Synthetic Metals 139 (2003) 705–709 Magnetic-field tunable photonic stop band in metallodielectric photonic crystals M. Golosovsky ∗ , Y. Neve-Oz, D. Davidov The Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Abstract We fabricated an artificial crystal consisting of a stack of containers with magnetizable ferromagnetic spheres. In the absence of external magnetic field the particles are in disordered state while in the presence of the field the particles self-assemble in almost a perfect hexagonal order. The degree of order is controlled by magnetic field. We study magnitude and phase of the frequency-dependent mm-wave transmission through the stack as a function of magnetic field. In the ordered state there are well-defined photonic stopbands separated by the regions where transmission is close to unity (“transparency windows”) while in the disordered state these regions mostly disappear. By varying magnetic field we achieve effective tuning of the mm-wave transmission through the stack. © 2003 Elsevier B.V. All rights reserved. Keywords: Photonic crystal; Stopband; Self-assembly; Tunability; Magnetic field Photonic crystals are two-dimensional (2D) or three- dimensional (3D) ordered structures which exhibit stop- band, or even complete bandgap [1]. Many fabrication routes of such crystals have been reported recently and the important challenge now is tunability. This is usually achieved by varying the lattice constant or symmetry of the perfectly ordered crystal using different agents such as tem- perature, elastic stress and magnetic field [2–10]. We report here a novel photonic crystal where tunability is achieved by magnetic field induced order–disorder transition. Our building block is a transparent hexagonally-shaped plexiglas plate filled with 2 mm diameter magnetizable steel spheres which can move freely in lateral directions (Fig. 1). The motion of the spheres is limited by the walls made of magnetizable rods. The friction force between the spheres and the substrate is small but not negligible. The num- ber of particles in each container corresponds to the per- fect hexagonal packing, namely Z = 1 + 3s(s + 1), where s = 1, 2, 3,... . Several containers are arranged in a stack mounted inside the Helmholtz coils (Fig. 2). Perpendicular magnetic field of ∼10 mT magnetizes the spheres and in- duces magnetic interaction between them. The out-of-plane attraction between the spheres is too weak to induce or- dering across the layers, while the in-plane repulsion is strong enough to drive each 2D-array into a well-ordered ∗ Corresponding author. Tel.: +972-2-658-5139/6551; fax: +972-2-5617-805. E-mail address: golos@vms.huji.ac.il (M. Golosovsky). “crystalline” state with almost constant density (Fig. 1a). Intermediate field (1-10 mT) drives each array into homo- geneous “amorphous” state (Fig. 1b) exhibiting short-range rather than long-range order. In the absence of magnetic field this structure exhibits disordered “aggregated” state (Fig. 1c) with inhomogeneous in-plane particle density. The right panel of Fig. 1 shows Fourier transform of the corre- sponding images. Note sharp spots in the “crystalline” state (Fig. 1a); spots and a ring in the “amorphous” state (Fig. 1b); a wide “diffuse” ring in the “aggregated” state (Fig. 1c). Similar systems consisting of colloidal magnetic particles on liquid surface were studied recently [12,13]. The mm-wave transmission through our structures was measured using a HP850C Vector Network Analyzer and two standard gain horn antennae (Fig. 2) to which we attached home-made collimating Teflon lenses. We stud- ied transmission in the 20–50 GHz frequency range as a function of number of layers, layer spacing, and magnetic field. The measurements were performed as follows. We measured frequency dependence of the mm-wave trans- mission at fixed value of magnetic field and then moved to the next value of the field. In between the measure- ments we either sent a short and strong magnetic pulse or vibrated the stack in order to erase the memory of the previous state. At small values of magnetic field, when the particles are only partially ordered, there are many particular realizations of the configuration corresponding to a certain value of the field. The mm-wave transmis- sion through these configurations differ only in minor de- 0379-6779/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0379-6779(03)00331-X