Nanoscale optical integration is nowadays a strategic technological challenge and the ability of generating and manipulating nonlinear optical processes in sub-wavelength volumes is a key- enabling asset for the realization of efficient nonlinear optical sensing probes and photonic sources for the next-generation technology. Yet, to date, confining nonlinear processes beyond the diffraction limit remains a challenging task because phase-matching conditions, which ensure efficient energy transfer from the fundamental to the nonlinear wave in the bulk, cannot be exploited at the nanoscale.
Significant efforts have been recently devoted to the investigation of nonlinear optical processes in plasmonic nanoantennas [1,2], since the strong local field enhancements they produce represent, to date, the most promising solution to compensate for the lack of phase-matching at the nanoscale. I will present an approach we have recently devised to enhance the second harmonic generation (SHG) in non-centrosymmetric plasmonic nanoantennas [3]. In these optimized structures, higher- order nonlinear processes, e.g. third harmonic generation (THG), yield evidence of cascaded second-order processes [4]. I will also show an application idea based on an array of non- centrosymmetric nanoantennas employed as a sensor [5].
Dielectric nanoantennas have recently emerged as an alternative solution to plasmonics to attain nonlinear emission at the nanoscale, thanks to their sharp magnetic and electric resonances along with the low ohmic losses visible/near-infrared region of the spectrum. I will present our most recent studies on AlGaAs nanopillars, which demonstrate an extremely high SHG efficiency associated with the bulk second-order susceptibility of the material [6,7]. In this frame, I will also disclose a few key strategies we have recently adopted to improve the SHG collection efficiency, based on either tilted excitation or asymmetric gratings surrounding the antenna, as well as a hybrid configuration based on a plasmonic ring antenna [8,9].
[1] M. Kauranen, A. V. Zayats. Nature Photonics 6, 737–748, 2012. [2] J. Butèt et al. ACS Nano 9, 10545–10562, 2015.
[3] M. Celebrano et al. Nature Nanotechnology 10, 412–417, 2015. [4] M. Celebrano et al. Arxiv:1803.03617
[5] L. Ghirardini et al. J. Phys. Chem. C ASAP 2018. DOI: 10.1021/acs.jpcc.8b03148 [6] V. Gili et al. Opt. Exp. 24, 15965, 2016.
[7] L. Ghirardini et al. Opt. Lett. 42, 559-562, 2017.
[8] T. Shibanuma et al. Nano Lett. 17, 2647–2651, 2017.
[9] V. Gili et al. Submitted