  Using a recently developed strong-coupling method, we present a comprehensive
theory for doublon production processes in modulation spectroscopy of a
three-dimensional system of ultracold fermionic atoms in an optical lattice
with a trap. The theoretical predictions compare well to the experimental time
traces of doublon production. For experimentally feasible conditions, we
provide a quantitative prediction for the presence of a nonlinear "two-photon"
excitation at strong modulation amplitudes.
