Page 139 - Photoreactive Organic Thin Films

I jg ZOUHEIR SEKKAT AND WOLFGANG KNOLL

an a-helix that is stabilized by hydrogen bonds, with the side chains pointing
to the outside. These rigid-rod polyglutamates with flexible side chains
constitute a type of the so-called hairy rod polymers designed by Wegner. 53
57
The synthesis is described elsewhere.
We used the waveguide spectroscopy technique to study the optical
properties of the LBK azo-polyglutamate polymers. Waveguide films could be
prepared by transferring up to 156 monolayers of azo-polyglutamates onto
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the silver-coated glass substrate using the vertical dipping method. Both TE-
and TM-guided light modes could be coupled into these LBK structures by
means of the ATR method. When an LBK azo-polyglutamate sample is irradiated,
as shown in Figure 4.7, by unpolarized UV light (360 nm}, trans—»cis photo-
isomerization takes place, and a waveguide mode shifts its angular position
to lower incidence angles (see Figure 4.8). After blue light (450 nm) irradiation
and the subsequent cis—»trans back photo-reaction, the mode recovers exactly
its initial angular position before UV irradiation. The transient behavior of
the photoinduced switching process is reported in Figure 4.8 which shows the
time evolution of a TM mode guided in an LBK structure made out of 156
monolayers of P^IQ. A similar modulation behavior (not shown) of a surface
plasmon mode was observed when 20 LBK monolayers of P 2;io were
transferred, as a thin coating, to the silver-evaporated glass substrate. The
transients were accomplished by alternating the irradiation between UV and
blue light. Figure 4.8 exhibits the efficiency of the optical switching process
and shows that it can be repeated for several cycles without fatigue of the
LBK structures.
Figure 4.9 shows the evolution of in-plane (n m n y) and out-of-plane (« z)
refractive indices of the 0.37 um (156 monolayers)-thick P 2,io LKK structure,
under successive UV and blue unpolarized light irradiation cycles. The mean
refractive index (n = (n x + n y + n z)/3) is also reported in this figure. In all the
columns in Figure 4.9 (labeled New, UV, and B, and corresponding,
respectively, to the LBK structure before any irradiation, and after UV and
blue-light irradiations), a small and persistent in-plane anisotropy (n y-n x) can
be noted between the dipping direction (y), and the direction x perpendicular

FIG. 4.7 Waveguide spectroscopy experimental arrangement in the ATR-Kretschmann setup. The
probe is a 632.8 nm He-Ne laser beam, and the reflectivity of the sample is recorded as a function of
the incidence angle. The irradiation (pump) beam direction of propagation is perpendicular to the plane
of the sample.