ABSTRACT
The width of the reflection band is limited by the high and low refractive indices used in the periodic stack. Within the high-reflection band, the electric field has a decaying characteristic, somewhat similar as evanescent fields. Hence, this region is also known as the stop band or as the photonic bandgap. The width of the reflection spectrum can be defined by the points where the fields change from decaying to propagating. In regions where the high reflection of both regions overlap, there is a potential of creating resonant modes that can create high transmission peaks inside the high-reflection band. One possible technique to extend the width of the reflection band is to use two periodic stacks. However, this has to be done with caution, because in general, the performance of a combined structure is not a simple sum or product of the performances of the individual structures in isolation.
