  The decay of Rabi oscillations provides direct information about coherence of
electron spins. When observed in EPR experiments, it is often shortened by
spatial inhomogeneity of the microwave field amplitude in a bulk sample. In
order to suppress this undesired loss of coherence, we propose an additional
dressing of spin states by a weak longitudinal continuous radiofrequency field.
Our calculations of the Rabi oscillations between the doubly dressed spin
states show that the maximum suppression of decoherence is achieved at the
so-called Rabi resonance when the radio-field frequency is in resonance with
the Rabi frequency of spins in the microwave field. This effect is feasible
even in the absence of phase matching in the radiofrequency field and for
different types of inhomogeneity of the microwave field. The manifestations of
such suppression in the published EPR experiments with the bichromatic driving
are discussed. The realization of the Rabi resonance using the radiofrequency
field could open new possibilities for separating the contributions of
relaxation mechanisms from those due to the inhomogeneous driving in spin
decoherence.
