  The band structure of an Si inverse diamond structure whose lattice point
shape was vacant regular octahedrons, was calculated using plane wave expansion
method. A complete photonic band gap was theoretically confirmed at around 0.4
THz. 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. However, it was experimentally confirmed that the
polarization orientation of a reflected wave was different from that of an
incident wave, [I$(X,Y)$], where $(X,Y)$ is the coordinate system fixed in the
photonic crystal. It was studied on a plane (001) at around X point's photonic
band gap (0.36 $-$ 0.44 THz) for incident wave direction [001] by rotating a
sample in the plane (001), relatively. The polarization orientation of the
reflected wave was parallel to that of the incident wave when that of the
incident wave was I(1, 1) or I(1, $-$1). In contrast, the former was
perpendicular to the latter when that of the incident wave was I(1, 0) or I(0,
$-$1) at around 0.38 THz. As far as the photonic crystal in this work is
concerned, method of resolution and synthesis of the incident polarization
vector is not able to apply to the analyses of rotation of the measured
reflected spectra in appearance.
