Mode-Matching in Multiresonant Plasmonic Nanoantennas for Enhanced Second Harmonic Generation

Michele Celebrano Department of Physics, Politecnico di Milano, Milano, Italy Xiaofei Wu Department of Physics - Experimental Physics 5, University of Wuerzburg, Wuerzburg, Germany Department of Physics - Experimental Physics III, University of Bayreuth, Bayreuth, Germany Milena Baselli Department of Physics, Politecnico di Milano, Milano, Italy Swen Grossmann Department of Physics - Experimental Physics 5, University of Wuerzburg, Wuerzburg, Germany Paolo Biagioni Department of Physics, Politecnico di Milano, Milano, Italy Andrea Locatelli Department of Information Engineering, University of Brescia, Brescia, Italy Costantino De Angelis Department of Information Engineering, University of Brescia, Brescia, Italy Giulio Cerullo Department of Physics, Politecnico di Milano, Milano, Italy Department of Physics, IFN-CNR - Politecnico di Milano, Milano, Italy Roberto Osellame Department of Physics, Politecnico di Milano, Milano, Italy Department of Physics, IFN-CNR - Politecnico di Milano, Milano, Italy Bert Hecht Department of Physics - Experimental Physics 5, University of Wuerzburg, Wuerzburg, Germany Lamberto Duò Department of Physics, Politecnico di Milano, Milano, Italy Franco Ciccacci Department of Physics, Politecnico di Milano, Milano, Italy Marco Finazzi Department of Physics, Politecnico di Milano, Milano, Italy

Second Harmonic Generation (SHG) is well known to be a powerful imaging tool for background-free and non-damaging live tissues investigation. Field enhancements in plasmonic nanostructures are often exploited to effectively compensate for the lack of phase-matching in confined volumes with the aim of obtaining brighter nanoscale nonlinear probes. Recently, nanoantenna designs featuring a double resonance at both the excitation and the emission wavelengths have been proposed to improve SHG [1]. However, the high degree of symmetry in plasmonic materials at the atomic scale and in nanoantenna designs have so far limited SHG efficiency [2]. Here we report on especially engineered gold single-crystalline nanoantennas (see Figure inset) working in the near-infrared that show unprecedented SHG efficiency thanks to (i) a multi-resonant response occurring at both the excitation and SH wavelength, (ii) a significant spatial overlap of the localized fields at the wavelengths of interest and (iii) a broken-symmetry geometry to achieve dipole-allowed SHG. The effective combination of these key features in a single plasmonic antenna, characterized by the absence of local defects, allows optimizing SHG efficiency in a well-controlled fashion.

Once the antenna geometry is properly tuned to display a double resonance matching simultaneously the laser excitation (1560 nm) and the SHG wavelength (see Figure), it demonstrates a SHG efficiency β_SH = P_SH /P_FW² ≈ 5×10^-5, which is well above the values reported in literature for the same spectral region. [3]

These results shed new light on the optimization of nanoscale SHG via metal nanoantennas, paving the way to a new class of tunable molecular sensing devices and nanoscale coherent light sources based on nonlinear plasmonic platforms.

[1] K. Thyagarajan et al., Opt. Express 20, 12860 (2012).

[2] M. Finazzi et al., Phys. Rev. B 76, 125414 (2007).

[3] H. Aouani et al., Nano Lett. 12, 4997 (2012).

paolo.biagioni@polimi.it