Optical response and emission waveguiding in rubrene crystals

The optical response of rubrene crystals is described by providing the full UV-VIS dielectric tensor. Consistently with the molecular and crystal symmetries, the lowest crystal transition originates from an $A_u$ molecular transition, it is polarized along the $c$ axis (normal to the larger crystal face), and it is the origin of the emission, which is self-guided towards the edge where the corresponding polarization and intensity angular distribution are detected. By contrast, the $B_u$ molecular transitions give rise to Davydov states described by the other two components of the diagonal dielectric tensor.