Excited-state four-wave mixing for nonresonant holographic storage in azo-dye-doped polymers

In the past, four-wave mixing for real-time holography storage based on ground-state absorption at the resonant wavelength has been extensively studied in azo-dye-doped polymer films. We report theoreti- cal and experimental studies on the four-wave mixing based on the excited-state absorption at a nonresonant wavelength using ethyl- orange-doped polyvinyl alcohol films. In our experiments, we use two laser beams: a 632.8-nm He-Ne laser as a nonresonant light source for excited-state four-wave mixing and a 488-nm Ar 1 laser as a preexcited light source to enhance the excited-state absorption. To explain the ex- perimental results, we propose a four-level model for the azo-molecule system. The steady state and dynamic behavior of phase conjugation signal in four-wave mixing are simulated by solving rate equations. The theoretical simulations, which are in accord with the experimental re- sults, show that using the preexcited method, four-wave mixing hologra- phy storage can be realized by using a weak laser beam with a nonreso- nant wavelength based on the excited-state process. © 2000 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(00)00903-X)