Tunable High-Q Photonic Bandgap Microwave Cavity*

A woodpile structure made out of dielectric rods exhibits an omnidirectional photonic bandgap (PBG) which forbids the propagation of electromagnetic wave with energy within a certain range in all directions. We designed an electromagnetic cavity by creating a defect inside the crystal, such that its frequency lies within the forbidden bandgap. Very high Q-factors can be achieved since the light has no way to escape because of the bandgap and is only limited by the dielectric loss in the material. We predict the quality factor of such a cavity to be close to 10⁸ near 10 GHz. The cavity frequency is tuned by sliding the rods in and out. One of the potential applications is in the axion dark matter search, which is currently limited by the use of low Q-factor copper cavities due to the presence of a strong magnetic field. We predict the Q-factor of a PBG cavity to increase in the presence of a large magnetic field due to the shift in the two-level system energies to a higher level.