Ellipsometry and polarimetry – classical measurement techniques with always new developments, concepts, and applications

Ellipsometry is a matured experimental method, whose roots reach back into the early phase of modern optics itself. It is often attributed to be invented by Paul Drude in the last decade of the 19th century, but similar techniques had already been applied for years before Drude started his work.

The actual term “ellipsometry” is in use since the 1940s. Interestingly, it started in context of a work describing a biological application. It is noteworthy that this was at a time when modern physics of solid phases, especially of semiconducting materials, was expanding rapidly. The ellipsometer was immediately welcomed by the solid state and surface research community as the ability to investigate surfaces, interfaces, and thin layers showed up to be essential. Ellipsometry is a method benefiting much from numerical calculations and modelling concepts. The connection between solid state physics and ellipsometry is an example of a self-strengthening innovation cycle in science and technology. Especially the strong increase in computational power was accompanied with an increase of ellipsometry in research and a rapid expansion of the community. Ellipsometry benefits from microelectronics and digital technology. Vice versa, it enables the development of better electronic devices. The hardware which is the foundation of the digital age would not exist without the development of ellipsometers and their manyfold applications decades earlier.

Ellipsometry is the realisation of polarimetry as a reflection experiment. All polarimetric techniques rely on the measurement of the change in polarisation state of electromagnetic waves upon transmission, reflection, or scattering. Ellipsometry is extremely sensitive towards thin layer properties through its ability to detect phase shifts. In ellipsometry, the change of a probe light’s polarization state is measured, giving access to the Jones or Mueller Matrix of a sample. By model analysis of the experimental data by means of transfer-matrix techniques, information on sample structure and the optical response functions (the complex dielectric tensor and even the optical gyration tensor) of the sample’s constituting materials can be derived. This way, the properties of very thin layers are accessible, together with structural information of layers and bulk phases. The advantages of ellipsometry and polarimetry are widely known: the relative ease of use as a measurement scheme, combined with unprecedented sensitivity to tiny changes in optical properties and structure, especially of the surface. The short measurement times make ellipsometry suitable for large-scale scanning and imaging as well as routine quality assessment.

The ellipsometry community has always been connected by their common methods, searching for new fields of applications, driven by better theoretical understanding and analysis capabilities. Ellipsometry has made the development from monochromatic to full variable angle-of-incidence spectroscopic measurements and to full Mueller-Matrix representation of the polarimetric data. The current development moves the method from single point measurements and scanning to full imaging. Today, the spectral range has expanded up the edge of soft X-rays to the THz radiation regime without significant gaps. The basic concepts and analysis methodologies are very similar over this wide spectral range, bridging the otherwise very diverse respective research fields as a strong unifying power of the polarimetry community. Contemporarily, data science driven methods and modelling-free approaches are on the horizon, providing new analytical opportunities for the easily obtainable large amounts of difficult to analyse data available today.

With this Special Issue about ellipsometry and related techniques, we hope to bring more attention to this method and advance and propagate it to be used by a broader community. We have collected a good mixture of articles: some texts are more in the line of users’ tutorial and best practice guides; others are intended to show recent developments of the method. With this collection, we also hope to show the generally rapidly expanding possibilities of ellipsometry and polarimetry to draw attention of new users and previously unrelated communities to this valuable tool.