Enhancing the Third-Order Nonlinear Optical Properties of Porphyrins and Molecular Wires

The third-order nonlinear optical (NLO) properties of indium tin oxide (ITO) thin films, Fe3+, Mn3+, Co2+ 5,10,15,20-tetrakis-4hydroxytetraphenyl)porphyrin (TPP) films, and a series of ethynyl-linked azobenzene oligomers were investigated using degenerate four wave mixing (DFWM) with 100 fs laser pulses. To measure the NLO of ITO thin films, A DFWM method for measuring thin films on thick substrates was refined for the characterization of films less than 100 nm thick and applied to ITO films ~25 nm thick. It was found that the third-order nonlinear susceptibility of ITO, χ(3)ITO, is purely electronic at 900 - 1300 nm (11000 - 7700 cm-1) and has a value of (2.16 ± 0.18) x 10-l8 m2 V-2. The χ(3)IT0 value reaches (3.36 ± 0.28) x 10-l8 m2 V(sup>-2 at 1500 nm (6700 cm-1) due to two-photon absorption by free carriers (electrons). Ultrafast electron relaxation was also observed. The ~100 fs lifetime of this process could reflect electron scattering in the conduction band. This DFWM method was also used to investigate the two-photon properties of ~500 nm thick electropolymerized films of Fe3+, Mn3+, and Co2+ TPP in the near-IR spectral region. Metalloporphyrins with strong charge transfer (CT) transitions inthe linear absorption spectra also show enhanced two-photon absorption. (Metalloporphyrin two-photon absorption cross section, δ, increases >10 times over that for the metal free porphyrin.) This effect was attributed to a two-photon induced charge transfer between the metal ion's d orbitals and the π-system of the porphyrin. Correlationof one- and two-photon absorption properties of transition metal porphyrins suggests a new and simple approach to improve organic materials for photonic applications. Finally, a series of oligomers consisting of ethynyl-linked azobenzene units was prepared using Pd-catalyzed cross coupling. The linear and nonlinear optical properties of the oligomers were investigated. The molecular second hyperpolarizability, γ, followed the power law γσ n2.12±0.05 (n is number of repeat units) for unusually large molecular lengths. The exceptional exciton delocalization length exceeds 360 conjugated bonds (>49 nm) and is attributed to the rigidity of the conjugated backbone.

Name thesis:degree_name

Doctor of Philosophy

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Chemistry

Year dc:date.available

2006

Author dc:creator

• Humphrey, Jonathan L.

Contributors dc:contributor

• Dr. Darius Kuciauskas

• nonlinear optics
• laser
• degenerate four wave mixing
• porphyrin
• Chemistry
• Physical Sciences and Mathematics

Statement dc:rights

• © The Author

Identifier

https://scholarscompass.vcu.edu/etd/969

OAI identifier oai:identifier

oai:scholarscompass.vcu.edu:etd-1968