The relations between incident intensities and behaviors of two-wave coupled dynamic diffraction in a thick bacteriorhodopsin film

When two coherent red beams (λ = 632.8 nm) are coupled in bacteriorhodopsin (bR) film with a small angle, diffraction beams occur. The diffraction intensities as a function of exposure time are quite different for different incident intensities. With low incident intensity, the diffraction intensity rises from zero to a maximum value as exposure time increasing. Conversely, for the higher incident intensity, the diffraction intensity increases from zero to maximum value and then decays till it tends to be a stable value. We found that the incident intensity which is corresponding to the maximum diffraction efficiency in a stable state is an optimal intensity, while incident intensity is lower than it, the dynamic diffraction intensities have no an overshooting peak; and if incident intensity is higher than it, an overshooting peak occurs in the diffraction intensities. Experimental and theoretical proofs were presented.