  All-optical, mirrorless switching and bistability is demonstrated with
bacteriorhodopsin (bR). A low-power, 532 nm laser beam modulates the
transmission of a cw laser beam at 635 nm that corresponds to peak absorption
of the O-excited state in the bR photocycle. The switch has features like a
NOT-gate; its switching contrast depends on the pulse width and average power
of the modulating laser. The phase of the switching characteristics can be
controlled by varying the pulse width and frequency of the modulating laser.
Simulations based on a rate equation approach considering a six-state model of
the bR photocycle successfully reproduce the experimental results.
