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Nat Nanotechnol. 2015 May;10(5):412-7. doi: 10.1038/nnano.2015.69. Epub 2015 Apr 20.

Mode matching in multiresonant plasmonic nanoantennas for enhanced second harmonic generation.

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Physics Department, Politecnico di Milano, Piazza Leonardo Da Vinci 32, Milano 20133, Italy.
Nano-Optics &Biophotonics Group - Department of Physics - Experimental Physics 5, University of Würzburg, Am Hubland, Würzburg 97074, Germany.
Department of Information Engineering, University of Brescia, Via Branze 38, Brescia 25123, Italy.
1] Physics Department, Politecnico di Milano, Piazza Leonardo Da Vinci 32, Milano 20133, Italy [2] Institute for Photonics and Nanotechnologies (IFN)-CNR, Piazza Leonardo Da Vinci 32, Milano 20133, Italy.


Boosting nonlinear frequency conversion in extremely confined volumes remains a challenge in nano-optics research, but can enable applications in nanomedicine, photocatalysis and background-free biosensing. To obtain brighter nonlinear nanoscale sources, approaches that enhance the electromagnetic field intensity and counter the lack of phase matching in nanoplasmonic systems are often employed. However, the high degree of symmetry in the crystalline structure of plasmonic materials (metals in particular) and in nanoantenna designs strongly quenches second harmonic generation. Here, we describe doubly-resonant single-crystalline gold nanostructures with no axial symmetry displaying spatial mode overlap at both the excitation and second harmonic wavelengths. The combination of these features allows the attainment of a nonlinear coefficient for second harmonic generation of ∼5 × 10(-10) W(-1), enabling a second harmonic photon yield higher than 3 × 10(6) photons per second. Theoretical estimations point toward the use of our nonlinear plasmonic nanoantennas as efficient platforms for label-free molecular sensing.


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