Plane wave method for out-of-plane propagation in 2D photonic crystals

In this research we investigate the behavior of the plane wave method (PWM) for calculations of the band structure of 2D photonic crystals with low optical contrasts. By considering the out-of-plane propagation, we have shown that in limiting case of homogeneous medium (epsiv_a=epsiv_b) the PWM returns some higher curves additionally to usual straight light line. Also, we have shown that these additional curves precisely coincide with the modes of ID cavity with perfectly reflecting walls. The nature of this effect is periodic boundary condition used in the PWM. Thus, we conclude that PWM is inappropriate for calculations of 2D photonic crystals with low optical contrast