Lewis B. Whitbourn

Equivalent-circuit formulas for metal grid reflectors at a dielectric boundary

This paper points out a number of errors that have appeared in the literature concerning transmission-line models for metal grid reflectors (strip gratings and meshes) especially in regard to the design of laser mirrors and filter elements for use at submillimeter wavelengths. General formulas are given for the transmittance of lossy grids and for the equivalent circuit impedances to be used in these formulas for strip gratings and meshes at a plane boundary between two lossless dielectrics. The results apply for normal incidence and for wavelengths in both dielectrics greater than the grid period. Limitations of the transmission-line models for meshes at dielectric boundaries are discussed.

Strip gratings at a dielectric interface and application of Babinet’s principle: erratum

R. C. Compton, L. B. Whitbourn, and R. C. McPhedran R. C. McPhedran is with University of Sydney, School of Physics, Sydney NSW 2006, Australia. When this work was done the other authors were with CSIRO Division of Applied Physics, Lindfield, NSW 2070, Australia; R. C. Compton is now with California Institute of Technology, Pasadena, California 91225 and L. B. Whitbourn is with CSIRO Division of Mineral Physics and Mineralogy, P.O Box 218, Lindfield, NSW 2070 Australia. Received 5 August 1986. 0003-6935/86/223974-01

Thin capacitive meshes on a dielectric boundary: theory and experiment

02.00/0.

PHASE-SHIFTS IN TRANSMISSION-LINE MODELS OF THIN PERIODIC METAL GRIDS

A rigorous formulation is used to calculate the transmission properties of a thin, perfectly conducting biperiodic capacitive mesh on a dielectric boundary. The formulation is analogous to the well-known modal method used for inductive meshes, with the modal electric fields replaced by modal currents. Measurements made at submillimeter wavelengths are presented for square capacitive meshes on a crystal quartz substrate (n = 2.1). These measurements are shown to be in good agreement with the theory. The applicability of simple equivalent circuit models is investigated and the variation of the equivalent circuit parameters with the refractive index of the substrate is discussed. A modified expression of Babinets principle is presented which is valid in the nondiffracting region for thin meshes on a dielectric interface.

Mapping surface mineralogy and scattering behavior using backscattered reflectance from a hyperspectral midinfrared airborne CO/sub 2/ laser system (MIRACO/sub 2/LAS)

This paper describes simple formulas for the phase shifts associated with reflection and transmission of electromagnetic waves by a thin periodic metal grid on a plane boundary between two lossless dielectrics, for the case where the wavelengths on both sides of the boundary are greater than the grid period. The phase shifts are expressed in terms of the zero-order intensity reflectance and transmittance of the grid and are correct even when the grid has loss. They correspond exactly to the phase shifts that can be calculated using the well-known equivalent-circuit models for thin metallic grids but are applicable for any thin nondiffracting layer on a dielectric boundary, for normal incidence.

Simple formulae for the transmittance of strip gratings

Airborne, high-spectral resolution, thermal-infrared (TIR) MIRACO/sub 2/LAS reflectance data were evaluated for mapping surface mineralogy and scattering behavior for a variety of semi-arid, geological test sites in Australia. MIRACO/sub 2/LAS is a rapidly tuned, airborne CO/sub 2/ laser system that measures backscattered (bidirectional) reflectance at 100 wavelengths between 9.1 and 11.2 /spl mu/m for 2-m footprints in line profile mode. An operational methodology is described that permits reduction of the raw airborne signal-to-ground reflectance. This ground reflectance has two major properties, namely, wavelength-dependent mineralogical variations and reflection albedo variations related to surface roughness. Comparisons between the airborne MIRACO/sub 2/LAS spectra and laboratory directional hemispherical reflectance (DHR) spectra show the same spectral shapes, though differences in average reflectance (albedo) occur for some types of rocks. The minerals identified using MIRACO/sub 2/LAS include silicates (for example, quartz, microcline, plagiodase, almandine, spessartine, talc, tremolite, and kaolinite) and carbonates (dolomite and magnesite) as well as vegetation (dry and green). Many of the diagnostic spectral features that allow identification of these materials are narrow (<0.2 /spl mu/m), making them difficult to detect with broadband TIR systems, like the airborne TIMS and satelliteborne ASTER. Based on an empirical relationship between the minimum and maximum reflectance established using laboratory DHR spectra, a method is proposed that allows the use of MIRACO/sub 2/LAS data to identify surfaces that are characterized by Lambertian or specular scattering. The MIRACO/sub 2/LAS results show that Lambertian-type scatterers include soils and many types of isotropic rocks.

Strip gratings on dielectric substrates as output couplers for submillimeter lasers

We present simple but accurate formulae for the transmittance through strip gratings of infinitesimal thickness. The formulae are valid irrespective of the angle of incidence of radiation on the gratings and can take into account the effects of a dielectric substrate. We demonstrate the range of applicability of the formulae by comparing their predictions with the results of rigorous calculations.

Synergy of active and passive airborne thermal infrared systems for surface compositional mapping

This paper describes the use and advantages of metallic strip gratings on dielectric substrates as output couplers for both optically pumped and discharge-excited submillimeter lasers. Formulas are presented for the calculation of transmittance and loss of such couplers, taking account of loss in the strip grating as well as loss and multiple reflections in the substrate. Included are expressions for the phase shifts on reflection and transmission by an arbitrary lossy grid on a plane boundary between two dielectrics according to a transmission-line model that is applicable for wavelengths in both dielectrics longer than the grid period. In relation to these phase shifts attention is drawn to an important sign convention. The theory is shown to agree well with measured transmittance of a typical device between 500 and 1600 GHz as well as spot measurements at 891 (337-microm HCN laser), 1540, and 1578 GHz (195- and 190-microm DCN laser). Finally, the theory is used to design a low-loss coupler for the low-gain 119-microm line of discharge excited H2O.

Equivalent thin film of a periodic metal grid

NASA thermal infrared multispectral scanner (TIMS) and Commonwealth Scientific Industrial Research Organisation mid-infrared airborne CO2 laser spectrometer (MIRACO2LAS) data were acquired over the Mount Fitton area, South Australia, in order to evaluate their combined use for geological mapping and mineral exploration. TIMS is a passive, imaging system with six spectral bands in the thermal infrared wavelength region (8–12 μm), whereas MIRACO2LAS is an active, profiling system with ∼100 spectral bands in the 9–11 μm spectral range. The TIMS and CO2 laser data (emissivity variations for TIMS and apparent reflectance for MIRACO2LAS) were processed to enhance spectral information related to the surface composition. This spectral information was compared with existing geological maps and field emissivity spectra. Known geological units were well discriminated in the TIMS imagery, including a range of quartz-rich and carbonate-rich sedimentary units, as well as several previously unmapped areas of alteration in the carbonate rocks. However, the broadband spectral resolution of TIMS did not allow identification of discrete mineral constituents. In contrast, the high spectral resolution MIRACO2LAS data provided diagnostic spectral information about a range of minerals present including quartz, dolomite, talc, and tremolite, albeit, along discrete profiles. The widths of some of these diagnostic spectral features were less than 0.2 μm wide, which is half the resolution of the TIMS band passes (0.4 μm). The MIRACO2LAS spectra closely matched the shape, location, and depth of spectra of field samples measured by both a field emission spectrometer and a laboratory laser spectrometer. Pure mineral spectra measured by the (bidirectional) laboratory laser spectrometer also closely matched those measured by a conventional laboratory spectrometer measuring directional hemispherical reflectance. These results indicate that future remote thermal infrared systems designed for improved geological mapping and mineral exploration should incorporate both an imager for mapping lithological units and a high spectral resolution profiler for identifying dominant mineral constituents. Recently, a hyperspectral imaging thermal infrared instrument has been developed termed Spatially Enhanced Broadband Array Spectrograph System (SEBASS) [Hackwell and Warren, 1997]. This instrument should allow spectral identification of an entire scene [Gillespie et al., 1997], although the swath width is currently limited to 128 pixels as opposed to the 752 pixels of TIMS after panoramic correction.

Carbon dioxide laser tuning through 110 lines in 3 ms for airborne remote sensing

This paper establishes that any thin periodic metal grid on a plane boundary between two semi-infinite media, irradiated normally at any specified wavelength larger than its period, can be represented by an equivalent thin film. Explicit formulas for the parameters of this film are derived, and a numerical example of the equivalence is given.