Photonic band gaps and localization in two-dimensional metallic quasicrystals

– We report on experimental observation of photonic stop bands in two-dimensional metallic Penrose lattice. We investigated the defect characteristics, and observed strongly localized cavity modes below the plasma frequency. The absence of the translational symmetry allowed to change the defect frequencies and localization properties of the defect modes. Propagation of photons along highly localized coupled-cavity modes was experimentally demonstrated in quasiperiodic metallic structures. The artificially created periodic structures, photonic band gap (PBG) materials, inhibit the propagation of electromagnetic (EM) waves in certain frequency ranges in analogy with electronic band gaps in semiconductors [1, 2]. In semiconductors, only short-ranged order is necessary for the formation of electronic band gaps [3]. Therefore, the existence of stop bands for photons in quasiperiodic or random structures has a fundamental importance. Recently, it was shown that the propagation of EM waves through two-dimensional (2D) quasiperiodic dielectric photonic crystals is forbidden for certain wavelengths [4–10]. Even if the quasiperiodic structures are characterized by a lack of long-range periodic translational order, these systems have long-range bond-orientational order. Therefore, a quasiperiodic system can be considered as an intermediate between periodic and completely random systems [11,12]. Photonic crystals have inspired great interest because of their scientific and technological applications. However, most of the attention has been concentrated on dielectric-based structures. Although metals exhibit high absorption at optical frequencies, some specific properties of metals can be used at microwave and millimeter-wave applications [13–18]. In this letter, we experimentally show that a 2D Penrose lattice exhibits photonic stop bands for certain frequency ranges and strong photon localization in the vicinity of the plasma frequency. Moreover, we investigate the guiding of EM waves through a coupled-cavity array which was formed by removing rods from the Penrose lattice. The Penrose tiles [19, 20] are composed of fat and skinny rhombic unit cells, and fill the 2D plane nonperiodically as illustrated in fig. 1. The Penrose tiling is self-similar and has many interesting properties such as macroscopic tenfold symmetry, long-range decagonal