ONE-DIRECTIONAL GAIN AND FREQUENCY SELECTION OF ELECTROMAGNETIC-WAVESIN A MEDIUM WITH A RUNNING GRATING OF INVERSE POPULATION

The effects of one-directional amplification, frequency selection and transformation of time structure of electromagnetic waves (the generat ion of ultra-short pulses) are considered theoretically for a medium, where the;creation of a running wave of atomic states inverse populati on is possible under excitation by coherent light beams with different frequencies and directions. These effects are due to the possibility of synchronism of a group velocity of an amplified electromagnetic wav e with the phase velocity of a wave of inverse population. The general theory is applied for describing two specific cases which are observe d in CdS crystals where the gain effects are caused by: 1) radiative t ransitions on lines generating the P-band of recombination radiation ( the visible range and 2) radiative transitions between different excit ion subbands (the far IR range).