BENJAMIN-FEIR-TYPE INSTABILITY IN A SATURATED FERRITE - TRANSITION BETWEEN FOCUSING AND DEFOCUSING REGIMES FOR POLARIZED ELECTROMAGNETIC-WAVES

We prove the existence of an oscillatory instability of the Benjamin-F eir type for electromagnetic propagation in a saturated ferrite. We do this by reducing the nonlinear equations governing the propagation of electromagnetic waves in such a medium to the nonlinear Schrodinger e quation. We characterize regions where focusing or defocusing of the i nitial carrier envelope occurs in a function of three physical paramet ers: the phase velocity, the quotient between the external magnetic he ld and the magnetization of saturation, and a third one related to the angle between the direction of propagation of the carrier wave and th e external magnetic field. We show that there exist points of transiti on between focusing and defocusing regimes for left elliptically polar ized waves. No such point exists for right elliptically polarized wave s. We show that all circularly polarized waves propagating parallel to the external magnetic field are stable (unstable) if they have negati ve (positive) helicity.