In Situ Characterizations of Porous Media for Applications in Biofuel Cells: Issues and Challenges - Porous Media: Applications in Biological Systems and Biotechnology - page 503

P A

p

s

Polarizer

Ana lyser

Compensator

Sample

FIGURE 13.9
Schematic of an imaging ellipsometer, showing the polarized laser source and
the detection methods. When operating under null ellipsometric condition,
the ellipsometric angles (Ψ and ∆) can be measured with a range of AOI to
derive d and N . (Courtesy of Accurion, Inc. 2009.)

these potential diculties, ellipsometry remains as an attractive approach to
study interfacial properties and thin porous media, due to the molecular-level
sensitivity, noninvasiveness, and in situ capability, in combination with well-
established optical model, that allows monitoring the dynamic changes at the
surface/interface, without labeling the species of interest.
Successful practice of ellipsometry relies on a proper interpretation of the
measured results. Although the ellipsometric measurements directly determine
the changes in the optics at the interface/surface, the understanding of the
phenomenon however comes from the optical model interpretation. Indepen-
dent verification of the interpreted results from other supplementary data is
essential. Such an independent verification is, however, dicult due to limited
availability of supplementary data from few other techniques. Fortunately,
some earlier work (Benesch et al. 2000) showed that ellipsometry is at least
as accurate as other state-of-the-art techniques.
Figure 13.9 shows an imaging ellipsometer schematic, which details the
light source and detection mechanism. In the ellipsometric measurements, a
pair of ellipsometric angles (Ψ and ∆) is often measured along with wave-
length, AOI, time, and spatial references. A monochromatic linearly polarized
beam is radiated on and reflected from the sample, while polarization and
intensity of the incident and reflected beams determine the sample's Ψ and
∆. Delta (∆) specifies the phase shift ( δ ) between the parallel-to-surface (s)
and perpendicular (p) components of the incident ( δ i ) and reflected ( δ r ) beam.

∆= δ i − δ r