Nonlinear optical waveguide spectroscopy of a conjugated polymer: poly(p-phenylenevinylene)

We have measured intensity-dependent prism coupling of poly(p-phenylenevinylene) thin films with picosecond laser pulses in the range 750–1600 nm. We developed a refined evaluation method that yields the signs and absolute values of the nonlinear refractive index $n_2$ and the nonlinear absorption coefficient ${\alpha }_2.$ Strong two-photon absorption levels were found at 2.8 and 3.1 eV. We observed a resonance at 1.3 eV that behaves as saturable absorption, and several additional resonances and sign changes of $n_2$ at even lower photon energies. We demonstrate reversible refractive-index changes $\mathrm{\Delta }n=+0.002$ at 910–950 nm on a picosecond time scale. Even larger $\mathrm{\Delta }n$ appear feasible.