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angles, but there is a certain angle at which the resonance conditions for the
excitation of surface plasmons are fulfilled. If the metal layer is very thin, the
resonance conditions are influenced by the material present at the back side
of the metal layer; in particular, the resonance conditions are sensitive to the
refractive index n z (perpendicular to the surface) of the surrounding medium.
The trans to cis isomerization in photoactive LBK films results in a change
of the refractive index, and therefore, the photoisomerization results in a
change of the resonance conditions. Irradiated and nonirradiated areas can be
distinguished with high lateral resolution with this method. The use of
photoactive LBK films as material and surface plasmon microscopy as read-
out technique have been suggested for application in optical data storage. 46
Observation of the reflected intensity at a fixed angle allows the monitor-
ing of changes in the refractive index with time, and with that the structural
changes. In such an experiment, the reflectivity of a glass/metal interface
covered with an LBK film of a photochromic poly(L-glutamate) 38 (n = 6)
shows an increase in the very beginning of UV irradiation (see Figure 6.22),
As the irradiation proceeds, however, the reflectivity sharply drops.
The increase in the beginning is due to the trans to cis isomerization and
the concomitant change in the refractive index. As long as the content of the
as-isomer is not too high, the structure is not changed; but as soon as the
as-content exceeds a critical value, the structural changes set in. At this
point, the optical properties of the LBK film change in such a way that the
reflectivity of the glass/metal interface drops. At the photostationary state,
the reflectivity reaches a plateau. Upon cis to trans isomerization by irradia-
tion with visible light, the reflectivity is restored partially, but it does not
reach the original state. In subsequent irradiation cycles, it can be switched
92
between the plateaus. These results indicate that the change of the LBK film

1.2
1.0

"5" 0.8
CO
-t 0.6
<D
•§ °- 4
01
0.2

0.0
100 200 300 400
Time [s]
FIG. 6.22 Reflectivity of a glass/metal interface covered with 12 layers of poly(t-glutamate) 38 (n =
6) in a surface plasmon experiment with fixed angles corresponding to the resonance condition for the
nonirradiated films as a function of the irradiation time at the first and subsequent irradiation cycles
(UV = irradiation at 365 ± 50 nm; vis = irradiation at 440 ± 50 nm). (Reproduced with permission from
reference 92).