An extremal problem related to negative refraction

We solve an extremal problem that arises in the study of the refractive indices of passive metamaterials. The problem concerns Hermitian functions in $H^2$ of the upper half-plane, i.e., $H^2$ functions satisfying $f(-x)=\bar{f(x)}$. An additional requirement is that the imaginary part of $f$ be nonnegative for nonnegative arguments. We parameterize the class of such functions whose real part is constant on an interval, and solve the problem of minimizing the imaginary part on the interval on which the function's real part takes a given constant value.