  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.
