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Materials for Freeform Fabrication of GHz Tunable Dielectric Photonic Crystals Paul G. Clem, James F. Carroll III1 , Michael K. Niehaus2, Joseph Cesarano III, James E. Smay3, Jennifer A. Lewis4, and Shawn-Yu Lin Sandia National Laboratories, Albuquerque, NM 87185-1411 1 Also Alfred University, Alfred, NY 2 Also New Mexico Institute of Mining and Technology, Socorro, NM 3 Also University of Illinois, Urbana, IL 4 Department of Materials Science and Engineering, University of Illinois, Urbana, IL ABSTRACT Photonic crystals are of interest for GHz transmission applications, including rapid switching, GHz filters, and phased-array technology. 3D fabrication by Robocasting enables moldless printing of high solid loading slurries into structures such as the “woodpile” structures used to fabricate dielectric photonic band gap crystals. In this work, tunable dielectric materials were developed and printed into woodpile structures via solid freefrom fabrication (SFF) toward demonstration of tunable photonic crystals. Barium strontium titanate ceramics possess interesting electrical properties including high permittivity, low loss, and high tunability. This paper discusses the processing route and dielectric characterization of (BaxSr1-XTiO3):MgO ceramic composites, toward fabrication of tunable dielectric photonic band gap crystals.

INTRODUCTION The material system of barium strontium titanate (BST) has been widely studied for low loss, tunable dielectric applications. Such applications include phased array antennas, photonic crystal waveguides, and photonic crystal filters. Phased array antennas enable beam sweeping without mechanically oscillating the antenna by altering the capacitance, and hence phase, of individual array elements and the vector of the array wavefront. Photonic crystal filters are 3D lattice structures that forbid the propagation of electromagnetic waves at distinct wavelengths or frequencies, thus creating a photonic band gap (PBG).1,2 The unique ability of PBGs to create directions of zero effective photonic density of states enables waveguiding, switching, and filtering of a variety of electromagnetic radiation frequencies, from optical to GHz. Introduction of defects into these crystals enables photonic crystal waveguides, which allow propagation of light around 90° bends for the designed wavelength. To date, passive PBGs made of non-tunable materials have generally been produced, while introduction of tunable materials could enable high speed PBG switching and reconfigurable waveguide architectures. Developing a directwrite, tunable 3D PBG filter has been the focus of this research study, as a first step toward waveguides and more complicated devices. Three methods were combined in this work: (1) fabrication of PBGs by the 3D SFF method of Robocasting3-7, (2) modeling of PBG transmission and reflection as a function of dielectric permittivity and loss, and (3) development of novel tunable dielectric materials. The combination of these three efforts is aimed at enabling 3D fabrication of ac