In the two-beam coupling experiment in the limit of high beam intensity ratio (βp = Ir0/Is0 >> 1, where Is0 (Ir0) is the incident signal (reference) beam intensity), the intensity of the transmitted signal beam Is is given by Is=Is0 exp(ΓL), where Γ is the two-beam coupling gain coefficient, and L is the interaction length in the sample. Therefore, if in a conventional two-beam coupling geometry (image on the right), an object is inserted in the signal beam path, its image will be enhanced after passage through the sample due to the energy transfer from the reference beam. Up to an order of magnitude amplification, with a time constant of ~30 ms, has been demonstrated in 100-μm-thick PR polymer composite film.A. Goonesekera et al., Appl. Phys. Lett. 76, 3358 (2000)

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