Page 181 - Photoreactive Organic Thin Films

MIKHAIL V. KOZLOVSKY, LEV M. BLINOV, AND WOLFGANG HAASE

within 1 to 2 minutes but disappears if the irradiation is continued further.
On the other hand, the birefringence induced by blue-green light (band C)
shows no saturation, due to repeated ds-trans-ds transformations. The sensi-
tivity of films to visible light can be substantially increased by preiilumination
with nonpolarized UV light. Photorecording during the phase formation (on
cooling, or also during the kinetically determined isothermal mesophase for-
mation for copolymers of the KM series) results in much higher An imi values
as compared with photorecording in a completely organized mesophase.
The combination of these features presents photochromic IsoSm* films
as interesting and diverse media for optical data storage. In Section 5.4,
we consider holographic grating recording in copolymer films and related
(combined) photorecording techniques.

5.4 HOLOGRAPHIC GRATING RECORDING

Optical holography provides unique opportunities for information storage and
visualization. To date, most investigations have focused on doped inorganic
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crystals, the polymer organic materials attract more and more attention as
holographic media, however, because of easy processing and "custom-tailored"
adjustment of properties with well-developed polymer technology techniques.
Among those, amorphous and liquid crystalline azobenzene side-chain poly-
9 42
mers " are distinguished for high diffraction efficiency, easy erasing/
rewriting of recorded data, and reasonably short response time.
Generally, there are three mechanisms for writing holograms in azo-dye
polymers: (1) (rotational) molecular reorientation; (2) redistribution of ds-
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and £ra«s-isomer population; and (3) formation of surface relief grating. '
The first two mechanisms represent bulk effects, and the latter is related to
the surface. The contribution of these mechanisms into the total diffraction
efficiency depends on many parameters, including the extinction coefficient at
a particular wavelength of a laser beam, the quantum yield of the photo-
chemical isomerization, the temperature-dependent relaxation time of the ds-
isomers, the angular diffusion, or even the film confinement conditions. To
elucidate the mechanism of photorecording, and to optimize the conditions
for readout of recorded data, different polarization geometries of the interfer-
84 87
ing pump beams can be chosen. '
Holographic grating recording in the chiral photochromic copolymers of
the SK-, KW-, and KM-series has been studied for several polarization
configurations. Figure 5.16A shows the experimental setup for the grating
recording, and the light-field interference patterns for $,s-, p,p-, s,p-, and R,L-
configurations of laser beams are presented in Figure 5.16B. Here, s- and p-
polarizations correspond to linear polarization with the electric vector of the
incident beam perpendicular and parallel to the plane of light incidence,
whereas the R- and L-polarizations correspond to right- and left-handed
circularly polarized light. The experimental scheme of Figure 5.16A allows
for all four types of recording with the proper choice of waveplates, WP.
Some examples of holographic gratings recorded in an IsoSm* copolymer
film are shown in Figure 5.17.