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

Abstract

Four-wave mixing in plasmas using ponderomotive and thermal nonlinearities has been extensively studied. Plasmas have received considerable attention since they become more efficient four-wave-mixing and phase-conjugation media at longer wavelengths (far infrared, 10 \ensuremath{\mu}m, to radio wave, 10 m). In this paper, the prospect of using an ionization nonlinearity in weakly ionized plasmas for degenerate four-wave mixing and phase conjugation is discussed. Like the thermal pressure nonlinearity, the ionization nonlinearity results from the heating of the plasma by the beat wave. However, as the local temperature increases, more neutral species are ionized by electron-impact ionization to form a beat-wave grating structure; instead of pushing the electrons into a grating structure (as with the thermal and ponderomotive nonlinearities), the electron density grating is created directly by ionization. Numerical estimates of the phase-conjugate reflectivity indicate reflectivities in the range of ${10}^{\mathrm{\ensuremath{-}}4--}$${10}^{\mathrm{\ensuremath{-}}3}$ are possible in a weakly ionized steady-state gas-discharge plasma. The reflectivity is limited to this range due to the onset of an ionization instability (striations). Larger reflectivities may be possible for pulsed plasma sources.