What can light-based techniques do ever better? Although it is not widely known, recently Mueller matrix ellipsometry (MME) is being applied as a powerful tool for the characterization of depolarization effects, optically anisotropic materials (both natural and artificial), diffractive nanostructures, critical dimension (the critical dimension is the smallest feature size in an electronic device), remote sensing and biomedical diagnosis. MME is an exciting example of how the interaction of light with matter can be exploited to unveil the properties of materials spanning a range from the inorganic world to bio-tissues.
Mueller matrix ellipsometry is an excellent non-destructive and non-invasive method that provides a complete description of the polarizing properties of a sample, including the description of depolarization, diattenuation and retardance, and makes it possible to determine structural parameters by solving 16 linearly independent equations for polarization states.
The design of instruments for MME began in late 1970 and since then notable progress has been made developing instruments that enable real time spectroscopic measurements, widening the spectral range from the visible-ultraviolet to the infrared, allowing imaging, and approaching nowadays tomographic Mueller matrix scatterometry. Simultaneously, significant advances have been made in electromagnetic as well as phenomenological modelling for interpreting Mueller matrix measurements.
Within this impressive progress, challenges are still posed by the sensitivity of the method to complex characteristics of ultrathin films and nanostructures.
In this Optics Express article Ndoing and co-authors propose a very interesting approach to enhance the sensitivity of Mueller ellipsometry to ultrathin polymeric films in the mid-infrared, using an appropriate space layer. After surveying the principles of polarization measurement, a formulation of the polarimetric measurement and data reduction process is presented, which readily handles test samples of ultrathin polystyrene on silicon substrates with a SiO2 spacer. It is a very simple but effective method to circumvent the low absorption values of the vibrational resonances and the low signal to noise ratio.
My hope is that experimentalists will uncover more and more the potential of Mueller spectroscopic studies of vibronic properties of ultrathin films.
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