Holographic recording by excitation of metastable electronic states in Na-2[Fe(CN)(5)NO]center dot 2H(2)O: a new photorefractive effect

Elementary holographic phase gratings can be written in single crystals of Na-2[Fe(CN)(5)NO]. 2H(2)O, sodiumnitroprusside, by excitation of metastable electronic states in the blue-green spectral range. For light polarized pa rallel to the crystallographic a and b axes of the orthorhombic crystal the light-induced modulation of the refractive index reaches Delta(n) approxim ate to 2 x 10(-3) at lambda = 514.5 nm. Although the largest population of the metastable states is reached for light polarized parallel to the crysta llographic c axis, a photorefractive response is not observed. In contrast to electro-optic photorefractive materials the photorefractive effect depen ds mono-exponentially on the exposure and on the modulation of the incident light interference pattern. Beam-coupling experiments demonstrate that wri tten gratings are in phase with the interference pattern in correspondence with the fact that the excitation of the metastable electronic states is lo cal. The width of the rocking curve shows that the holographic gratings are written completely over the volume of the crystal. Variations of the wavel engths within the excitation range as well as of the crystal thickness do n ot influence the maximum photorefractive response. Investigations on the gr ating vector of the written gratings show unambiguously that charge migrati on is not responsible for the photorefractive effect.