Bacteriorhodopsin: a natural, efficient (nonlinear) photonic crystal

From the angular dependence of the second-order nonlinear light scattering (hyper-Rayleigh scattering) from a suspension of purple membrane, bacteriorhodopsin was recently shown to exhibit (nonlinear) photonic crystal properties [Opt. Lett. 25, 1391 (2000)]. The optical nonlinearity, i.e., the first hyperpolarizability beta, is localized in the small retinal moiety, whereas the optically linear refractive index n is relevant to the large membrane protein. The combination of the nonlinear hyperpolarizability of the retinal decoupled from the linear refractive index of the protein explains the observed angular dependence. The temporal evolution of this angular dependence has now been analyzed. The disappearance of the angular dependence of the nonlinear scattering is shown to be a consequence of the solubilization of the large purple membrane patches into individual protein monomers. This result strongly suggests that decoupling of the optical nonlinearity from the phase-matching condition for coherent second-harmonic generation will result in highly efficient coherent second-harmonic generation in bacteriorhodopsin crystals. In addition, a simulation of the bandgap properties was made for an abstracted structure with a large refractive index for the retinal and with a small refractive index for the protein matrix. (C) 2001 Optical Society of America.