Microphotometric investigation of fixed holograms

Abstract

Holographic gratings are written in copper-doped LiNbO3 crystals at temperatures of about 180 degrees C. At room temperature the gratings are fixed. The subsequent development process under homogeneous illumination is investigated by the microphotometric method. Measurements of the light intensity pattern at the exit face of the crystal reveal not only the change of the amplitude but also a phase shift of nearly pi . The dynamics of the development process can be fully understood by the assumption of a quasi-permanent modulated charge distribution and a second modulated charge distribution of optically excitable ions.

Stability of fixed holograms in LiNbO3

“…[11][12][13]16 As we have shown here, those values result lower than the true thermal erasure time constant values. Those measurements were interpreted on the assumption that the electronic charges are not thermally activated at temperatures below ϳ150°C.…”

“…After these authors, the thermal fixing was studied experimentally as well as theoretically by a number of researchers. [12][13][14][15][16][17][18][19][20] This method consists in heating the sample temporarily during or after optical recording at a temperature of above 100°C, and illuminating it subsequently at room temperature. The explanation of the process is as follows: The initial hologram is stored as a nonhomogeneous distribution of electronic charges ͑Fe 2ϩ /Fe 3ϩ ), which generate a refractive index modulation through the electro-optical effect.…”

Holographic storage dynamics in lithium niobate: theory and experiment

“…(11) happens when i ӷ D i K 2 ; i.e., the density of ions is sufficiently large, n i0 ӷ (⑀kT/e 2 )K 2 [inequality (12)]. This condition is, in fact, most usually satisfied because of the large density of conducting ions at elevated temperature.…”

“…4,9,[11][12][13][14][18][19][20][21][22] In general, it is agreed that above 70-80°C the ionic conductivity in this crystal largely prevails over the dark electronic conduction because of Fe 2ϩ -electron detrapping. At temperatures somewhat below 60°C, however, the dark conductivity is due predominantly to electrons and is characterized by a small activation energy (0.1-0.4 eV) that is due to the thermally assisted small polaron electronic conduction associated with the Nb Li 4ϩ defect center.…”

“…[8][9][10][11][12][13][14] What differentiates our study is the fact that, by taking advantage of the great disparity between the ionic and the electronic transport time constants that are involved in typical crystals, especially lithium niobate (LiNbO 3 ), we are able to obtain simple analytic expressions 9 for the time-dependent field in each of the phases defined above under realistic conditions.…”