Nonlinear optics of conjugated polymers

doi:10.1016/0379-6779(89)90756-X

Synthetic Metals

Volume 28, Issue 3, 20 February 1989, Pages D645-D653

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Abstract

We present the results of two types of measurements of the nonlinear optical properties of conjugated polymers. First, we have used third harmonic generation (THG) to probe the nonlinear susceptibility of polyacetylene. The magnitude of $δ^{(3)(3ω;ω,ω,ω)} is (4±2)× 10^{−10} esu with h ̵ ω = 1.17 eV$ the only important component of χ⁽³⁾ is that associated with π-electron motion along the polymer backbone. Comparison of $cis-$ and $trans$ -(CH)_x shows that χ⁽³⁾|trans is 15–20 time larger than χ⁽³⁾|cis. Second, we have employed time-resolved waveguide modulation to measure the magnitude and sign of the nonlinear refractive index (n₂) of polydiacetylene-4BCMU. We find that n₂ is negative, with magnitude of approximately 10⁻⁷(MW/cm²)⁻¹. The response time of this nonlinearity shows a fast (resolution-limited) decay, followed by a slower (∼2.5 ps) decay time. These results are consistent with bleaching of the excitonic absorption as the mechanism of the nonlinearity.

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The effect of side chain substituents on third-order optical nonlinearity of conjugated polymers: A theoretical study
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We present a detailed investigation concerning the influence of phenyl side groups, adjacent to a carbon backbone, on the third-order nonlinear optical susceptibility χ⁽³⁾(ω) of conjugated polymers. A theoretical calculation of χ⁽³⁾(ω is carried out within the simple two-band model of the electronic structure of the polymer based on a tight-binding approximation. Our treatment takes into consideration two sources of optical nonlinearity: the interband transitions of π-electrons and combined (interband—intraband) ones. The influence of the substituent groups upon the electronic structure of the polymer is modeled by changing the Coulomb integral on the sites of the main chain to which the side groups are attached. We adopt the Genkin—Mednis approach to derive analytical expressions for χ⁽³⁾(ω) responsible for the third-harmonic generation, the intensity-dependent index of refraction and the two-photon absorption. The spectra χ⁽³⁾(ω) are evaluated numerically for all these effects and the resonance structure of the spectra is analyzed depending on the distance between the side groups and their coupling strength with the backbone. It is found that the interaction between the side groups and the backbone results in a change of the sign of χ⁽³⁾ (0) from positive (which is predicted for polyacetylene) to negative and leads to an enhancement of the χ⁽³⁾(0) coefficient with increasing side-group separation along the backbone. The resonance behavior of χ⁽³⁾(ω) in the transparency region of the polymer also reveals remarkable differences from that observed experimentally in polyacetylene. Our results show that the optimal positioning of the substituent groups along the backbone may be promising for engineering new polymer materials with significantly enhanced nonlinear optical properties.
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The synthesis of poly(3, 3″- and 3′, 4′-dialkyl-2, 2′:5′, 2″-terthiophene)s possessing a full ordered regiochemistry of bonding and substitution, as well as a conjugated chain with the minimum possible amount of steric interactions between the substituents, is described. The synthesis was achieved through the simplest chemical method, namely the oxidative polymerization reaction based on FeCl₃ as oxidant. The structural regularities of the polymers have been confirmed by Fourier transform infra-red (FTi.r.) and ¹H nuclear magnetic resonance (n.m.r.) spectroscopy. Experimental conditions of special interest for improving the yields and molecular weights of the neutral soluble polymers are discussed.
Random disorder and nonlinear susceptibility of conjugated polymers
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Non-linear optical properties of conjugated polymers in the SSH model: third harmonic generation
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Time-resolved photoinduced absorption spectra of vacuum-evaporated thin films of α-conjugated thiophene oligomer (α-nT) exhibit a transient bleaching with strong oscillations of almost constant energy spacing (ΔA=0.2 eV) in the intra-bandgap region that can be related to the fine structures currently observed in the unperturbed absorption spectra. Exponential decay of the time-resolved absorbance change with time constants of 1.3±0.1 μs, 550±20 ns and 310±10 ns, due to the lowest triplet state in the α-4T, α-5T and α-6T respectively, was identified.

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^★: Permanent Address: Sandia National Laboratories, Albuquerque, NM, 87185

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Polydiacetylenes and nonlinear opticsNonlinear optics of conjugated polymers

Abstract

Synth. Met.

Opt. Commun.

Phys. Rev. Lett.

Phys. Rev. Lett.

Phys. Rev. Lett.

Phys. Rev.

Synth. Met.

Polydiacetylenes and nonlinear optics
Nonlinear optics of conjugated polymers