  Coherent, optically dressed media composed of two-level molecular systems
without inversion symmetry are considered as all-optically tunable sources of
coherent radiation in the microwave domain. A theoretical model and a numerical
toolbox are developed to confirm the main finding: the generation of a
low-frequency radiation, and the buildup and propagation dynamics of such
low-frequency signals in a medium of polar molecules in a gas phase. The
physical mechanism of the signal generation relies on the permanent dipole
moment characterizing systems without inversion symmetry. The molecules are
polarized with a DC electric field yielding a permanent electric dipole moment
in the laboratory frame; the direction and magnitude of the moment depend on
the molecular state. As the system is resonantly driven, the dipole moment
oscillates at the Rabi frequency and, hence, generates microwave radiation. We
demonstrate the tuning capability of the output signal frequency with the drive
amplitude and detuning. We find that even though decoherence mechanisms such as
spontaneous emission may damp the output field, a scenario based on pulsed
illumination yields a coherent, pulsed output of tunable temporal width.
Finally, we discuss experimental scenarios exploiting rotational levels of
gaseous ensembles of heteronuclear diatomic molecules.
