A heuristic view of NLO processes in conjugated polymers

Shahab Etemad

doi:10.1117/12.2322311

1 February 1992 A heuristic view of NLO processes in conjugated polymers

Shahab Etemad

Proceedings Volume 1599, Recent Advances in the Uses of Light in Physics, Chemistry, Engineering, and Medicine; (1992) https://doi.org/10.1117/12.2322311
Event: Recent Advances in the Uses of Light in Physics, Chemistry, Engineering, and Medicine, 1991, New York, NY, United States

Abstract

We have used the nonlinear optical response of conjugated polymers to study the electronic structure of this class of one-dimensional semiconductors. These studies fall into two categories: Pump-probe experiments where a weak probe monitors the change in the linear optical response, and multiphoton processes such as third harmonic generation (X(3)) and two-photon absorption (TPA). In the first category of experiments we discuss a set of picosecond experiments where we measure the electronic response of the 1-D exciton in polydiacetylene thin films to a pump which is tuned to in, near and off resonance with respect to the excitonic absorption. These experiments provide information on the lifetime and the size of the 1-D exciton, and demonstrate how it couples to the optical field through the optical and phononmediated Stark effects. In the second category of experiment we discuss the consequences of intensity dependent change in the phase (n2) and amplitude (TPA) of optical field as high intensity electromagnetic field propagates through the nonlinear polydiacetylene waveguides. We find TPA is a sensitive measure of the two-photon accessible (covalent) states in a similar manner that the linear absorption is a sensitive measure of the one-photon accessible (ionic) states. Comparison of the two- and one-photon absorption spectra in polydiacetylenes indicate that the covalent gap is smaller than the optical gap and the optical nonlinearity is dominated by states well above the optical gap. These results are strikingly different from predictions of theoretical models that treat Coulomb interaction perturbatively. We conclude that the presence of an excitonic absorption in polydiacetylenes implies only a nonvanishing Coulomb interaction, and NLO spectroscopy is essential in uncovering the extent of many body effects in this class of 1-D semiconductors.

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Shahab Etemad "A heuristic view of NLO processes in conjugated polymers", Proc. SPIE 1599, Recent Advances in the Uses of Light in Physics, Chemistry, Engineering, and Medicine, (1 February 1992); https://doi.org/10.1117/12.2322311

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KEYWORDS

Nonlinear optics

Absorption

Polymers

Excitons

Semiconductors

Harmonic generation

Multiphoton processes

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