AN ULTRAWIDEBAND PHOTONIC CRYSTAL ANTENNA K. Agil, M. Mojahedit , K.J. Malloyl and E. Schamiloglu2 lCenter for High Technology Materials University of New Mexico 1313 Goddard SE Albuquerque, NM 87106 kagi@?chtm.unm.edu 2Pulsed Power and Plasma Science Laboratory University of New Mexico EECE Bldg./Rrn. 125 Albuquerque, NM 87131 Abstract We present the development of a novel ultrawideband antenna structure that uses photonic crystals as a substrate to reflect the energy normally trapped within a substrate into free space. Photonic crystals are three-dimensional periodic structures that exhibit spatial three- dimensional filter properties. In order to achieve an ultrawideband photonic crystal, two photonic crystals with different periodicities are stacked in tandem. The stop band of the photonic crystal is used as a distributed reflector of a short pulse that is generated at the surface of the crystal by photoconductively switched planar antennas. Introduction Photonic crystals (PCs) are three dimensional periodic structures that exhibit pass and stop bands in their frequency response. 1 In the most general case, the PC can be fabricated using any host material that can be made to have a spatial periodicity on the order of the wavelength. For example, the original PCs were fabricated by drilling periodically spaced air holes in a host dielectric. 2 Typically, the stop band width is approximately 20% of the center frequency. For applications as an ultrawideband antenna substrate, where ultrawideband (UWB) is defined as bandwidths exceeding 25% of the center fi-equency,3 a single periodicity air/dielectric PC cannot cover the entire bandwidth. Currently, PCs are being fabricated using dielectrics and placing metal spheres at the periodic lattice sites to increase the bandwidth of the stop bands beyond 25%.4 In the work described in this paper, the focus is on PCs fabricated with air holes embedded in a dielectric host.5 The UWB-PC is achieved by stacking two PCs with different periodicities in tandem.6 The overall response is the superposition of the component responses in given directions. Recently, Brown, et. al. showed that one application of PCs is their use as efficient substrates for narrowband antennas .7 A standard narrowband antenna printed on a substrate radiates a fair amount of energy into the substrate.8 Because the periodicity in the PC provides a 0-78034214-3/97/$1 0,0001997 IEEE 753