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 theoretical and experimental studies on the four-wave mixing based on the excited-state absorption at a nonresonant wavelength using ethylorange-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{sup}+ laser as a preexcited light source to enhance the excited-state absorption. To explain the experimental 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 results, show that using the preexcited method, four-wave mixing holography storage can be realized by using a weak laser beam with a nonresonant wavelength based on the excited-state process.
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