Index-tunable photonic crystals based on ferroelectric materials provide a means for active modulation of optical signals,
and hold promises for novel device applications. In this study, (Ba,Sr)TiO3 (BST)-based planar photonic crystals with
different cavity geometries were modeled. Photonic crystals with square-shaped air rod geometry, which can be prepared
in a straight-forward manner by interference lithography, were compared with photonic crystals having circular air rods.
Calculations were performed on square lattice, with either square or circular air rods, by the plane wave expansion
method. Simulation results suggested comparable bandstructures and gap maps for square or circular air rod photonic
crystal, if (1) the dimension of the air rod was small compared to the electromagnetic wavelengths inside the photonic
crystal being considered, or (2) the frequencies of the electromagnetic waves were less than 0.35(2πc/a). A better
correlation in bandstructures and gap maps between the square and circular air rod photonic crystals can be achieved, if
we compare them by the volume fractions of the photonic crystals in stead of the characteristic lengths of the rods (i.e.
diameter of the circular rod and width of the square rod).