Dielectric Tensor of Microtextured Squaraine Thin Films Obtained by Imaging Mueller Matrix Ellipsometry

Manuela Schiek, S. Funke, M. Duwe, P. H. Thiesen, K. Hingerl, F. Balzer.

di-Iso-Butyl-anilino-Squaraine SQIB

transition dipole moment (TDM) along long molecular axis

Frenkel excitons incl. Davydov splitting ≥ 2 inequivalent molecules per unit cell

orthorhombic polymorph Pbcn CCDC 1567104 (4 inequivalent molecules in primitive unit cell) spincasting + thermal annealing @ 180°C formation of rotational domains with (110) orientation

Davydov splitting: 4 inequivalent molecules = 4 Davydov bands (2x UDC & 2x LDC)

linear combination of TDMs within one stack + interstack combination

SUM => LDC1 along b-axis & LDC2 along a-axis
DIFFERENCE => UDC1 along c-axis & UDC2 = 0 (dark state)

diagonal tensor for orthorhombic crystals: optical indicatrix is congruent with crystallographic axes

oscillator model to ensure Kramers-Kronig consistency negative real permittivity in vicinity of the strong resonances explains quasi-metallic shine of SQIB platelets

projected view onto (110) plane Davydov bands are polarized mutually perpendicular projected Davydov splitting ≈ 250 meV

spatially resolved polarized absorbance measured (circles) is well reproduced by calculated (solid lines) spectra transfer matrix optical calculations using ε_a and projection of ε_B and ε_y onto (110) plane

UDC along c-axis, effective LDC along projected a-b-axes

NanoFilm_EP4 imaging Mueller matrix ellipsometry system, Accurion GmbH, Göttingen

single compensator (PCSA-configuration): 11 normalized MM elements knife-edge illumination to suppress incoherent backside reflections collimated light beam spot diameter 1 mm lateral resolution 2 µm (λ = 400 nm) through microscopy objective, no focusing of incident beam selection of multiple ROIs reflection & transmission scans Theta Stage (TS): azimuthal Φ-rotation

Funding & Acknowledgement

Linz Institute of Technology (LIT-2019-7-INC-313 SEAMBIOF) PRO RETINA Foundation (especially Franz Badura)
We are indebted to Dr. Matthias Schulz and Prof. Dr. Arne Lützen, University of Bonn, for providing SQIB powder.