IR-Mueller matrix ellipsometry of self-assembled nanopatterned gold grid polarizer

摘要

Graphical abstract A) Schematic view of the optical configuration of the spectroscopic infrared Mueller ellipsometer used in this study. B) AFM image of the surface of the sample, revealing the presence of highly oriented gold nanowires. C) Polar color map of the experimental spectroscopic Mueller matrices. Radial and angular coordinates correspond to the wavenumber and the sample azimuth, respectively. The spectral features appearing are due to the strong anisotropic dichroism of the sample. Display Omitted Highlights • Highly anisotropic gold nanowires on glass substrate are studied. • Spectroscopic Mueller matrices are collected in the mid-IR spectral range. • The effective dielectric function in the mid-IR is characterized. • Three approaches are used to model the optical response of the sample. • Ellipsometric measurements are sensitive to the “average” features of the sample. Abstract A gold nanowired grid polarizer is studied in the mid-IR range by means of spectroscopic Mueller matrix ellipsometry. This material is also an example of a so-called 2-dimensional hyperbolic metamaterial/metasurface, and produced by low-cost large area scalable manufacturing techniques. We present the IR spectroscopic Mueller matrix optical response of such sample for a full azimuthal rotation. It is observed strong in-plane anisotropy due to the gold wire grid. The main objective is to show a characterization methodology to reveal the complete polarimetric response over a large spectral range, providing information about the effective optical properties of the sample. Three different optical models are proposed that catch the main features of experimental data. The first one is based on the Rigorous Coupled-Wave Analysis (RCWA) assuming the profile of the sample to be periodic, second, a Generalized Effective Medium Approximation (G-EMA) assuming oriented gold ellipsoids aligned along the lines of the wire grid, and third, a biaxial dispersion model. Experimental data is compared with simulated data obtained from the three optical models and a comparative analysis between them is presented. The different models allow to retrieve information on the characteristic dimensions of the nanowires, and also to obtain the two in-plane components of the effective dielectric tensor of the nanostructured layer. The tensor component parallel to the nanowires shows a metallic response whereas the component perpendicular to them behaves as a transparent dielectric. Moreover, we have also shown that the nanowired grid behaves as a hyperbolic material.