  We investigate rectified currents in response to oscillating electric fields
in systems lacking inversion and time-reversal symmetries. These currents, in
second-order perturbation theory, are inversely proportional to the relaxation
rate, and, therefore, naively diverge in the ideal clean limit. Employing a
combination of the non-equilibrium Green function technique and Floquet theory,
we show that this is an artifact of perturbation theory, and that there is a
well-defined periodic steady-state akin to Rabi oscillations leading to finite
rectified currents in the limit of weak coupling to a thermal bath. In this
Rabi regime the rectified current scales as the square root of the radiation
intensity, in contrast with the linear scaling of the perturbative regime,
allowing to readily diagnose it in experiments. More generally, our description
provides a smooth interpolation from the ideal Periodic Gibbs Ensemble
describing the Rabi oscillations of a closed system to the perturbative regime
of rapid relaxation due to strong coupling to a thermal bath.
