  In this paper we consider a quantum harmonic oscillator interacting with the
electromagnetic radiation field in the presence of a boundary condition
preserving the continuous spectrum of the field, such as an infinite perfectly
conducting plate. Using an appropriate Bogoliubov-type transformation we can
diagonalize exactly the Hamiltonian of our system in the continuum limit and
obtain non-perturbative expressions for its ground-state energy. From the
expressions found, the atom-wall Casimir-Polder interaction energy can be
obtained, and well-know lowest-order results are recovered as a limiting case.
Use and advantage of this method for dealing with other systems where
perturbation theory cannot be used is also discussed.
