  Axions are a well-motivated dark matter candidate particle. Haloscopes aim to
detect axions in the galactic halo by measuring the photon signal resulting
from axions interacting with a strong magnetic field. Existing haloscopes are
primarily targeting axion masses which produce microwave-range photons and rely
on microwave resonators to enhance the signal power. Only a limited subset of
resonator modes are useful for this process, and current cylindrical-style
cavities suffer from mode mixing and crowding from other fundamental modes. The
majority of these modes can be eliminated by using photonic band gap (PBG)
resonators. The band gap behavior of these structures allows for a resonator
with mode selectivity based on frequency. We present results from the first
tunable PBG resonator, a proof-of-concept design with a footprint compatible
with axion haloscopes. We have thoroughly characterized the tuning range of two
versions of the structure and report the successful confinement of the
operating TM$_{010}$ mode and the elimination of all TE modes within the tuning
range.
