224 Bionanotechnology G O 1 I O 2 FIGURE 11.5 An optically switched optical switch. The basic construction is a slab of bacteriorhodopsin-containing purple membrane fragments sandwiched between two optical waveguides. Initially, let us suppose that an optical wave introduced at the input I is guided through the structure to emerge at output O 2 . If a grating (indicated by the stripes) is holographically generated in the bacteriorhodopsin slab by illuminating with light of 570 nm (from G, the "gate"), light will be coupled out from the lower waveguide and coupled into the upper waveguide, emerging at output O 1 . Destroying the grating by illuminating with light of 410 nm will cause the output to revert to O 2 .