ABSTRACT
The basic idea behind the Herpin equivalence principle is that a layer sandwiched between two other identical layers, such as (ABA), can be described as having an equivalent index η and an equivalent phase thickness θ as illustrated in Figure 7.1. Therefore, repeating periods of this three-layer unit cell, such as (ABA) N , will also have the same equivalent index η with a phase thickness of Nθ. In other words, adding more unit cells only changes the effective phase length of the structure. This is a very useful mathematical construction because treating a periodic structure as if it had a uniform refractive index allows us to use it as a building block to synthesize more complex structures.
