Diffraction of focus wave modes at a perfectly conducting screen

We discuss the diffraction of scalar focus wave modes normally incident on the edge of a perfectly conducting half-plane S. The focus wave modes are Gaussian-like beams with parameters depending on space-time. In the region of the beam where the profile is nearly Gaussian (the analogue of the optical region for Gaussian beams) we get the total field nu made of the incident, reflected and diffracted fields as a solution of the wave equation, continuous outside S and satisfying on S the boundary condition of either (nu)(s) = 0 or (delta(n) nu)(s) = 0, where delta(n) denotes the normal derivative. We also discuss the conditions making the diffraction possible and the generalization to electromagnetic focus wave modes.