Resonant four-wave mixing of laser radiation in plasmas

Optical phase conjugation is an area of nonlinear optics with a wide variety of potential applications. 2 One method of generating a phase-conjugate signal is with four-wave mixing (FWM). In FWM three input beams interact in a nonlinear medium and a fourth beam is produced that is the phase conjugate of one of the input waves. Degenerate four-wave mixing (DFWM) is a special case of FWM in which all the beams are at the same frequency.3 In a plasma DFWM is an effective technique for phase conjugation in high-density, low-temperature plasmas.4 6 One method of enhancing the phase-conjugate signal over and above the DFWM level is with resonant four-wave mixing (RFWM), in which two of the input beams beat at a plasma resonance.7 '0 In addition to enhancing the generated wave, RFWM can also serve as a diagnostic for many plasma parameters, such as electron and ion temperatures, ion acoustic velocity, and damping rate. In this paper experimental evidence of RFWM in plasmas with C02 laser radiation (10.6 m) is presented and data are compared with theoretical predictions.