Page 135 - Photoreactive Organic Thin Films

I I 4 ZOUHEIR SEK.KAT AND WOLFGANG KNOLL

interchanged when the initial UV polarization was rotated from vertical to
horizontal (see Figure 4.3B). The same amount of dichroism was observed for
the same amount of UV irradiation, independent of the direction of UV
polarization. This crossed dichroism could then be erased and rewritten as
described previously.
The light-polarization sensitivity of this photoisomerization reaction is
discussed earlier in this book. It is worth recalling that, in principle, for high
intensities of irradiating light, or for long irradiation times, even the molecules
aligned perpendicular to the polarization of the irradiating light will be
excited. The system will then be saturated and isotropy will be restored. We
have accounted for this phenomenon in our experiments by recording that
the amount of photoinduced dichroism increases with increasing irradiation
times until a maximum value is reached. Longer irradiation times progressively
produce less dichroism until isotropy is finally regained.

4.2.3 Photo-Modulation of the Optical Thickness of Molecularly Thin Layers
We used surface plasmons (SPs) to estimate the thickness of the azo-silane
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SAMs. Details about surface plasmons and guided waves can be found
1 51
elsewhere. ' Briefly, surface plasmons are transverse magnetic waves that
propagate along a metal dielectric interface, their field amplitude decaying
exponentially perpendicular to the interface. SPs and guided waves can be
introduced by the Kretschmann configuration setup, wherein a thin metal
film (~ 50 nm) is evaporated on the base of a glass prism. The metal film acts
as an oscillator that can be driven by the electromagnetic wave impinging upon
that interface, and a resonance phenomenon that depends on the incidence
angle made by the wave with the interface can occur in the attenuated total
reflection (ATR) scan.
In the ATR scan, the incident light is totally reflected above a critical angle
9 C. Dips in the reflectivity curve above 9 C indicate the resonant excitation of
SPs at the metal/air interface, or guided waves in films deposited on top of
the metal layer (see Figure 4.4). The coupling angle depends on the resonance
condition for SPs and guided waves; it is possible to excite two sets of guided
modes in waveguide films. The transverse electric (TE) modes are sensitive to
the in-plane refractive index, n y, of the waveguide, and the transverse magnetic
(TM) modes are sensitive to both the in-plane refractive index in the guided
wave propagation direction (n x, x perpendicular to y) and the out-of-plane
refractive index, n z, in the direction normal to the waveguide plane. Study of
the resonance angles enables accurate determination of the optical thickness
(refractive index x thickness) of thin coatings by SPs, and the anisotropic
refractive indices, n x, n y, and n z, and thickness, d, of thicker coatings by waveguide
modes. Waveguide spectroscopy is used later in this chapter to study thick
layers of azo-polyglutamates and azo-polyimides.
For the azo-silane layers, assuming a value equal to 1.45, at 632.8 nm, for
the refractive index, « z, normal to the plane of the layer, our SAM could best
be described by a layer thickness of 9 A (i.e., an optical thickness of 13.1 A).
This is considerably thinner than would be expected for a fully extended azo-
silane molecule (ca. 30 A). This may be better understood by comparing the