Optical characterisation of 6T and 4T single crystals by ellipsometry; Anisotropy and crystalline structure

Ellipsometry and absorption spectroscopies have been carried out on sexithiophene (alpha-6T), and quaterthiophene (alpha-4T) single crystals frown at High Temperature (HT) or Low Temperature (LT). Effective Optical Indices (EOI) neff and keff have been obtained in two positions of the crystals, b axis being either perpendicular to the yz incidence plane (alpha=0 degrees) or parallel to Oy (alpha=90 degrees). The spectra contain only a strong absorption feature around 3.4eV and 3.7eV For 6T and 4T respectively. We have compared experimental data and those obtained by simulation of EOI considering a biaxial semi-infinite medium with its three orthogonal principal axis parallel to Ox, Oy, Oz directions. The strong anisotropy of the 4T and 6T LT forms induces an anomalous dispersion curve of neff and a blue shift of keff (similar to 0.1-0.3eV) regardless of the crystal set. The weak anisotropy of the HT form, leads to an anomalous dispersion and a blue shift for keff (similar to 0.1-0.3eV) only for the crystal set alpha=0 degrees. The absorption anisotropy or (kz-kx,y) is related to the angle between the L molecular axis and the crystal plane (b,c).