Inversionless amplification and laser-induced transparency at the   discrete transitions and the transitions to continuum
  The effects of coherence of quantum transitions and the interference of resonant nonlinear optical processes on the spectra of absorption, amplification, and nonlinear-optical generation are considered. The most favorable conditions are discussed for the inversionless amplification, resonant refraction in the absence of absorption and for resonant enhancement of nonlinear-optical generation at the discrete transitions and the transitions to continuum. 
A. K. Popov