David D. Allred received Alumni Professorship Award

Abstract: Near-field photodetection optical microscopy (NPOM) is a fundamentally new approach to near-field optical microscopy. This scanning probe technique uses a nanometer-scale photodiode detector which absorbs optical power directly as it is scanned in the near held of an illuminated sample surface. We have applied NPOM to measure the visible absorption spectrum of. dye molecules embedded in a single 300 nm polystyrene sphere. The near-held absorption spectrum is obtained by measuring the NPOM probe photocurrent while the wavelength of the illumination pump beam is scanned from 450 to 800 nm. Peaks-are identified at 567, 608, and 657 nn in the near-field spectrum of the single-dyed polystyrene sphere. These peak. positions are in good agreement with far-field absorption measurements performed on many dyed polystyrene spheres. (C) 1996 American Institute of Physics.

Abstract: Near-field photodetection optical microscopy (NPOM) is a fundamentally new approach to near-field optical microscopy. This scanning-probe technique uses a nanometer-scale photodiode detector as a near-field optical probe. We have fabricated probes for NPOM that have optically sensitive areas as small as 100 nm x 100 nm. These new NPOM probes have been employed to image light transmitted through holes in an aluminum film. Near-surface optical interference is observed near defects and edges of the aluminum film. The optical edge response is shown to be of the order of 100 nm. (C) 1996 Optical Society of America

Abstract: A submicrometer photodiode probe with a sub‐50 nanometer tip radius has been developed for optical surface characterization on a nanometer scale. The nanoprobe is built to detect subwavelength optical intensity variations in the near field of an illuminated surface. The probe consists of an Al–Si Schottky diode constructed near the end of a micromachined pyramidal silicon tip. The process for batch fabrication of the nanoprobes is described. Electrical and optical characterization measurements of the nanoprobe are presented. The diode has a submicrometer optically sensitive area with a 150 fW sensitivity.

Abstract:

A new deposition method developed by Goddard Space Flight Center fluorinates Al mirrors with XeF2 followed by a LiF coating to create what they term Al+XeLiF. This in-situ, room temperature process produces mirrors with high reflectivity in a broad spectral range, from the FUV to the IR, and is reported to be stable in relative humidities of 30% and lower. These mirrors are envisioned for missions requiring sensitivity down to 100 nm wavelength such as the habitable worlds observatory. Because most mission integration and testing campaigns require prolonged exposure to lab environments, and launch sites experience high relative humidities (RH) on average, some at 80% RH. We investigate Al+XeLiF stability in a wider range of temperatures and humidities along with employing additional characterization techniques including atomic force microscopy and x-ray photoelectron spectroscopy. We found that Al+XeLiF is stable in environments up to 82%RH when kept at cooler temperatures (3°C and 21°C). However, this material is unstable when stored at 60°C, experiencing roughening and loss in reflection from resulting Al surface plasmon excitation.

Abstract:

Astronomical instrumentation for measurements in the Far Ultraviolet (FUV, 90−200 nm) have historically considered aluminum (Al) thin film mirrors due to this material high reflectance over this wavelength range. However, the native aluminum oxide layer that forms on Al upon exposure to the atmosphere is strongly absorbing in this wavelength range, requiring that the films be protected with a dielectric that inhibits oxidation. Typically, magnesium fluoride (MgF₂) or lithium fluoride (LiF) coatings are used as protective layers, but each has shortcomings. For example, MgF₂ has an absorption cutoff at 115 nm that reduces performance below this wavelength, which is a critical part of the FUV spectrum for observational astrophysics. The use of LiF as a protection for Al provides a lower absorption cutoff at 100 nm, but it is hygroscopic and thus susceptible to degradation in humid conditions. Our team at GSFC has developed a new reactive Physical Vapor Deposition (rPVD) process that consists of a fluorination process with XeF₂ gas combined with our traditional PVD process. We have found that this new rPVD process produces Al+XeF₂+LiF (XeLiF) and Al+XeF₂+MgF₂ (XeMgF₂) mirror coatings with unprecedented reflectance. In addition, the rPVD process seems to produce much more environmentally stable coatings (when compared to the conventional process without the XeF₂ fluorination). We report on IR/Vis/UV reflectance of XeLiF and XeMgF₂ mirrors. The surface roughness as well as the FUV reflectance measured over a period of 8 months for a XeLiF sample with a relatively thin (≃ 30 nm) Al layer are also reported. We have also been investigating the compatibility of this rPVD coating process for potential efficiency enhancements of Si-based gratings. Since it is known that the XeF₂ vapor is a strong Si etchant, we are investigating if the native SiO₂ layer on Si is sufficient to protect the groove profile of E-beam-ruled Si gratings from degradation. Preliminary results indicate that the native SiO₂ layer is an effective barrier against etching of Si by XeF₂.