  A previous work [1] experimentally confirmed that the special polarization
characteristic features of a three-dimensional terahertz (THz) photonic crystal
with a silicon inverse diamond structure whose lattice point shape was vacant
regular octahedrons, did not apply to general physical and optical basic rules
in appearance. One of the basic rules is that, according to Maxwell's
equations, the electric-field direction of the reflected wave rotates by 180
degrees from that of the incident wave in the case of complete reflection
(Bragg reflection) and normal incidence. It is said that three-dimensional
photonic crystals have no polarization anisotropy within photonic band gap
(stop gap, stop band) of high symmetry points in normal incidence. Experimental
results, however, confirmed that the polarization orientation of a reflected
wave was rotated by 90 degrees for that of an incident wave at a frequency
within photonic band gap. In this work, measured reflected spectra [1] were
semi-quantitatively evaluated by being compared with reflection ones by using a
FEM (finite element method). These results suggest that measured polarization
anisotropy does not apply to Maxwell's equations in appearance but applies to
these equations in essential as expected.
