  We study the scattering of photons from periodically modulated
quantum-optical systems. For excitation-number conserving quantum optical
systems, we connect the analytic structure of the frequency-domain N-photon
scattering matrix of the system to the Floquet decomposition of its effective
Hamiltonian. Furthermore, it is shown that the first order contribution to the
transmission or equal-time N-photon correlation spectrum with respect to the
modulation frequency is completely geometric in nature i.e. it only depends on
the Hamiltonian trajectory and not on the precise nature of the modulation
being applied.
