Full-Tensor Magic Angle Spectroscopy

Linear Dichroism (LD) optical absorption spectroscopy historically has had substantial but limited application in various domains of science. In particular, full-tensor reconstruction has been tedious and difficult, usually requiring extensive measurements on single crystals at many orientations using a four-circle goniometer. As a consequence it is very seldom done. Here we propose, and test by numerical simulation, a simple, novel method of determining the full dipole optical absorption tensor of homogeneous planar films in real-time as a function of energy (or wavelength), while requiring only minimal additional time and instrumentation. The full-tensor spectrum, after construction from the experimental data, allows one to instantly calculate the absorption for any selected polarization direction, even those that are physically inaccessible to experimental measurement. Although our specific application in this paper is to X-ray Absorption Fine Structure Spectroscopy, the method should be adaptable to UV-Vis, IR, THz, microwave, and other wavelengths. A strength of this measurement modality is that full tensor data can be acquired using essentially the same sort of scanning geometry that is normally used for XAFS, with only a one discrete shift in spin axis orientation between groups of scans. The additional instrumentation needed to determine the five Fourier components of the signal at each energy is minimal; two angles gives up to ten parameters, while six are needed, and the others can be put to good use. Outside of XAFS, FTMAS also should be applicable to diverse scientific and technological areas such as oriented bio-molecular films, semiconductor and materials physics, and process control of thin-film photovoltaics and semiconductors.