Special polarization characteristic features of a three dimensional terahertz photonic crystal not apparently apply to physical and optical basic rules

A silicon inverse diamond structure whose lattice point shape was vacant regular octahedrons had a complete photonic band gap at around 0.4 THz and X point's photonic band gap (0.36 THz to 0.44 THz) by plane wave expansion method. It is said that three-dimensional photonic crystals have no polarization anisotropy in photonic band gap (stop gap, stop band) of high symmetry points in normal incidence. Experimental results, however, confirmed that the polarization orientation (electric-field direction) of a reflected wave was different from that of an incident wave whose direction was [001]. The polarization orientation of the incident wave was parallel to the surface (001) of the photonic crystal, and it was set in the orientation defined as $\theta$ $^\circ$ (degree). The angle, $\theta$ was 0$^\circ$ to 90$^\circ$ per 15$^\circ$. A sample was rotated in plane (001) instead of the incident wave, relatively. The polarization orientation of the reflected wave was parallel to that of the incident wave for $\theta$ = 0$^\circ$ and 90$^\circ$, in contrast, the former was perpendicular to the latter for $\theta$ = 45$^\circ$ in the vicinity of 0.42 THz. For an intermediate $\theta$, the former was an intermediate orientation. As far as the photonic crystal in this work is concerned, these phenomena do not apply to physical and optical basic rules in appearance.