A Novel Fluorescence Based Method of Assessing Subsurface Damage in Optical Materials
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Abstract
Lapping and polishing are loose abrasive finishing processes that have been used to achieve critical surface parameters in optical materials for centuries. These processes remove material from the surface through a variety of mechanical and chemical interactions. These interactions influence not only the surface of the finished material, but also the subsurface, the region immediately beneath the surface. These processes may induce a damaged layer of cracks, voids and stressed material below the surface. This subsurface damage (SSD) can create optical aberrations due to diffraction, propagate to the surface, and reduce the laser induced damage threshold of the material. It is difficult to detect SSD, as these defects lie beneath the surface. Methods have been developed to detect SSD, but they can have notable limitations regarding sample size and material, preparation time, or they can be destructive in nature. The author tested a non-destructive method for assessing SSD that consisted of tagging the abrasive slurries used in loose abrasive finishing with quantum dots (nano-sized fluorescent particles). Subsequent detection of fluorescence on the processed surface is hypothesized to indicate SSD. Quantum dots present during the lapping process were retained in the glass sample through subsequent polishing and cleaning processes. The quantum dots were successfully imaged by both wide field and confocal fluorescence microscopy techniques. The detected fluorescence highlighted defects that were not observable with optical or interferometric microscopy. Analysis indicates that most dots are firmly embedded in the surface, however examination of confocal fluorescence scans beneath the surface did show incidences of quantum dots at depths up to 10 µm beneath the surface. The incidence of these deep features was less than 20% of the sites examined. Etching of the samples exhibiting fluorescence confirmed the presence of SSD and provided a conservative SSD depth estimate of 10 µm. These etching results confirm the hypothesis that quantum dots can tag SSD. Further testing demonstrated that for quantum dots to be embedded in the surface they must experience the dynamics of the lapping process, and that quantum dots can only tag brittle fracture sites.Quantum dots that were introduced to YAG samples during loose abrasive finishing were only retained on the surface and at levels consistent with simple exposure to quantum dots prior to cleaning, possibly highlighting surface defects that were not apparent with conventional microscopy. Subsequent etching of the YAG samples showed low levels of fracture in the subsurface region, indicating few suitable defects to house the quantum dots. In addition to the research above, an instrument was design and built to measure the axial and torque loads during loose abrasive finishing. Experiments with this measurement head showed expected increases in material removal rate and surface roughness with increased axial load. Results from these tests were also used to corroborate SSD depth estimates from glass samples finished with quantum dot laden slurries.