“…Optical filters have also been produced using liquid crystals [ 14 – 16 ], porous silicon [ 9 , 12 ], anodic aluminina [ 17 ], polymers [ 4 , 18 ], photochromic films [ 19 , 20 ], nonlinear optical chromophore thin films [ 21 ], and RbCdF 3 :Mn 2+ single crystals [ 22 ]. Optically switchable diffraction gratings would be particularly useful for all-optical switching and multiplexing [ 5 ] and could be used, for example, in add/drop [ 4 ] modules in optical communication systems that are needed for dense wavelength division multiplexing.…”