Upper limits of dielectric permittivity modulation in bacteriorhodopsin films

A theoretical study of light-induced modulation of the dielectric permittivity in bacteriorhodopsin films has been done (including B--> M and B --> Q transitions). Analysis of dielectric permittivity modulation enables us to determine the fundamental limits of BR to be used in a holographic data storage system, together with the optimum experimental and material conditions. In order to carry out this analysis, the macroscopic dielectric permittivity was related to the microscopic polarizability of the three states of BR considered (B, M and Q). This parameter was calculated using a modelization procedure that includes the effect of ASP85, TRP86, and TYR185 amino acid residues (the B3LYP/6-31+ G(*) method was used for the calculations). Good concordance between theoretical calculations and experimental data was found for the linear optical properties (absorption wavelength, transition dipole moment, and dielectric permittivity modulation). The theoretical upper limits of Deltaepsilson at 750 nm (far from the resonance of the molecule) in a randomly oriented material are about 0.01 and 0.012 for B--> M and B--> Q transitions, respectively. The values of Deltaepsilon obtained were used to simulate diffraction efficiencies (eta) of a volume phase hologram recorded in a BR film. The high absorptive losses at low wavelengths (about 625 nm ) cause an interesting behavior, since the highest Deltaepsilom do not produce the greatest eta. The highest eta is produced for a hologram thickness in the range of 900-1000 microm and working wavelength of 700-750 nm.