Nanomanufacturing of dielectric metasurfaces supported by spectroscopic Mueller Matrix Ellipsometry

Abstract

Metamaterials are artificially structured composites. They can be used to modulate electromagnetic waves. The structural dimensions must be determined by being smaller than the working wavelength. Metasurfaces are multi- or single layer nanostructured surfaces. They can be used for the application as e.g. polarization beam splitter or lens. As planar optical objects, metasurfaces can substitute conventional optical elements. Also more complex applications are possible, like lens and polarization beam splitter combined and inducing an orbital angular momentum on a light beam. This project focuses on the fabrication of metasurfaces with the application as polarization beam splitter. Introducing a phase gradient along a surface like a silicon-air interface enables the control of the angle of diffraction. Varying the relative phase shift of adjacent meta-atoms in a manner that there is a different phase gradient resulting for orthogonal directions, allows to diffract orthogonal states of polarizations at different angles. Also right and left circularly polarized light can be generated through a metasurface and diffracted at different angles. Design, characterization and fabrication of such devices requires precise metrology. Spectroscopic ellipsometry is the main tool for optical characterization. It is a method relying itself on analysing how different states of polarizations are interacting with a sample. The working material for metasurfaces fabricated in this project is amorphous silicon and also to be named - air. A high permittivity contrast between these two allows that nanopillars are exhibiting waveguiding of light. Transverse electric and transverse magnetic resonance modes can exist within the nanopillars which are dependent of the pillars geometrical dimensions beside the refractive index. Based on these effects, one metasurface is for instance designed in a way that it shall diffract linear polarization of a principle orientation at -6.74° while the orthogonal linear polarization to the first on is diffracted at 0° at normal incidence. In order to realize such metasurfaces, a multi-step fabrication process needs to be carried out. Therefore ''ISO 5'' clean room work is necessary and the use of expensive machines like electron beam lithography. A fabrication process is successfully established. Multiple issues need to be solved through optimization of process parameters and the adaption; E.g. a chromium hardmask is eventually used although it was initially not intended. The fabrication approach established here can be directly transferred for the manufacturing of other amorphous silicon based metasurfaces. Electromagnetic field simulations are essential for the design of metasurfaces. These are typically non-analytical calculations and require some computational effort. Different commercial and open-source software exist which are using a variety of methods. The design is carried out by the project supervisor Morten Kildemo via finite element method. But also Fourier modal methods are carried out in order to verify the metasurface design by using another simulation method.