Four-wave mixing and phase conjugation in plasmas using ionization nonlinearities

Plasmas have recently received considerable attention as potentially efficient long wavelength (infrared to microwave) phase conjugators using degenerate four-wave mixing (DFWM). In such processes, two electromagnetic (EM) pump waves (‘forward’ and ‘backward’) plus a ‘probe’ electromagnetic wave interact through the nonlinear low frequency susceptibility of the medium to produce a fourth ‘signal’ or ‘conjugate’ electromagnetic wave. The process may also be viewed as an Bragg scattering of an EM wave from a low frequency density grating. The density grating is generated by the coupling of one pump wave to the probe wave via pondermotive or thermal pressure forces. In this paper, the possibility of using ionization nonlinearities in weakly ionized plasmas for wave mixing applications is discussed. By locallv heating the weakly ionized plasma, one may selectively ionized portions of the plasma to produce a low frequency density grating. The applicable wavelength regimes, plasma parameters, and response times for ionization nonlinearities will be discussed. ã spie.