Characterisation 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 extended from 10 ms for the native BR protein to ~20 minutes for our BR films, by chemical and physical modifications of the material. Optical cache memory characteristics of thin films are studied in excite-probe experiments in which nanosecond-duration pulses are used to excite a BR film and cw monochromatic light used to monitor the transmission dynamics. The memory read-time is found to vary significantly with the probe wavelength; it is of the order of milliseconds at 633 nm and of the order of 100 µs for probe wavelengths of 600 nm and below. The memory erase-time is ~50 µs. Contrast ratios and insertion losses are measured at different probe wavelengths, with a maximum contrast ratio following nanosecond excitation of ~3:1 obtained in the peak absorption region of the B-state ( ~570 nm), at an excitation fluence of 30 mJcm-2. The implications of these results for the application of BR films as optical cache memory devices are discussed.