EFFECTIVE PRINCIPAL REFRACTIVE-INDEXES AND COLUMN ANGLES FOR PERIODICSTACKS OF THIN BIREFRINGENT FILMS

An effective medium theory is developed for periodic stacks of thin fi lms A and B, one or both of which may be birefringent because of an ef fect such as columnar microstructure. The equivalent medium is itself described as columnlike, and the theory permits calculation of the ang le of the effective columns together with the magnitudes of the effect ive principal refractive indices. Several examples that relate to poss ible applications of the method are discussed, and it is shown that op tical media with predetermined refractive indices and birefringence ma y be engineered. For example, the layered combination of a low-index m aterial A with zero or small normal-incidence refractive anisotropy an d a high-index material B with large anisotropy may be used to make a medium-index material with considerable normal-incidence anisotropy.