Threaded-chiral media: reflectors for elliptically polarized light

Effective medium theory is applied to nanoengineered inorganic coatings in which a primary chiral medium (A) threads through a second medium (B) that may be isotropic, birefringent or chiral. In general A and B are represented by thin locally-birefringent layers with axes that rotate through the thickness of the coating. Local properties of the complex medium are represented by a Herpin birefringent period (A/2)B(A/2), with effective principal refractive indices and with thickness much less than the chiral period. The effect of changing the fraction fB of material B is considered. With simplicity of fabrication in mind, the special case in which A and B are identical apart from thickness and azimuthal orientation is developed. Then, by maintaining a constant optical thickness for the Herpin period, the circular Bragg resonant wavelength of the primary medium can be retained. As an example of the potential use of threaded-chiral materials, handed resonant reflectors for elliptically-polarized light are considered.