The optical edges of a starshade define its outermost perimeter. During astronomical measurements, the edges are exposed to sunlight resulting in glint seen by the telescope. Clean, sharply etched edges are capable of meeting the stringent solar glint scatter requirement. We report on the increased glint that arises from particulate contamination clinging to the edges. We measure the relationship between surface contamination and edge contamination and compare to a simple edge contaminant population model where the edge acts like a line drawn across the continuous surface. We correlate the level of edge contamination to the degree of increased scatter and derive an on-orbit contamination requirement that is compatible with the detection of exo-Earths.
Tabletop extreme ultraviolet (EUV) sources based on high harmonic generation (HHG) have been used as a powerful tool for probing magnetism. Obtaining magnetic information via magneto-optical contrast often requires the energy of the light to be tuned to magnetic resonance energies of the magnetic element present in the material; therefore, it is essential to calibrate the HHG spectrum to well defined absorption energies of materials. We have designed and assembled a HHG based EUV source for studying transition metal magnetic materials at their resonant M-absorption edges (35-75 eV of photon energy). One material of interest is iron, for which the iron M2,3 edge is 52.7 eV (23.5 nm wavelength) according to CXRO. We prepared and characterized a thin sample of iron for absorption spectroscopy and calibration of the absorption edge with beamline 6.3.2 at the Advance Light Source (ALS) in Lawrence Berkeley National Laboratory. This well characterized sample was capped with gold to prevent oxidation. From these measurements we extracted the absorption part of the index of refraction β spectrally and confirmed that the absorption edge of iron is 52.7 eV. With this information, we can better calibrate the HHG spectrum of our tabletop EUV source. Calibration of the HHG spectrum was achieved using model fitting the HHG spectrum using the grating equation and law of cosines while taking account into the results of the ALS data. We have determined that driving wavelength of the HHG process to be 773 nm. We also conclude that the chirp of the driving laser pulse can cause an energy shift to a HHG spectrum.
First Contact (FC) Polymer™, developed by Photonic Cleaning Technologies, is used to clean and protect surfaces from contamination. The polymer creates a peelable coating that renders the surface clean while not leaving visible residues. To investigate the effectiveness of FC at the subnanometer level, we used variable-angle, spectroscopic ellipsometry (VASE) to measure sample top-layer thickness after repeated application/storage/removal cycles of standard (red) FC with three sample sets (CVD Si3N4 on Si, bare Si, and SiO2 on Si). The samples were measured via VASE after every FC removal to understand contaminant thickness changes with “peel-off” count. Control samples were also measured at each iteration. Ellipsometric analysis revealed FC removed, during the first peel-off, impurity from the surface of samples treated with impure isopropyl alcohol. Linear regressions and t-tests comparing samples with and without FC were employed for evaluating changes with peel-off counts. There is evidence for the very slight build-up of material which is not removed by iterative FC application/removal cycles on these samples. It is slight, <0.1 nm after 17 iterations, in the case of native oxide on Si.
Lithium fluoride (LiF) is difficult to work with because of its hygroscopic nature (it pulls water out of air). The stability limits of LiF thin films and the nature of their failure when exposed to humid air are poorly understood. We show that LiF films undergo irreversible changes in optical properties and microstructure as determined by ellipsometry and SEM when exposed to dew points greater than 6 C. On the other hand, samples stored at a dew point of -22 C (4% RH at room temperature), showed only small changes in ellipsometric parameters. The ones stored at intermediate humidity 6 C (21% RH at room temperature) showed larger changes in ellipsometric parameters. SEM shows that deliquescence as well as efflorescence is important in LiF thin films. In situ spectroscopic ellipsometric measurements using a controlled variable humidity environment illuminates the changes in LiF thin films moving from moisture absorption to complete deliquescence.
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