Wavelength Dependence of the Complex Third-Order Nonlinear Optical Susceptibility of Poly(3-hexylthiophene) Studied by Femtosecond Z-Scan in Solution and Thin Film

Comprehensive studies of the third-order susceptibility of regioregular poly(3-hexylthiophene) in the wavelength range from 530 to 1600 nm for both thin film (with stacked chains interactions) and dilute solution (no chains interactions) were performed with the Z -scan technique. Negative nonlinear refraction was observed in the whole wavelength range of the measurements. The nonlinear absorption exhibits regions of two-photon and three-photon absorption, and on approaching the linear absorption region, a saturable absorption process is seen. It was found that for thin film the effective multiphoton absorption cross-sections σ 2 and σ 3 are 1.5 and 3 times bigger than for solution, respectively. Influence of the elongated conjugation length in the stacked polythiophene chains in thin film on the red shift of the multiphoton absorption cross-section spectra in comparison with those for polythiophene in solution is discussed.