Two-wave mixing in sodium vapour

Abstract
We have observed a unidirectional oscillation in a ring cavity when a sodium atomic vapour interacts with a non resonant pump beam. We have checked that the frequency of the oscillating beam is almost the same as the frequency of the pump beam. We propose a theoretical interpretation of the origin of a two-wave mixing process in an atomic vapour. We show the necessity of a relaxation process and we emphasize the close relationship between this effect and the Bloembergen PIER 4 resonances