Jeffrey S. Tharp

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.

Fabrication of periodic microstructures on flexible polyimide membranes

Periodic metallic microstructures were fabricated on polyimide membranes. Techniques were developed to maintain flatness of the membrane during processing while still allowing for flexibility in the final structure. For proper functionality of the structures, it was necessary to first fabricate a continuous metallic film and a continuous dielectric layer on top of the flexible substrate, which underlaid the periodic microstructure. Flexibility of the overall structure was maintained by using a polymer as the dielectric layer, which was constrained to have high optical transmission over the infrared wavelength range of 6–14μm. Three candidate polymers were evaluated, and their measured optical properties are presented. Benzocyclobutene was found to be the best choice for this application. The final structure fully populated a 10cm (4in.) diameter flexible membrane with microstructures of excellent uniformity.

Off-axis behavior of an infrared meander-line waveplate

An infrared meander-line waveplate has been modeled and measured over the 8 to 12 microm spectral band in terms of its differential phase delay, axial ratio of the output polarization ellipse, and power throughput for angles of incidence between 0 degrees and 60 degrees. The study has been performed for planes of incidence parallel and perpendicular to the meander-line axis. The main significance is that the phase delay remains almost unaffected by the angle of incidence. Infrared meander-line retarders can thus be used well beyond the paraxial range as in low-f/# optical systems and in non-normal-incidence applications.

Electron-beam lithography of multiple-layer submicrometer periodic arrays on a barium fluoride substrate

Direct-write electron-beam fabrication of periodic arrays of submicrometer metallic structures onto Si wafers has been demonstrated for use as infrared IR frequency selective surfaces FSSs . Typically these fabrications have used Si for convenience and because of its broad spectral range of IR transparency from 2 to past 14 m. However, the high refractive index 3.4 of Si is not ideal for some applications because it implies a high surface reflectance and also because of the resulting shrinkage of structure sizes to maintain a given electrical size when fabricated on the Si/air interface. The goal of this work was to explore the feasibility of using BaF2 as a low-index substrate material in multiple-layer FSS designs. While BaF2 has low index 1.4 and good transmission in the IR, it presents unique fabrication issues including difficulties of obtaining a return from a laser-based height monitor, susceptibility to thermal shock, and attack by etchants. We present procedures we used to successfully fabricate submicron metallic arrays on BaF2 in both single-layer and multiple-layer configurations.

Distributed loading effect for infrared FSS

Frequency selective surfaces (FSS) are traditionally resonant periodic antenna structures patterned on a surface for the purpose of spectral filtering. At infrared (IR) frequencies, FSS structures have been successfully demonstrated as both transmission filters and surface emissivity modifiers. In this paper, distributed loading effect for infrared FSS was investigated using four pure metals as distributed load: aluminum, gold, nickel, and titanium.

Demonstration of a multilayer meanderline at IR

This paper extends these initial efforts and attempts to increase the transmission and bandwidth by using multiple meanderline layers. A new ellipsometric method of characterization allows spectral measurements from 6-14 mum. The use of multiple layers reduces impedance mismatches at each meanderline layer. Therefore less reflection is seen at each interface. A dielectric superstrate layer is also deposited so that an anti-reflection coating may be used to increase the overall transmittance.