Schlieren optics for leak detection

The purpose of this research was to develop an optical method of leak detection. Various modifications of schlieren optics were explored with initial emphasis on leak detection of the plumbing within the orbital maneuvering system of the space shuttle (OMS pod). The schlieren scheme envisioned for OMS pod leak detection was that of a high contrast pattern on flexible reflecting material imaged onto a negative of the same pattern. We find that the OMS pod geometry constrains the characteristic length scale of the pattern to the order of 0.001 inch. Our experiments suggest that optical modulation transfer efficiency will be very low for such patterns, which will limit the sensitivity of the technique. Optical elements which allow a negative of the scene to be reversibly recorded using light from the scene itself were explored for their potential in adaptive single-ended schlieren systems. Elements studied include photochromic glass, bacteriorhodopsin, and a transmissive liquid crystal display. The dynamics of writing and reading patterns were studied using intensity profiles from recorded images. Schlieren detection of index gradients in air was demonstrated

Micro electro mechanical cantilever with electrostatically controlled tip contact

A micro-electro-mechanical system (MEMS) cantilever that lifts from the surface by electrostatic force is described. The design is composed of three conductors: a fixed buried plate, a fixed surface plate, and a moveable cantilever. All have the same square shape and are arranged parallel in a vertical stack with aligned edges. The surface plate and cantilever are biased at the same potential, and the buried plate is oppositely biased. Theoretical analysis based on values of position-dependent coefficients of capacitance and electrostatic induction from finite element method demonstrates the sign of the force on the cantilever and determines its magnitude. Video microscopy and electrical measurements demonstrate the electrostatic lifting of the cantilever in a fabricated MEMS device. The vertical displacement of the cantilever is quantified from changes in optical interference fringes, and the displacement magnitude agrees with expectations based on estimated strengths of upward electrostatic force and downward elastic restoring force

Optical Salisbury screen with design-tunable resonant absorption bands

A thin-film selective absorber at visible and near infra-red wavelengths is demonstrated. The structure consists of an optically thick layer of gold, a SiO2 dielectric spacer and a partially transparent gold film on top. The optical cavity so formed traps and absorbs light at a resonance wavelength determined by the film thicknesses. Observed fundamental-resonance absorption strengths are in the range 93%-97%. The absorption red-shifts and broadens as the thickness of the top gold layer is decreased with little change in absorption strength. Thus, strong absorption with design-tunable wavelength and width is achieved easily by unstructured blanket depositions. Observed angle-dependent spectra agree well with the recent three-layer analytical model of Shu et al. [Opt. Express 21, 25307 (2013)], if effective medium approximation is used to calculate the permittivity of the top gold film when it becomes discontinuous at the lowest thicknesses

Surface and grain-boundary scattering in nanometric Cu films

We report a quantitative analysis of both surface and grain-boundary scattering in Cu thin films with independent variation in film thickness (27 to 158 nm) and grain size (35 to 425 nm) in samples prepared by subambient temperature film deposition followed by annealing. Film resistivities of carefully characterized samples were measured at both room temperature and at 4.2 K and were compared with physical models that include the effects of surface and grain-boundary scattering. Grain-boundary scattering is found to provide the strongest contribution to the resistivity increase. However, a weaker, but significant, role is observed for surface scattering. We find that the data are best fit when the Mayadas and Shatzkes\u27 model of grain-boundary scattering and the Fuchs and Sondheimer\u27s model of surface scattering resistivity contributions are combined using Matthiessen\u27s rule (simple addition of resistivities). This finding implies that grain-boundary scattering preserves the component of electron momentum parallel to the grain-boundary plane. Using Matthiessen\u27s rule, we find our data are well described by a grain-boundary reflection coefficient of 0.43 and a surface specularity coefficient of 0.52. This analysis finds a significantly lower contribution from surface scattering than has been reported in previous works and we attribute this difference to the careful quantitative microstructural characterization performed on our samples. The effects of surface roughness, impurities, voids, and interactions between surface and grain-boundary scattering are also examined and their importance is evaluated

Stress Analysis of Free-Standing Silicon Oxide Films Using Optical Interference

ABSTRACT We report a method for stress measurement and analysis in silicon oxide thin films using optical interference. Effects of design and fabrication on stress have been studied by fabricating submicron-thick slabs of oxide anchored at one end and extending over a reflective surface. Optical interference occurs between reflections from the surface and the oxide slab, giving rise to light and dark fringes that may be imaged with a microscope. Analysis of the interference pattern at different wavelengths gives the radius of curvature and means of stress mapping. The accuracy exceeds non-interferometric profilometry using optical or confocal microscopes, and it can be more quantitative than scanning electron microscopy. This nondestructive profilometry method can aid the stress optimization of silicon oxide or other transparent thin films to achieve specific mechanical characteristics in MEMS devices

Infrared surface polaritons on bismuth

Optical constants for evaporated bismuth (Bi) films were measured by ellipsometry and compared with those published for single crystal and melt-cast polycrystalline Bi in the wavelength range of 1 to 40 mu m. The bulk plasma frequency omega(p) and high-frequency limit to the permittivity epsilon(infinity) were determined from the long-wave portion of the permittivity spectrum, taking previously published values for the relaxation time tau and effective mass m*. This part of the complex permittivity spectrum was confirmed by comparing calculated and measured reflectivity spectra in the far-infrared. Properties of surface polaritons (SPs) in the long-wave infrared were calculated to evaluate the potential of Bi for applications in infrared plasmonics. Measured excitation resonances for SPs on Bi lamellar gratings agree well with calculated resonance spectra based on grating geometry and complex permittivity

Infrared surface plasmons on heavily doped silicon

Conductors with infrared plasma frequencies are potentially useful hosts of surface plasmon polaritons (SPP) with sub-wavelength mode confinement for sensing applications. A challenge is to identify such a conductor that also has sharp SPP excitation resonances and the capability to be functionalized for biosensor applications. In this paper we present experimental and theoretical investigations of IR SPPs on doped silicon and their excitation resonances on doped-silicon gratings. The measured complex permittivity spectra for p-type silicon with carrier concentration 6 x 10(19) and 1 x 10(20) cm(-3) show that these materials should support SPPs beyond 11 and 6 mu m wavelengths, respectively. The permittivity spectra were used to calculate SPP mode heights above the silicon surface and SPP propagation lengths. Reasonable merit criteria applied to these quantities suggest that only the heaviest doped material has sensor potential, and then mainly within the wavelength range 6 to 10 mu m. Photon-to-plasmon coupling resonances, a necessary condition for sensing, were demonstrated near 10 mu m wavelength for this material. The shape and position of these resonances agree well with simple analytic calculations based on the theory of Hessel and Oliner (1965)

Finesse of silicon-based terahertz Fabry-Perot spectrometer

ABSTRACT This paper considers factors that affect achievable finesse for a recently demonstrated silicon-based scanning FabryPerot transmission filter at millimeter and sub-millimeter wavelengths. The mirrors are formed by alternating quarterwave optical thicknesses of silicon and air in the usual Bragg configuration. Fundamental loss by lattice and free carrier absorption are considered. Technological factors such as surface roughness, bowing, and misalignment are considered for various proposed manufacturing schemes

Platinum germanides for mid- and long-wave infrared plasmonics

Platinum germanides (PtGe) were investigated for infrared plasmonic applications. Layers of Pt and Ge were deposited and annealed. X-ray diffraction identified PtGe2 and Pt2Ge3 phases, and x-ray photoelectron spectroscopy determined vertical atomic composition profiles for the films. Complex permittivity spectra were measured by ellipsometry over the 2 to 15 mu m wavelength range. Surface plasmon polariton (SPP) characteristics such as propagation length and field penetration depth were calculated. Photon-to-SPP couplers in the form of 1D lamellar gratings were fabricated and characterized in the range 9 - 10.5 mu m via wavelength-dependent specular reflection spectra for multiple angles of incidence. The observed resonances compare well with calculated spectra for SPP excitation on PtGe2. Platinum germanides are CMOS compatible and may serve as SPP hosts for on-chip mid-IR plasmonic components with tighter field confinement than noble-metal hosts

The low‐degree shape of Mercury

The shape of Mercury, particularly when combined with its geoid, provides clues to the planet's internal structure, thermal evolution, and rotational history. Elevation measurements of the northern hemisphere acquired by the Mercury Laser Altimeter on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft, combined with 378 occultations of radio signals from the spacecraft in the planet's southern hemisphere, reveal the low‐degree shape of Mercury. Mercury's mean radius is 2439.36 ± 0.02 km, and there is a 0.14 km offset between the planet's centers of mass and figure. Mercury is oblate, with a polar radius 1.65 km less than the mean equatorial radius. The difference between the semimajor and semiminor equatorial axes is 1.25 km, with the long axis oriented 15° west of Mercury's dynamically defined principal axis. Mercury's geoid is also oblate and elongated, but it deviates from a sphere by a factor of 10 less than Mercury's shape, implying compensation of elevation variations on a global scale