Measurements of holographic properties of bacteriorhodopsin films

Several different bacteriorhodopsin (BR) films are characterized with respect to general holographic properties. Experimental measurements include diffraction efficiency and sensitivity as functions of the writing intensity and grating frequency, hologram thermal-decay behavior, diffraction efficiency as a function of the grating tilt within the film and the modulation depth, and estimates of the refractive-index change from the diffraction-efficiency data. The films studied include those made from wildtype BR and the genetic variants D96N and D96N/T46V. The film holographic properties were found to be relatively insensitive to the grating frequency and the grating-tilt angle. The diffraction efficiency dropped off more sharply as a function of the modulation depth than did a purely linear medium, and only the hydrated wildtype film exhibited significant behavior variation with different writing intensities because of its short M-state lifetime. The maximum diffraction efficiency measured was approximately 7.5% for a hydrated D96N BR film. We also find that the hydrated BR films exhibit significantly higher refractive-index modulation than do dry films.