Page 183 - Photoreactive Organic Thin Films

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

10-
s,s -grating
8-
6-
4-
^ o O
c 2-
• 2n . " 0
'o 1 o 8 o
0- i ' ' i • • ' ' i ' i • *
W 6-
c p,p -grating Q
.2 I 1 ^"
tj O
cd 4-
fi
i3
2- m
- m *
O
0- fi •
100 200 300 4(
Writing Beam Intensity, P, mW
FIG. 5.18 First-order s- and p-diffraction efficiencies (• and O, correspondingly) of s,s- and p,p-
gratings recorded with a fixed exposure time, t^ = 15 s, as a function of the power of the initial
s-polarized (top) and p-polarized (bottom) Ar laser beam (for the 0 1.6-mm spot).

increase in the beam intensity, for a fixed exposure time, as shown in Figure
5,18. The superlinearity is especially pronounced at higher writing powers.
The data presented in the figure show unambiguously that in the cases of
both s,s- and p,|>-gratings, the diffraction efficiency of the probe beam is
higher for polarization coinciding with that of the writing beams. On first
sight this looks strange because, in uniaxially symmetric materials, the differ-
ence W| - « iso is always larger than n L - n iso (w iso, n^ and n ± are refraction
indices for the nonoriented phase, and parallel and perpendicular to the
director, respectively), and if the chromophores are deviated outwards of the
light electric vector into the perpendicular position, the difference §« between
illuminated and dark parts would be greater for the perpendicular polariza-
tion. However, a more precise analysis of a simple three-dimensional model 88
shows that what we observe here is a typical situation when the angular
distribution of chromophores in darkness is close to isotropic (the case of the
chiral optically isotropic TGB A* phase, IsoSm*).
At the same time, only low-contrast s,s- and p,p-gratings could be written
at X = 514 nm in a virgin 3-um-thick film of another copolymer, KW40, using
2
a power of the output Ar laser beam as high as 25 W/cm (writing time
7S
t w-2s). However, as was mentioned in Section 5.3, pretreatment with un-
polarized UV light increases the proportion of long-living os-isomers and
amplifies therewith the photo-optical response, the effect being especially
pronounced for the copolymers of KW series with lower longitudinal dipole
moment of the photochromic groups. Thus, film sensibilization with UV pre-