Large third-order nonlinear optical response of porphyrin J-aggregates oriented in self-assembled thin films

• Published in: Journal of materials chemistry 2006-01, Vol.16 (16), p.1573-1578
• Main Authors: COLLINI, Elisabetta, FERRANTE, Camilla, BOZIO, Renato, LODI, Andrea, PONTERINI, Glauco
• Format: Article
• Language: English
• Subjects: Exact sciences and technology; Fundamental areas of phenomenology (including applications); Nonlinear optics; Optics; Physics
• ISSN: 0959-9428; 1364-5501
• DOI: 10.1039/b517591g

The preparation and characterization of a self-assembled material showing a high nonlinear response and good photostability to ultrashort laser pulses is presented. The material is built by alternate deposition of tetrakis(4-sulfonatophenyl)porphyrin diacid (H4TPPS2-) and poly(diallyldimethylammonium chloride) (PDDA) forming electrostatically self-assembled multilayers (ESAMs). UV-visible absorption and emission experiments show that in this matrix H4TPPS2- is present mainly in its J-aggregated form. Furthermore, linear dichroism experiments on a 3 bilayer film show a preferential alignment of the porphyrin aggregate with the J-band transition dipole moment parallel to the film surface. The two photon absorption (TPA) properties of these films are investigated with the Z-scan technique at 806 nm, employing 130 fs pulses. The samples exhibit strong nonlinearities with a very large two-photon absorption coefficient /-IPA of 50 cm GW-1. The origin of this large response is investigated. It has been already demonstrated that aggregation enhances the molecular TPA cross section of H4TPPS2- from 30 to 1000 GM in water solution thanks to cooperative effects. In a 20 bilayer film a further increase by a factor of 1.7 is observed and explained in terms of preferential alignment of J-aggregates in the multilayers.