  Analytic and passivity properties of reflection and transmission coefficients
of thin-film multilayered stacks are investigated. Using a rigorous formalism
based on the inverse Helmholtz operator, properties associated to causality
principle and passivity are established when both temporal frequency and
spatial wavevector are continued in the complex plane. This result extends the
range of situations where the Kramers-Kronig relations can be used to deduce
the phase from the intensity. In particular, it is rigorously shown that
Kramers-Kronig relations for reflection and transmission coefficients remain
valid at a fixed angle of incidence. Possibilities to exploit the new
relationships are discussed.
