CHARACTERIZATION OF LONG M-STATE LIFETIME BACTERIORHODOPSIN THIN-FILMS FOR OPTICAL CACHE MEMORY

We report the optical characterisation of bacteriorhodopsin (BR) films of long intermediate-state lifetimes. The M-state lifetime is extende d from 10 ms for the native BR protein to similar to 20 minutes for ou r BR films, by chemical and physical modifications of the material. Op tical cache memory characteristics of thin films are studied in excite -probe experiments in which nanosecond-duration pulses are used to exc ite a BR film and cw monochromatic light used to monitor the transmiss ion dynamics. The memory read-time is found to vary significantly with the probe wavelength; it is of the order of milliseconds at 633 nm an d of the order of 100 mu s for probe wavelengths of 600 nm and below. The memory erase-time is similar to 50 mu s. Contrast ratios and inser tion losses are measured at different probe wavelengths, with a maximu m contrast ratio following nanosecond excitation of similar to 3:1 obt ained in the peak absorption region of the B-state (similar to 570 nm) , at an excitation fluence of 30 mJcm(-2). The implications of these r esults for the application of BR films as optical cache memory devices are discussed.