The absorption change versus time after switching off the illumination on a polymeric film that contained a bacteriorhodopsin (BR) mutant has been measured. The M-lifetime of this BR mutant is ∼320 s. A pertinent Z-scan was performed to study the BR optical nonlinearity. A physical model with multi-level transitions in the Z-scan was suggested. The minimum saturated light intensity measured at 633 nm is ∼0.9 mW/cm2. A low intensity of 70 μW/cm2 has been used for recording in this film. The change in refraction index Δn633 is −3.0×10−3 and Δn476 is 8.5×10−3 with the intensity all at ∼100 mW/cm2.

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