Ellipsometric investigations of interfacial phase transitions
Ellipsometry measures the change of the polarization state of light during reflection at an interface. This is expressed with two angles, Ψ and Δ, which measure the amplitude ratio and the phase shift of the normal (rs) and parallel (rp) reflection coefficient. Ψ and Δ are very sensitive to changes of the optical parameters at an interface:
Ellipsometry can be used on the one hand to measure the complex dielectric constants of a bulk media (medium 1 in the figure above). On the other hand it allows access to thicknesses and optical properties of thin films on top of a bulk medium. This was utilized for the study of the complete wetting transition in the binary metallic alloy system gallium-bismuth.
As examples we present studies of a complete wetting transition in metallic alloy systems (like Ga-Bi) and the oscillatory instability of a complete wetting layer exposed to a temperature gradient.
Complete wetting in Ga-Bi
See the following document for an introduction to wetting transitions: Benetzungsuebergang.pdf
The phase diagram of Ga-Bi is characterized by a liquid-liquid demixing regime. Although the Bi-rich phase has the higher density it completely wets the Ga-rich phase: a thin Bi-rich film spreads on top of the Ga-rich bulk
It is of interest to study the way how the Bi-rich wetting film develops as the demixing regime is approached from higher temperatures at a given composition (see the experimental path in the phase diagram). The results are shown in terms of the ellipsometric angles Ψ and Δ:
According the three-phase model the wetting film thickness d can be calculated:
The data show a continuous divergence of d as the temperature approaches two phase coexistence. The red line is a fit according d∞ln(Δμ-1) which is predicted for screended coulomb interactions.
S. Dogel, D. Nattland, W. Freyland, Phys. Rev. B 72 (2005) 085403
Oscillatory interfacial instability
A Ga-Bi sample was prepared in the two phase regime (see the green dot in the phase diagram). Under these conditions a Bi-rich thin layer completely wets the Ga-rich bulk. If such a system is observed continuously Psi and Delta show a surprising time dependence:
The oscillations of Psi and Delta can be related to an oscillation of the wetting film thickness as can be seen in the next figure (data from the dashed frame):
Interpretation of the phenomenon: Since the sample is only heated from below the radiation equilibrium and, thus, the temperature of the sample depends on the wetting film thickness. This is due to the higher emissivity of Bi in comparison to Ga. As the wetting film develops the sample temperature decreases a little bit. This in turn causes the film to thicken as far as it becomes unstable and thin again. Now a new cycle starts.
S. Dogel et al., J. Phys.: Condens. Matter 17 (2005) S3289