Degenerate four-wave mixing and phase conjugation in a collisional plasma

Although degenerate four-wave mixing (DFWM) has many practical applications in the visible regime, no successful attempt has been made to study or demonstrate DFWM for wavelengths longer than 10 pm. Recently, Steel and Lam [Opt. Lett. 4, 363 (1979)] established plasma as a viable DFWM and phase-conjugation (PC) medium for infrared and far-infrared waves and microwaves. However, their analysis is incomplete since collisional effects were not included. Using a fluid description, our results demonstrate that when collisional absorption is small and the collisional mean free path is shorter than the nonlinear density grating scale length, collisional heating generates a thermal force that substantially enhances the phase-conjugate reflectivity. When the collisional attenuation length becomes comparable with the length of the plasma, the dominant effect is collisional absorption of the pump waves. Numerical estimates of the phase-conjugate reflectivity indicate that for modest power levels, gains S;1 are possible in the submillimeter- to centimeter-wavelength range. This suggests that a plasma is a viable PC medium at those long wavelengths. In addition, doubly DFWM is discussed.