  We present an explicit analytic calculation of the energy-level shift of an
atom in front of a non-dispersive and non-dissipative dielectric slab. We work
with the fully quantized electromagnetic field, taking retardation into
account. We give the shift as a two-dimensional integral and use asymptotic
analysis to find expressions for it in various retarded and non-retarded
limiting cases. The results can be used to estimate the energy shift of an atom
close to layered microstructures.
