  Resonant nonlinear-optical interference processes in four-level
Doppler-broadened media are studied. Specific features of amplification and
optical switching of short-wavelength radiation in a strongly-absorbing
resonant gas under coherent quantum control with two longer wavelength
radiations, are investigated. The major outcomes are illustrated with virtual
experiments aimed at inversionless short-wavelength amplification, which also
address deficiencies in this regard in recent experiments. With numerical
simulations related to the proposed experiment in optically-dense sodium dimer
vapor, we show optimal condition for optical switching and the expected gain of
the probe radiation, which is above the oscillation threshold.
