Metamaterial Filters and Low-Mass Focusing Optics for Millimeter, Sub-millimeter and Terahertz Applications

Large aperture far infrared filters are an essential component for virtually all far infrared, submillimeter and millimeter wave instrument, from space observatories to ground based experiments. But currently, high performance large aperture filters are available from only a single non-US source. In addition, extensive work over the past decades have reduced the mass, power and volume required for terahertz detectors and their associated backend electronics and cryogenics. Optical elements, however, remain largely untouched by this improvement. Particularly for smallsat applications, the mass and volume of antennas and optical elements provide significant restrictions. We propose to develop very thin, lightweight planar filters and focusing optics based on metamaterial structures to address this performance and availability gap. Quasioptical filters based on stacks of metal screen sandwiched between dielectric layers are commonly used in terahertz instruments. Our concept is based on a similar idea, but uses a different fabrication technology. The optics will be made using lithographic techniques at the Arizona State University Flexible Display Center, founded to fabricate liquid crystal displays on flexible dielectrics. The ASU FEDC process uses spin cast polyimide to generate high quality dielectric layers, with in situ metal layer deposition and etching. Circuits are built up one layer at a time, optically registered to the previous layer with each deposition, guaranteeing micron level alignment. Their fabrication technology is well matched to THz optics of this type with a minimum trace and space of 2-3um on dielectrics down to 3um thickness. Their technology can fabricate structures in 150mm diameter circular or 370 mm x 470 mm rectangular formats, allowing the fabrication of a wide variety of optics. This technology allows for the straightforward fabrication of filters, but also allows the fabrication of metamaterial focusing optics. A lens or mirror can be thought of as a phase transformer with a radially varying phase transformation. We divide the surface of the optic up into sub-wavelength cells and optimize the transmission line in that cell to provide the desired phase shift, while simultaneously optimizing transmission through the stack. Focusing optics can either be transmissive (a lens) or reflective (a mirror). In the transmissive case, the anti-reflective coating is “built in” to the design of the metamaterial optic. The FEDC process allows micron accuracy alignment of each layer, ensuring the features of each transmission line cell are aligned throughout the optic. A complete THz optic (with ~10 layers) is under 0.3mm thick. We propose to fabricate two generations of 150mm diameter planar filters, lenses and mirrors using the FEDC 150mm prototyping process on Kapton dielectric. In addition we will investigate the use of two novel low loss dielectrics in our designs: a proprietary low loss alumina film from Corning Inc., and cyclic olefin copolymer, a spin castable dielectric with loss comparable to polypropylene.

The Astrophysics Research and Analysis program (APRA) supports suborbital and suborbital-class investigations, development of detectors and supporting technology, laboratory astrophysics, and limited ground based observing. Basic research proposals in these areas are solicited for investigations that are relevant to NASA's programs in astronomy and astrophysics, including the entire range of photons, gravitational waves, and particle astrophysics. The emphasis of this solicitation is on technologies and investigations that advance NASA astrophysics missions and goals.

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