Large Area Conformal Infrared Frequency Selective Surfaces

Frequency selective surfaces (FSS) were originally developed for electromagnetic filtering applications at microwave frequencies. Electron-beam lithography has enabled the extension of FSS to infrared frequencies; however, these techniques create sample sizes that are seldom appropriate for real world applications due to the size and rigidity of the substrate. A new method of fabricating large area conformal infrared FSS is introduced, which involves releasing miniature FSS arrays from a substrate for implementation in a coating. A selective etching process is proposed and executed to create FSS particles from crossed-dipole and square-loop FSS arrays. When the fill-factor of the particles in the measurement area is accounted for, the spectral properties of the FSS flakes are similar to the full array from which they were created. As a step toward scalability of the process, a square-patch design is presented and formed into FSS flakes with geometry within the capability of ultraviolet optical lithography. Square-loop infrared FSS designs are investigated both in quasi-infinite arrays and in truncated sub-arrays. First, scattering-scanning near-field optical microscopy (s-SNOM) is introduced as a characterization method for square-loop arrays, and the near-field amplitude and phase results are discussed in terms of the resonant behavior observed in far-field measurements. Since the creation of FSS particles toward a large area coating inherently truncates the arrays, array truncation effects are investigated for square-loop arrays both in the near- and far-field. As an extension of the truncation study, small geometric changes in the design of square-loop arrays are introduced as a method to tune the resonant far-field wavelength back to that of the quasi-infinite arrays

Near- and far-field measurements of phase-ramped frequency selective surfaces at infrared wavelengths

Near- and far-field measurements of phase-ramped loop and patch structures are presented and compared to simulations. The far-field deflection measurements show that the phase-ramped structures can deflect a beam away from specular reflection, consistent with simulations. Scattering scanning near-field optical microscopy of the elements comprising the phase ramped structures reveals part of the underlying near-field phase contribution that dictates the far-field deflection, which correlates with the far-field phase behavior that was expected. These measurements provide insight into the resonances, coupling, and spatial phase variation among phase-ramped frequency selective surface (FSS) elements, which are important for the performance of FSS reflectarrays

Near-field mapping of dipole nano-antenna-coupled bolometers

The near-field characteristics of single, double, and arrays of connected dipole nano-antennas coupled to bolometers were studied by infrared scattering scanning near-field optical microscopy (s-SNOM) and analyzed by numerical simulations. Results were consistent with classical antenna theory showing the expected pi phase difference across the terminals of the dipoles. However, according to the observed differences between the measurements and simulations, the symmetry of the amplitude signal appeared to be sensitive with respect to the position of the bolometric element relative to the dipoles. The effect of the position of the bolometer on the associated near-field distribution suggests an influence on the coupling and efficiency of energy transfer into these detectors, which could be important for determining tolerances in the fabrication of such devices. These results show how near-field measurements in general can provide critical information to guide the design of nano-antennas, nano-antenna-phased arrays, and integrated photonic devices

Near- And Far-Field Investigation Of Dark And Bright Higher Order Resonances In Square Loop Elements At Mid-Infrared Wavelengths

Three different size gold square loop structures were fabricated as arrays on ZnS over a ground plane and designed to have absorptive fundamental, second order, and third order resonances at a wavelength of 10.6 ìm and 60° off-normal. The angular dependent farfield spectral absorptivity was investigated over the mid-infrared for each size loop array. It was found that the second order modes were dark at normal incidence, but became excited at off-normal incidence, which is consistent with previous work for similar geometry structures. Furthermore, near-field measurements and simulations at a wavelength of 10.6 ìm and 60° off-normal showed that the second order mode (quadrupolar) of the medium size loop yielded a near-field response similar in magnitude to the fundamental mode (dipolar) of the small size loop, which can be important for sensing related applications where both strong near-field enhancement and more uniform or less localized field is beneficial

Optical Dielectric Function Of Silver

The dielectric function of silver is a fundamental quantity related to its electronic structure and describes its optical properties. However, results published over the past six decades are in part inconsistent and exhibit significant discrepancies. The measurement is experimentally challenging with the values of dielectric function spanning over five orders of magnitude from the mid-infrared to the visible/ultraviolet spectral range. Using broadband spectroscopic ellipsometry, we determine the complex-valued dielectric function of evaporated and template stripped polycrystalline silver films from 0.05 eV (λ=25μm) to 4.14 eV (λ=300 nm) with a statistical uncertainty of less than 1%. From Drude analysis of the 0.1-3 eV range, values of the plasma frequency ωp=8.9±0.2 eV, dielectric function at infinite frequency ε =5±2, and relaxation time τ=1/Γ=17±3 fs are obtained, with the absolute uncertainties estimated from systematic errors and experimental repeatability. Further analysis based on the extended Drude model reveals an increase in τ with decreasing frequency in agreement with Fermi liquid theory, and extrapolates to τ≃22 fs for zero frequency. A deviation from simple Fermi liquid behavior is suggested at energies below 0.1 eV (λ=12μm) with the onset of a further increase in τ connecting to the DC value from transport measurements of ∼40 fs. The results are consistent with a wide range of optical and plasmonic experiments throughout the infrared and visible/ultraviolet spectral range. However, due to the polycrystalline nature of our sample, the values measured are not likely reaching the intrinsic limit of silver. The influence of grain boundaries, defect scattering, and surface oxidation is discussed. The results are compared with our previous measurements of the dielectric function of gold [Olmon, Phys. Rev. B 86, 235147 (2012)]PRBMDO1098-012110.1103/PhysRevB.86.235147

Directional Thermal Emission From A Leaky-Wave Frequency Selective Surface

We design, fabricate, and characterize a Frequency Selective Surface (FSS) with directional thermal emission and absorption for long-wave infrared wavelengths (LWIR). The FSS consists of an array of patch antennas connected by microstrips, the ensemble of which supports leaky-wave type modes with forward and backward propagating branches. The branches are designed to intersect at 9.8 μm, and have a broadside beam with 20° FWHM at this wavelength. The absorption along these branches is near-unity. Measurement of the hemispherical directional reflectometer (HDR) shows good agreement with simulation. The ability to control the spectral and directional emittance/absortpance profiles of surfaces has significant applications for radiation heat transfer and sensing. © 2013 SPIE