Four-Wave Mixing And Phase Conjugation In Plasmas Using Ionization Nonlinearities

Abstract

Four -Wave Mixing and Phase Conjugation in Plasmas Using Ionization NonlinearitiesJohn F. Federici' and Ernie J. ValeoPrinceton Plasma Physics Laboratory. Princeton UniversityP.O. Box 451, Princeton. New Jersey 08544ABSTRACTPlasmas have recently received considerable attention as potentially efficient long wavelength (infrared to mi-crowave) 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 lowfrequency susceptibility of the medium to produce a fourth `signal' or `conjugate' electromagnetic wave. The processmay also be viewed as an Bragg scattering of an EM wave from a low frequency density grating. The density gratingis generated by the coupling of one pump wave to the probe wave via pondermotive or thermal pressure forces. Inthis paper, the possibility of using ionization nonlinearities in weakly ionized plasmas for wave mixing applicationsis discussed. By locally heating the weakly ionized plasma, one may selectively ionized portions of the plasma toproduce a low frequency density grating. The applicable wavelength regimes, plasma parameters, and response timesfor ionization nonlinearities will be discussed.I. INTRODUCTIONPhase conjugation has many applications for signal processing, beam combining, tracking and pointing, laserbeam clean up, phase distortion compensation, etc..1,2 It has long been recognized that plasmas are efficient longwavelength (infrared through microwaves to radio waves) nonlinear optical mediums for phase conjugation by four -wave mixing (FWM).3 -6 Recent experiments have demonstrated nonlinear thermal forces' as well as pondermotiveforces8 for FWM in plasmas. This paper focuses on a new nonlinearity for plasmas based on laser- induced ionizationimbalances within the plasma.In collisionless plasmas, the pondermotive force dominates the thermal force. When the electron collisional mean -free -path is short compared to the beat -wave wavelength, thermal pressure forces become important. If a plasmawere weakly ionized, there would be another nonlinear force in plasmas - the ionization 'force' - which may beexploited for wave mixing and other nonlinear optical effects. Like the thermal pressure force, the ionization forceresults from the heating of the plasma by the beat -wave. However, as the weakly ionized plasma is heated, moreneutrals are ionized; instead of pushing the electrons into a grating structure, the electron grating is created directlyby ionization.The nonlinear mechanism in the plasma which generates the electron density grating for FWM is similar tothe mechanism for ionization instability driven electron -density waves. These waves are also called `striations' or`ionization waves'. A common type of striations occur naturally in discharge plasmas.''Semiconductors exhibit an analogous `ionization nonlinearity' for FWM.12 In semiconductors, the coupled pumpand probe waves generate a large concentration of free carriers (electron -hole plasma) by optically induced transitionsof electrons between the valance and conduction bands. For laser wavelengths near a semiconductor bandgap, the