Ultrafast Spectroscopic Holography in Conjugated Polymers

We present a detailed study of femtosecond transient holography in conjugated polymers and blends, using tunable nondegenerate four-wave mixing. Combined with spectral transient absorption studies, these experiments determine the complex photoinduced index of refraction over the photon energy range from 1–2.5 eV. By performing a Kramers-Kronig transformation of the broad-band photoinduced absorption spectrum, a reliable spectrum of the diffraction efficiency can be obtained. The samples used include luminescent polymers from the poly-phenylene-vinylene (PPV) family, as well as charge transfer blends of PPVʹs with soluble methanofullerenes. One can achieve diffraction efficiencies on the order of one percent for a single femtosecond pulse, and by controlling the degree of charge transfer it is possible to tune the time constant over the range from 1 to 100 ps. These materials represent a potential holographic information processing density up to 12 orders of magnitude higher than other holographic nonlinear optical materials.