| Abstract
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2909 |
| Title |
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Testing Quantum Dots as a Means of Assessing Subsurface Damage in Polished Glass |
| Category |
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Grinding, Polishing, and Lapping |
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| Content |
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Fabrication of an optical component through grinding and lapping can induce a layer of defects and stressed material that extends beneath the surface. This subsurface damage (SSD) is then obscured by the smoothing of the surface during polishing and undetectable through interferometry or contact probing (profilometer or atomic force microscope). These defects can cause a variety of problems such as propagating to the surface if subject to mechanical or thermal stresses, introducing optical aberrations in light passing through and serving as a site of absorption that lowers the laser induced damage threshold (LIDT). SSD is removed by polishing a component long enough to polish through the defect layer, but excessive polishing is time consuming and expensive.
Traditionally, SSD has been assessed by etching away the topmost layer of smoothed material to reveal the underlying damage. Other techniques polish a known geometry into the surface (a dimple or a taper) then etch the surface lightly to expose the defects and use measurements of the damaged region to calculate the depth of damage. These techniques however are destructive in nature and limited by the optical resolution of the microscope to defects 0.25 microns or larger. Newer techniques such a photothermal microscopy, ion channeling and Raman spectroscopy have been employed to assess SSD, but require a significant investment in equipment and expertise. As such, a production means of assessing SSD that is non-destructive would be of use to industry.
We have tested a means of assessing SSD that consists of adding quantum dots (semiconductor crystals with diameters of 7.8 nm) to the abrasive slurries used in lapping and polishing glass samples. These quantum dots are sufficiently small to be able to travel into the defects if they are open to the surface during the finishing process, which was designed to reliably induce SSD. These samples were imaged on a confocal fluorescence microscope where the fluorescence of various focal planes at and beneath the surface was measured. By utilizing the tunable emission spectra of the quantum dots as well as the ability of the confocal microscope to reject fluorescence from adjacent points and out of focus planes, 3-D maps of fluorescence associated with the quantum dots were built. The presence of quantum dots was taken to be a sign of defects on the surface or in the subsurface.
Samples which were lapped and polished in the presence of quantum dots prior to cleaning showed a much higher (10 times) incidence of fluorescence than samples which were simply immersed in quantum dots (for a comparable time) prior to cleaning. The peak fluorescence in these samples occurred within 2 microns of the surface. A subsequent pitch polishing step (without quantum dots) showed that samples where the SSD had been polished through (as verified by etching) did not retain quantum dots, while samples only briefly pitch polished did still register fluorescence on the confocal microscope. |
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