The electromagnetic response of an arbitrary array of multiple slits perforated on a metallic film and filled with different slit dielectric materials is studied in an analytical way. As a specific example, we consider a triple-slit structure perforated on a gold film, where the middle slit is used for the surface-plasmon excitation by a narrow Gaussian beam while the other two side slits are used for the detection of a transmitted surface-plasmon wave propagated from the middle opaque slit either at a particular wavelength or at double that wavelength, respectively. For this particular structure, we show that only one of the two side observation slits can be in a passing state for a particular wavelength, but the other blocked slit will change to a passing state at double that wavelength with a specific design for the slit depth, silt dielectric and inter-slit distance in the deep sub-wavelength regime. In this sense, we create a surface-plasmon mediated light diffraction in the near-filed regime.
For the scattering of an incident plane electromagnetic wave by a slotted metallic film, the previous analytical
calculation for a single slot is generalized into a model for an arbitrary linear array of slots with variable slot
width, slot separation and slot dielectric material. The advantage as well as the effectiveness of the generalized
model presented in this work are best described by enabling calculation of a continuous spatial distribution of an
electromagnetic field by inverting a small discrete coefficient matrix spanned by both the slot index and the sloteigenmode
index for a set of linear equations. In comparisons with well-known plane-wave and finite-difference
time-domain methods, inverting a large matrix, in wave number space for the former case and in real space at
each time step for the latter case, can be avoided to greatly speed up numerical calculations. In addition, based
on a partial-domain method, the formalism presented here can be employed to treat a composite surface (e.g., a
slotted metal film with different dielectric materials in the slots).
Numerical results for the spatial distributions of the light transmitted through metallic planar lenses composed
of symmetric nanogroove arrays on the surfaces of a gold film deposited on a dielectric substrate are presented
and explained. Both the near and far-field distributions of the intensity of light transmitted through such films,
which are modeled by two aligned and reversed one-dimensional surface profile functions, are calculated by
the use of a Green's function formalism. The focusing action obtained for different groove-width variations is
investigated thoroughly. Results for an optimal transverse focus based on a quadratic variation of groove width
across the array are also obtained, in addition to the effect of groove shape on the sharpness of planar lens
focusing. Meanwhile, a significant dependence of the focal length on the wavelength of light incident from the
air side through the gold film into a dielectric substrate is found for this detector configuration.
Under a contract with the Air Force Phillips Laboratory, two proposed methods for enhancing spectral
content of hyperspectral remote sensing data are discussed. The first method would use diffuse sky light in
which specular solar reflection from the 1st surface is absent. Characteristic material emission spectra would
be observed from a much wider range of view angles than for a sunlit scene. In diffuse light, the observed
polarization is reduced in comparison to the sun lit case, because the non-polarized volume radiance is
increased relative to the 1st surface scatter. Hence, diffraction grating artifacts due to polarized inputs are
reduced. In a second method, a polarizer positioned in front of a hyperspectral imager, is shown to enhance
spectral contrast by reducing polarized 1st surface reflections in forward scatter directions. A polarizer of
known orientation improves diffraction grating performance because changes in the polarization dependent
grating efficiency can be accounted for through calibration. In support of these methods, polarimetric
bidirectional reflectance distribution function (BRDF) data from the visible grating spectral polarimeter
(VGSP)1 with wavelength range (375nm to 900nm) is presented. This data indicates that angular scatter or
the BRDF from many outdoor materials may be decomposed into a polarimetric 1st surface single scatter
component with little spectral variation and a diffuse or multiple scattered component having a significant
spectral dependence.
Recent developments for long-wave infrared (LWIR) imaging polarimeters include incorporating a microgrid polarizer array onto the focal plane array. Inherent advantages over other classes of polarimeters include rugged packaging, inherent alignment of the optomechanical system, and temporal synchronization that facilitates instantaneous acquisition of both thermal and polarimetric information. On the other hand, the pixel-to-pixel instantaneous field-of-view error that is inherent in the microgrid strategy leads to false polarization signatures. Because of this error, residual pixel-to-pixel variations in the gain-corrected responsivity, the noise-equivalent input, and variations in the pixel-to-pixel micropolarizer performance are extremely important. The degree of linear polarization is highly sensitive to these parameters and is consequently used as a metric to explore instrument sensitivities. We explore the unpolarized calibration issues associated with this class of LWIR polarimeters and discuss the resulting false polarization signature for thermally flat test scenes.
In order to understand the phenomenology of optimum data acquisition and analysis and to
develop an understanding of capabilities, field measurements of multiband, polarimetric data can
substantially assist in developing a methodology to collect and to exploit feature signatures.
In 1999, Duggin showed that images obtained with an 8-bit camera used as a polarimeter could
yield additional information to that contained in a radiometric (S0) image. It should be noted that
Walraven and Curran had performed some very fine experiments almost two decades earlier,
using photographic film, and North performed careful polarimetric measurements of the
skydome using a four-lens polarimetric film camera and convex mirror in 1997. There have been
a number of papers dealing with polarimetric field measurements since that time. Recently,
commercial color cameras have become available that have 12-bit depth per channel. Here, we
perform radiometric and chromatic calibrations and examine the possible use of a Nikon D200
10.2 mega pixel, 3 channel, 12-bit per channel camera fitted with a zoom lens as a potential field
imaging polarimeter. We show that there are still difficulties in using off-the-shelf technology for
field applications, but list some reasons why we need to address these challenges, in order to
understand the phenomenology of data collection and analysis metrics for multiple data streams.
Recent developments for Long Wave InfraRed (LWIR) imaging polarimeters include incorporating a microgrid polarizer array onto the focal plane array (FPA). Inherent advantages over typical polarimeters include packaging and instantaneous acquisition of thermal and polarimetric information. This allows for real time video of thermal and polarimetric products. The microgrid approach has inherent polarization measurement error due to the spatial sampling of a non-uniform scene, residual pixel to pixel variations in the gain corrected responsivity and in the noise equivalent input (NEI), and variations in the pixel to pixel micro-polarizer performance. The Degree of Linear Polarization (DoLP) is highly sensitive to these parameters and is consequently used as a metric to explore instrument sensitivities. Image processing and fusion techniques are used to take advantage of the inherent thermal and polarimetric sensing capability of this FPA, providing additional scene information in real time. Optimal operating conditions are employed to improve FPA uniformity and sensitivity. Data from two DRS Infrared Technologies, L.P. (DRS) microgrid polarizer HgCdTe FPAs are presented. One FPA resides in a liquid nitrogen (LN2) pour filled dewar with a 80°K nominal operating temperature. The other FPA resides in a cryogenic (cryo) dewar with a 60° K nominal operating temperature.
Including polarization signatures of material samples in passive sensing may enhance target detection capabilities. To obtain more information on this potential improvement, a simulation is being developed to aid in interpreting IR polarization measurements in a complex environment. The simulation accounts for the background, or incident illumination, and the scattering and emission from the target into the sensor. MODTRAN, in combination with a dipole approximation to singly scattered radiance, is used to polarimetrically model the background, or sky conditions. The scattering and emission from rough surfaces are calculated using an energy conserving polarimetric Torrance and Sparrow BRDF model. The simulation can be used to examine the surface properties of materials in a laboratory environment, to investigate IR polarization signatures in the field, or a complex environment, and to predict trends in LWIR polarization data. In this paper we discuss the simulation architecture, the process for determining and roughness as a function of wavelength, which involves making polarization measurements of flat glass plates at various angles and temperatures in the laboratory at Kirtland AF Base, and the comparison of the simulation with field dat taken at Elgin Air Force Base. The later process entails using the extrapolated index of refraction and surface roughness, and a polarimetric incident sky dome generated by MODTRAN. We also present some parametric studies in which the sky condition, the sky temperature and the sensor declination angle were all varied.
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