Effective medium equivalent to a symmetric multilayer at oblique incidence

It is shown that a multilayer with repeated symmetric unit structure is equivalent to a homogeneous and optically biaxial medium for obliquely incident light, the three principal axes being normal to the plane of incidence, normal to the film plane, and normal to the other two. Unit thickness dependence and incident angle dependence of the dielectric tensor and the effective thickness are calculated using exact formulas. It is found that, at small unit structure thickness, the equivalent medium is almost uniaxial, the optical axis being normal to the film, and the effective thickness is very close to the mechanical one. Absorbing films are also envisaged and the domain of validity (in terms of unit structure thickness) of the uniaxial equivalent medium is defined by numerical examples.