Abstract

Crystals such as Barium Titanate and Bismuth Silicon Oxide can record holograms in the form of phase gratings by their photorefractive response to an interference pattern. Absorption of light in a photorefractive crystal generates electric charges which due to trapping in the dark regions effectively diffuse from the anti-nodes to the nodes of the interference pattern. The charge separation produces a local electric field which in turn modifies the refractive index by the Pockels effect thus yielding a phase grating [1,2]. Algae, which exhibit photo-induced taxis, the movement towards or away from a light stimulus [3], may be thought of as behaving in an analogous manner except that by their presence or absence they change the local transmission of a sample instead of its refractive index. In phototaxis, cells respond to non-uniform illumination by sensing the light intensity, whereas in chemotaxis, the cells migrate along an oxygen gradient due to photosynthesis caused by the non-uniform illumination. The investigation of taxis in biological cells has a long history, the term phototaxis first being used in 1878 [4], but most observations have concerned the dynamics and characteristics of the phenomenon as manifested for particular species. We report here for the first time, the temporal and intensity dependence of the optical density and the recording of an optical interference pattern in a suspension of the flagelette Dunaliella Primolacta.

© 1992 Optical Society of America