The Dielectric Tensor of Micro-Textured Organic Thin Films Obtained by Imaging Mueller Matrix Ellipsometry

Crystalline organic thin films often exhibit linear dichroism and birefringence. By structural design of the molecular building blocks, advanced functionality can be introduced such as asymmetry and chirality resulting in nonlinear optical properties or circular dichroism. Here, the linear optical properties of a single polymorph from a model anilino squaraine with isobutyl side chains (SQIB) are explored. The polymorph shows a distinct polycrystalline thin film texture and crystallizes into polycrystalline platelet-like domains with an orthorhombic unit cell, making it ideal for imaging ellipsometry. Atomic force microscopy provides local morphology and in-plane orientation. The anisotropic optical response of the platelets is dominated by the Davydov-split absorption, the lower Davydov component absorbing within the deep red. The full biaxial dielectric tensor is determined by imaging Mueller matrix ellipsometry. Simultaneous fitting of transmission and oblique incidence reflection Mueller matrix scans combined with the spatial resolution of an optical microscope allows to accurately determine the dielectric tensor components by analyzing multiple domains with different orientation. The latter allowed parameter decorrelation via scans with variable azimuthal rotation. Oscillator dispersion relations model well the tensor components. Strong intermolecular interactions cause the real permittivity for all three directions to become strongly negative near the excitonic resonances, which is appealing for nanophotonic applications. In addition, the optical properties are correlated to electronic properties on the nanoscale obtained by Kelvin probe force microscopy (KPFM).