Ben A. Munk

A low-profile broadband phased array antenna

A fundamentally different approach to broadband array design is introduced and validated with measured data. This design approach relies on mutual coupling to increase array bandwidth as opposed to the conventional approach of attempting to minimize coupling between array elements designed in isolation. The design leverages from multi-layer FSS design, resulting in a broadband, low profile, highly efficient, conformal array.

Infrared frequency selective surface based on circuit-analog square loop design

A frequency selective surface (FSS) was designed to have a resonant spectral signature in the infrared. The lithographically composed, layered structure of this infrared FSS yields a resonant response in absorption to infrared radiation at a wavelength determined by its FSS element structure and the structure of its substrate layers. The infrared spectral characteristics of this surface are studied via Fourier transform infrared spectroscopy and spectral radiometry in the 3 to 15 /spl mu/m region of the spectrum. The design is based on circuit-analog resonant behavior of square loop conducting elements.

Ohmic loss in frequency-selective surfaces

The present study was undertaken in order to quantify absorption effects due to ohmic loss in frequency-selective surfaces (FSS) at infrared frequencies. The structures considered in this work act as electromagnetic filters, and as such, are of interest for use as thermophotovoltaic spectral control devices. For this application, absorption is of primary concern since it leads to reduced filter efficiency. This work focuses on the behavior of single-layer, free-standing FSS arrays comprised of circular apertures (holes) and circular loop apertures (rings). Numerical calculations of the transmission, reflection, and absorption characteristics of various arrays were carried out for wavelengths between 1 and 15 μm using a commercial finite-element software package. Absorption effects were included using measured optical properties as input parameters to a surface impedance boundary condition. Analytical techniques were then employed to determine the absorption behavior in the static limit. An interesting res...

Effects of surface waves on the currents of truncated periodic arrays

The behavior of surface waves in truncated periodic arrays is examined through analysis of the currents. The surface waves to be studied are guided by the perfectly conducting elements of the array itself and are to be distinguished from the dielectric slab-guided surface waves encountered elsewhere in the literature. The conditions under which surface waves may arise are given. The surface wave currents are extracted from the method of moments solution for the finite by an infinite array using a least squares algorithm. Surface wave excitation and reflection coefficients are then be determined from the data and compared with the semi-infinite array solution.

Scattering from surface waves on finite FSS

This paper discusses the presence of surface waves on periodic structures in general and on frequency selective surfaces (FSS) in particular. While certain types of surface waves can exist on infinite as well as finite periodic structures, this investigation centers around the types that can exist only on finite FSS. Radiation caused by these surface waves may lead to a significant scattering increase in the backscatter as well as the bistatic directions. Thus, this paper is of both theoretical and practical interest.

Demonstration of a single-layer meanderline phase retarder at infrared

Meanderline wave plates are in common use at radio frequencies as polarization retarders. We present initial results of a gold meanderline structure on a silicon substrate that functions at a wavelength of 10.6 microm in the IR. The measured results show a distinct change in the polarization state of the incident beam after passing through the device, inducing a 74 degrees phase retardance between horizontal and vertical components. A high degree of polarization (88%) is maintained in the transmitted beam with an overall power transmittance of 38% and a beam profile that remains essentially unchanged.

Design and Demonstration of an Infrared Meanderline Phase Retarder

We compare design and measurements for a single-layer meanderline quarter-wave phase retarder, operating across the wavelength range from 8 to 12 micrometers (25 to 37.5 THz) in the infrared. The structure was fabricated using direct-write electron-beam lithography. With measured frequency-dependent material properties incorporated into a periodic-moment-method model, reasonable agreement is obtained for the spectral dependence of axial ratio and phase delay. As expected from theory, the single-layer meanderline design has relatively low throughput (23%), but with extension to multiple-layer designs, the meanderline approach offers significant potential benefits as compared to conventional birefringent crystalline waveplates in terms of spectral bandwidth, angular bandwidth, and cost. Simple changes in the lithographic geometry will allow designs to be developed for specific phase retardations over specified frequency ranges in the infrared, terahertz, or millimeter-wave bands, where custom-designed waveplates are not commercially available.

Infrared frequency selective surfaces: design, fabrication, and measurement

A frequency selective surface (FSS) is designed and fabricated to resonate in the infrared. This IR FSS is designed using Periodic Method of Moments (PMM) software and is based on circuit-analog resonance of square loop conducting elements. The FSS is fabricated via electron beam lithography. The spectral characteristics of this surface are studied in the mid-infrared employing a spectral radiometer. The IR FSS may operate as an emissive narrowband source or reflective bandpass filter centered at a wavelength of 6.5μm, sharply cutting off short wavelength radiation and gradually filtering longer wavelengths. The addition of a superstrate layer, intended to further shape the FSS spectral signature, is also studied and the results discussed.

Why Periodic Structures May Not Be Able to Synthesize Negative Indices of Refraction

In this paper we first list some of the features that are widely accepted as being facts regarding materials with simultaneously negative mu and epsiv, namely: (a) negative index of refraction; (b) advance of the phase of a signal as it moves away from the source; (c) an increase of the evanescent waves as they get further away from the source; and (d) while the E-and H-field in an ordinary material form a right handed triplet with the direction of phase propagation, they will in a material with negative mu and epsiv form a left handed triplet. Such materials have never been found in nature. However, numerous researchers have suggested ways to produce them artificially. Periodic structures of elements varying from simple straight wires to very elaborate concoctions have been claimed to produce negative index of refraction. Nevertheless, we shall here show that according to a well known theory based on expansion into inhomogeneous plane waves, it does not seem possible to obtain the features that are characteristic for materials with negative mu and epsiv as listed above. Thus, it seems logical to re-examine Veselagos original paper. We find that it is mathematically correct. However, when used in certain practical applications like, for example, the well known flat lens, it may lead to negative time. While such a solution might be mathematically acceptable, it will violate the causality principle from a physical point of view. So it should not surprise us that we so far have encountered difficulties when trying to create materials with negative mu and epsiv, in particular negative index of refraction.

Infrared frequency selective surfaces

A frequency selective surface (FSS) is designed and fabricated to resonate in the infrared. The FSS is designed via periodic method of moments (PMM) software and is based on circuit-analog resonance of square loop elements. The lithographically composed FSS resonates at the absorption of infrared radiation. The spectral characteristics of this surface are studied from 3 to 15 /spl mu/m.