can initiate plasma formation at fluences as low as 2 J/cm2. The intensity of the recorded plasma emission remains low for fluences up to approximately 5 J/cm2 but rapidly increases thereafter, accompanied by an increase of the size of the damage crater.

Depth profiling of polishing-induced contamination on fused silica surfaces

Laser-induced damage on optical surfaces is often associated with absorbing contaminants introduced by the polishing process. This is particularly the case for UV optics. In the present study, secondary ion mass spectroscopy (SIMS) was used to measure depth profiles of finishing-process contamination on fused silica surfaces. Contaminating detected include the major polishing compound components, Al present largely because of the use of Al2O3 in the final cleaning process, and other metals incorporated during the polishing step or earlier grinding steps. Depth profile data typically showed an exponential decay of contaminant concentration to a depth of 100-200 nm. This depth is consistent with a polishing redeposition layers formed during the chemo-mechanical polishing of fused silica. Peak contaminant levels are typically in the 10-10 pm range, except for Al which often exceeds 1000 ppm.

Photothermal characterization of optical thin film coatings

Photothermal techniques are widely used in thin film characterizations and are particularly useful in studying laser-induced damage in optical coatings. The specific applications include measuring weak absorption, characterizing thermal conductivity, detecting local defects, and monitoring laser-interaction dynamics and determining laser damage thresholds as well as thermal impedance at boundaries of multilayers. We take an overview of the principle of photothermal techniques, the various detection methods, and the progress made during the last decade in applying these techniques to optical thin films. The further potential and limitations of the techniques will also be discussed, with emphasis on {ital in situ} studies of laser interaction with thin films and local defects. {copyright} {ital 1997 Society of Photo-Optical Instrumentation Engineers.}

Characterization of defect geometries in multilayer optical coatings

Laser‐induced damage in optical coatings is generally associated with micrometer‐scale defects. A simple geometric model for nodule‐shaped defects is commonly used to describe defects in optical coatings. No systematic study has been done, however, to prove the applicability of that model to an optical coating deposition process. Not all defects have a classic nodule geometry. The present study uses atomic force microscopy (AFM) and scanning electron microscopy to characterize the topography of coating defects in a HfO2/SiO2 multilayer mirror system. Focused ion‐beam cross sectioning is then used to study the underlying defect structure. This work develops a model for defect shape such that the overall geometry of a coating defect, particularly the seed size and depth, can be inferred from nondestructive evaluation measurements such as AFM. The relative mechanical stabilities of nodular defects can be deduced based on the nodule’s geometry. Auger analysis showed that the seed material that causes nodular ...

Role of defects in laser damage of multilayer coatings

Laser induced damage to optical coatings is generally a localized phenomenon associated with coating defects. The most common of the defect types are the well-known nodule defect. This paper reviews the use of experiments and modeling to understand the formation of these defects and their interaction with laser light. Of particular interest are efforts to identify which defects are most susceptible to laser damage. Also discussed are possible methods for stabilizing these defects (laser conditioning) or preventing their initiation (source stabilization, spatter particle trapping).

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.

Modeling of electric-field enhancement at nodular defects in dielectric mirror coatings

In dielectric multilayer optical coatings, laser induced damage is often associated with micrometers -scale surface defects such as the well known nodule defect. The interaction mechanism of the laser light with the coating defects is not understood, however. Historically, laser damage has been associated with peaks in the standing-wave electric-field distribution within the multilayer films. In the present work we use a finite-difference time-domain electromagnetic modeling code to study the influence of 3-D nodule defects on the E-field distribution. The coating studied is a dielectric multilayer HR consisting of alternating quarter- wave layers of HfO2 and SiO2 at 1.06 micrometers . The nodule is modeled as a parabolic defect initiated at a spherical seed. The modeling results show that E-field enhancements as large as a factor of 4 can be present at the defects. The enhancement shows a complex dependence on the size, depth, and dielectric constant of the seed material. In general, defects initiated by large, shallow seeds produce the largest E-fields. Voids at the nodule boundary influence the E-field distribution, but have a small effect on the peak field.

A comparison of nodular defect seed geometeries from different deposition techniques

A focused ion-beam milling instrument, commonly utilized in the semiconductor industry for failure analysis and IC repair, is capable of cross-sectioning nodular defects. Utilizing the instruments scanning on beam, high-resolution imaging of the seeds that initiate nodular defect growth is possible. In an attempt to understand the origins of these seeds, HfO2/SiO2 and Ta2O5/SiO2 coatings were prepared by a variety of coating vendors and different deposition processes including e-beam, magnetron sputtering, and ion beam sputtering. By studying the shape, depth, and composition of the seed, inferences of its origin can be drawn. The boundaries between the nodule and thin film provide insight into the mechanical stability of the nodule. Significant differences in the seed composition, geometry of nodular growth and mechanical stability of the defects for sputtered versus e-beam coatings are reported. Differences in seed shape were also observed from different coating vendors using e-beam deposition of HfO2/SiO2 coatings.