Optical second harmonic generation in a low-bandgap polymer

•We characterize optical second harmonic generation in a low-bandgap polymer (PCPDT).•PCPDT displays unexpectedly large nonlinear optical susceptibilities.•PCPDT has a very low absorption in the relevant wavelength region.•We demonstrate favorable thermal and temporal stability of the material.•We show the promise of low-bandgap polymers for nonlinear optical devices.

Recently, much research has been performed on developing low-bandgap polymers for e.g. harvesting solar energy. In the quest to improve these properties, little attention has been paid to their nonlinear optical properties, despite their interesting linear optical spectra and structural similarities to certain nonlinear optically active compounds. We characterized the optical second harmonic generation of corona poled films of poly(cyclopenta[2,1-b;3,4-b′] dithiophen-4-ylidenedioctylmalonate). The unexpectedly large nonlinear optical susceptibilities and the thermal and temporal stability of the material compare favorably to other novel nonlinear optical materials despite the lack of a donor-acceptor dye. Additionally, the polymer displays a very low absorption in the relevant wavelength region. These results demonstrate the promise of these materials for nonlinear optical devices.

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