Phase conjugation in two photon absorbing dye films by degenerate four-wave mixing

Optical phase conjugation through degenerate four-wave mixing is observed in 4-[4-(Dimethylamino)styryl]-1-docosyl pyridinium bromide (DASPB) dye-doped in Polymethyl methacrylate - metacrylic acid (PMMA-PA) polymer films under low-power, continuous-wave laser irradiation. A maximum phase conjugate efficiency of 0.42% has been obtained for probe beam intensity at 2.5 W/cm². Phase conjugation is observed for both parallel- and orthogonally-polarized probe and pump beams. The maximum PC reflectivity is achieved when the angle between probe beam and forward pump beam is 8 degrees. The effects of dye concentration, inter beam angle between probe and forward pump beam on phase conjugation reflectivity are also studied. PC signal strength first increases and then decreases with time. PC reflectivity is also increased by increasing the intensity of the backward and forward pump beam. The polarization and intensity profile are verified to be preserved in the conjugate signal. The predominant phase conjugation signal is attributed due to the saturable absorption and two photon induced fluorescence property of the dye molecules.