Spontaneous pattern formation in a thin film of bacteriorhodopsin with mixed absorptive-dispersive nonlinearity

We have observed the spontaneous formation of transverse spatial patterns in a thin film of bacteriorhodopsin with a feedback mirror. Bacteriorhodopsin has a mixed absorptive-dispersive nonlinearity at the wavelength used in the experiments (633 nm). Threshold values of the incident intensity for observation of pattern formation are found from a linear stability analysis of a model that describes bacteriorhodopsin as a sluggish saturable nonlinear medium with a complex Kerr coefficient. The calculated threshold intensity is in good agreement with the experimental observations, and the patterns are predicted to be frequency offset from the pump radiation.