Figure 2. Contour plots of the experimentally measured transmission (a,b) and phase delay (c,d) as a function of frequency and incident angle for the square-lattice crystal (a,c) and HUD structure (b,d).

In the square lattice, Bragg scattering is responsible for forming stop bands (blocking regions), and their center frequency and width vary rapidly with the incident angle. In the HUD sample, a truly isotropic PBG is observed around 23.5 GHz despite the low index-contrast of 1.6:1 and the lack of Bragg scattering. The measured phase delay varies with frequency continuously for propagating modes outside of stop bands and appears to be random inside the stop bands where transmission is extremely low.