Light Propagating through a Helical Medium

The coordinate systems used to describe propagation of light in hehcal medium. 

Following the same process as for the uniform director structure described through (5.18)-(5.22), complicated by the fact that now we have to make transformations between two systems, we arrive at an equation for the wave number k 

We note that the iridescent colors of butterfly wings, fish scales and bird feathers originate in the interference of light after multiple reflections in the more or less regularly layered thin film stmcture of the material, which is a very similar phenomenon to that causing the color of the cholesteric liquid crystals. 

In the reflection band, the nonpropagating modes are the so-cdled evanescent waves, whose electric vector is exponentially damped (E °c ). We note that, in the infinite pitch limit, the regular wave transforms to the extraordinary wave, and the irregular wave turns into an ordinary one. 

Outside the bandgap the eigenmodes become elliptically polarized, and the ellipticity increases as the pitch becomes tighter. For a right-handed 

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