Page 182 - Photoreactive Organic Thin Films

5. CHIRAL POLYMERS WITH PHOTOAFFECTED PHASE BEHAVIOR FOR OPTICAL DATA STORAGE

FIG. 5.16 (A) Experimental setup for holographic grating recording; (B) schematic representation of
electric field patterns on a vertically installed sample.

FIG. 5.17 Microphotographs of an (A) s,s-grating and a (B) s,p-grating from SK8 {22-|U,m pressed
film) in crossed polarizers.

The quality of holographic grating is estimated by the efficiency of the
probe beam scattering from the grating, calculated as the ratio of light inten-
sities for the first diffraction order and for the incident beam, r\ - I,/I 0. The
He-Ne laser source is usually selected for the probe beam, because it is inac-
tive in the photochemical processes involved. The r\ value depends substan-
tially on the wavelength and intensity of the recording beam, the recording
time, and the prehistory of the sample.
It has been shown that the scattering efficiency of the grating recorded in
a 23 pm thick virgin film of SK8 grows linearly with the exposure time at
constant beam power but falls drastically with the wavelength of the record-
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ing beam. In contrast, the r\ value shows a superlinear growth upon