Third order nonlinear optical properties of polycrystalline octithiophene thin films studied by electroabsorption spectroscopy

Electroabsorption (EA) studies at room temperature on polycrystalline thin films of α-octithiophene (8T), the longest non-substituted model oligomer of polythiophene, are reported. The EA spectra obtained on isotropic 8T films deposited on top of coplanar electrodes exhibit several intense and narrow peaks in the absorption region and on the absorption edge. They are compared with the derivatives of absorption spectrum, and it is shown that the excitation in the longer wavelength absorption presents a Frenkel exciton character. Third-order nonlinear optical susceptibility χ⁽³⁾ (-ω ω, 0, 0) is evaluated to be in the order of 10^-10 esu at its maximum.