Magnetic-field-tunable photonic stop band in a three-dimensional array of conducting spheres

We explore the possibility of tuning photonic crystal properties via order-disorder transition. We fabricated a photonic bandgap material consisting of a three-dimensional array of conducting magnetizable spheres. The spheres self-assemble into ordered state under external magnetic field, in such a way that the crystalline order can be continuously controlled. We study mm-wave transmission through the array as a function of magnetic field, i.e., for different degrees of order. This was done for the regular crystal, as well for the crystal with the planar defect which demonstrates resonance transmission at a certain frequency. We observe that in the ordered, "crystalline" state there is a well-defined stop band, while in the completely disordered, glassy or "amorphous" state, the stop band nearly disappears. We relate the disappearance of the stop band in the disordered state to the fluctuations in the particle area density. We develop a model which predicts how these fluctuations depend on magnetic field and how they affect electrodynamic properties of the whole sample. The model describes our results fairly well.