  We report a strong thickness dependence of the complex frequency-dependent
optical dielectric function in epitaxial CaMnO3(001) thin films on SrTiO3(001),
LaAlO3(001), and SrLaAlO4(001) substrates. A doubling of the peak value of the
imaginary part of the dielectric function and spectral shifts of 0.5 eV for a
given magnitude of absorption are observed. On the basis of the experimental
data and first-principles density functional theory calculations of the
dielectric function, its evolution with thickness from 4 to 63 nm has several
regimes. In the thinnest, strain-coherent films, the response is characterized
by a significant contribution from the free surface that dominates strain
effects. However, at intermediate and larger thicknesses approaching the
bulk-like film, strain coherence and partial strain relaxation persist and in
influence the dielectric function.
