Imaging Mueller Matrix Ellipsometry
The Mueller-Stokes formalism is a mathematical representation of polarized light and polarizing properties of optical matter. The light intensity is described via the Stokes vector and the polarization changing properties are described by a 4 by 4 Mueller matrix. The Stokes vector describes the state of polarization based on time averaged intensity measurements, which allow the source of light to be partially incoherent.
Mueller matrix ellipsometry (MME) is a technique which can measure the complete Mueller matrix of a sample and is being developed and used in our laboratory to study the polarization response (retardance, dichroism, depolarization) of colloids, partially ordered (self assembled nanostructures, rough surfaces) and ordered surfaces (e.g. optical gra¬tings). The group aims at being able to develop research grade proto-types of the state-of the art of optical techniques, particularly related to spectroscopic and laser based methods in polarimetry. The latter involves opto-mechanical design, optical design, instrumentation/acquisition hardware and software in addition to the algorithms for analysis and related software. We here describe recent results with relevance for studies of solar cell materials and the related materials science.
The liquid crystal based systems have been found to be best suited for MME imaging, visible to NIR. A first prototype, implemented at the NTNU, of a Fe-LC based NIR-MME appears promising as the fastest and most robust system known to date.
Some recent examples on an application is shown in the Figure below. Here a force is applied to a transparent bulk glass material. A set of Mueller matrix images where each "sub-image" corresponds to an element of the Mueller matrix, is collected. From the raw data (middle 16x16 images) one can calculate the retardance and its direction distribution within the materials. One can observe how the the direction of the slow axis arranges vertically under pressure spot in the lower image.
This technique is currently developed for studies of wafers and 3G solar cell materials in collaboration with SINTEF and other researchers at NTNU.
Mueller matrix ellipsometry and optical engineering laboratory
There are two unique home-built Mueller matrix ellipsometers. One employs lasers (405, 532, 633, 1550 nm) useful for studies of specular and non-specular scattering processes. The other is an image based system allowing full video frame-rate Mueller matrix characterization at wavelengths between 800-1900 nm.