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10 May 2010 Controlling molecular organization at the nanoscale for localized second harmonic generation
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Using the electric field present inside a Scanning Tunnelling Microscope (STM) junction, we demonstrate the possibility to create a very local non-centrosymmetry via molecular orientation under the tip. We show this can be used to get localized light emission through second harmonic generation (SHG). Experiments were performed by coupling a femtosecond laser inside a metallic-substrate / metallic-tip junction immersed in concentrated solutions of highly nonlinear azo-dye molecules. The quadratic dependence of the SHG signal intensity with the tip voltage unambiguously shows that it comes from an electric field induced molecular polarisation under the tip. The potentialities of such effects are evoked as an original concept for scanning probe microscopy. Extrapolating the SH intensities that could be recorded, it is estimated that the minimum volume that can be detected in the present experimental configuration is about 1 μm3. In order to be able to decrease this limit, a new experimental set-up is developed towards better signal collection. The implementation of sharp metallic tips to induce amplifying nanoantenna effects (electrostatic lightning rod effects or localized surface plasmon resonances) is also discussed as a complementary way to increase the system lateral resolution.
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I. Berline, L. Douillard, F. Charra, and C. Fiorini-Debuisschert "Controlling molecular organization at the nanoscale for localized second harmonic generation", Proc. SPIE 7712, Nanophotonics III, 771236 (10 May 2010);

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