Electroabsorption study on octithiophene thin films towards third-order nonlinear properties

Octithiophene (8T) based thin films are investigated by electroabsorption spectroscopy in terms of influence of molecular orientation. Oriented 8T thin films are prepared to obtain either an average upright or horizontal orientation of molecules on the substrate. Changes in optical absorption coefficient of 8T based thin films are measured and analyzed under polarized light to characterize the dichroism induced by the dominant molecular orientation in both thin films. The results suggest an electroabsorption (EA) response of the weak Davydov component conferring nonlinear optical properties to both oriented thin films. The spectral profile of EA responses for an Isotropic film is analyzed in terms of derivatives of optical absorption spectra and the results suggest that the transitions at 500-600 nm present a neutral (Frenkel exciton) character. Third-order nonlinear susceptibilitiy χ⁽³⁾ (-ω; ω, 0, 0) is also evaluated, and 8T presents a high value of χ⁽³⁾ - 45 × 10^-12 esu.