Controlling optical gain in semiconducting polymers with nanoscale chain positioning and alignment

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Abstract

We control the chain conformation of a semiconducting polymer by encapsulating it within the aligned nanopores of a silica host. The confinement leads to polarized, low-threshold amplified spontaneous emission from the polymer chains. The polymer enters the porous silica film from only one face and the filling of the pores is therefore graded. As a result, the profile of the index of refraction in the film is also graded, in the direction normal to the pores, so that the composite film forms a low-loss, graded-index waveguide. The aligned polymer chains plus naturally formed waveguide are ideally configured for optical gain, with a threshold for amplified spontaneous emission that is twenty times lower than in comparable unoriented polymer films. Moreover, the optimal conditions for ASE are met in only one spatial orientation and with one polarization. The results show that nanometre-scale control of semiconducting polymer chain orientation and position leads to novel and desirable optical properties.

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Figure 1: Composite films of aligned silica nanopores containing the semiconducting polymer poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV).
Figure 2: Intensity-dependent emission spectra from an MEH-PPV/aligned mesoporous silica composite film.
Figure 3: The threshold for ASE depends on both the excitation polarization and the nanoscale alignment of the polymer.
Figure 4: Electric field intensity distribution calculations.

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Acknowledgements

This work was supported by Canon (S.H.T. and H.M.), by the Office of Naval Research under grant N00014-04-1-0410 (S.H.T. and B.J.S.), and by the National Science Foundation under grants DMR-0305254 (BJS) and CHE-0527015 (B.J.S. and S.H.T.).

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Correspondence to Sarah H. Tolbert or Benjamin J. Schwartz.

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The authors declare no competing financial interests.

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Martini, I., Craig, I., Molenkamp, W. et al. Controlling optical gain in semiconducting polymers with nanoscale chain positioning and alignment. Nature Nanotech 2, 647–652 (2007) doi:10.1038/nnano.2007.294

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