3 October 2006 Narrowing of nonlinear enhancements in near-field images
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In our attempt to reveal highly localized field enhancements on random metallic films using near-field scattering probe microscopy we experimentally demonstrated the existence of narrow peaks when using a monochromatic illumination. In order to get a better understanding of the second harmonic generation taking place on such films we have undertaken the same kind of near-field experiments using femtosecond lasers sources with high peak power able to induce the non linear response. These lasers have a spectral bandwidth associated with the pulse duration, which is in the femtosecond range. With such spectral broadening we have observed, as expected, a spatial broadening of the peaks at ω, which spread over distances in the 100-500 nm range. The behavior of the peaks is quite different at 2 ω: they are found to be always very well localized (~10 nm) despite of the polychromatic nature of the light; moreover there is no clear correlation between the peaks position at ω and those at 2 ω. This observation indicates, as often underlined in non linear processes, that coherent interactions involving a distribution of available frequencies in the lasers spectra take place. These frequencies ωn, coherently induce second harmonic generation as long as ωn + ωm = 2 ω.
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Samuel Grésillon, Samuel Grésillon, Laurent Williame, Laurent Williame, Eric le Moal, Eric le Moal, Emmanuel Fort, Emmanuel Fort, A. Claude Boccara, A. Claude Boccara, "Narrowing of nonlinear enhancements in near-field images", Proc. SPIE 6324, Plasmonics: Nanoimaging, Nanofabrication, and their Applications II, 632406 (3 October 2006); doi: 10.1117/12.679886; https://doi.org/10.1117/12.679886

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