On the Casimir energy for scalar fields with bulk inhomogeneities
  We study the field theoretical model of a real scalar field in presence of spacial inhomogeneity in form of a finite width mirror (material layer). The interaction of the scalar field with the defect is described with position-dependent mass term. We calculate the propagator of the theory, the Casimir energy and the pressure on the boundaries of the layer. We discuss the renormalization procedure for the model in dimensional regularization. 
I.V. Fialkovsky, V.N. Markov, Yu.M. Pis'mak