Multi-photon absorption in polydiacetylenes adsorbed on metal nanostructures

We report on the measurement of the multi-photon absorption dispersion of polydiacetylene-decorated silver nanoparticles in water solution. They were prepared by self assembly of the monomer 10,12-pentacosadiynoic acid (PCDA) onto pre-formed chitosan-stabilized Ag nanoparticles (Chit-AgNps) followed by photopolymerization of the diacetylenic outer shell. Z-Scan technique with fs pulses in open aperture configuration was employed: the spectral range covered the region between 1150 and 1350 nm. We tentatively attribute our results to a two photon state (2Ag) which peaks at 600 nm. This result is consistent with the fluorescence spectrum that shows an emitting state lying at about 650 nm, in addition to a peak at 568 nm which is attributed to the emission of an orange form of the polymer. However we cannot rule out the possibility that we are also dealing with higher order absorption processes and further studies are needed to clarify this point and to provide a deeper insight into the origin of the emission spectrum