INVITED TALK -> Second harmonic generation in monolithic AlGaAs nanoantennas

Metal-less nanophotonics has recently become a hot research topic because technologically available high-permittivity dielectric nanoparticles exhibit negligible dissipative losses and strong magnetic multipole resonances in... [ view full abstract ]

Metal-less nanophotonics has recently become a hot research topic because technologically available high-permittivity dielectric nanoparticles exhibit negligible dissipative losses and strong magnetic multipole resonances in the visible and near-IR [1,2]. Here we report on efficient second harmonic generation (SHG) in the optical telecom range, from monolithic AlGaAs nanocylinders that are transparent at both the pump and the SH frequency. Our samples are fabricated from a (100) GaAs wafer, with an Al0.18Ga0.82As layer (400nm) on top of an aluminum-rich substrate (1µm), and after lithography and oxidation steps we obtain high-χ(2) nanopillars on an aluminum-oxide substrate. In our experiment, we separately excite single nanopillars with an ultrafast Erbium-doped fiber laser centered at 1554 nm (150 fs pulses, 80 MHz repetition rate) and we collect the back-scattered SHG signal through the same microscope objective that we use for the pump beam. The dependence of the measured SHG on the radius of the nanocylinders (with r = 175-225 nm) shows a few resonances corresponding to distinct SH nanocavity modes, in striking agreement with our calculations. The latter, carried out with frequency-domain finite element simulations in COMSOL [3], are also in excellent agreement with the experimental SHG spectra and polarization, and with the conversion efficiency (7×10-5, measured for a 1.6 GW/cm2 pump [4]). Depending on the radius, the SHG is driven by either a magnetic-dipole (MD) or an electric-dipole (ED) resonance at the pump wavelength. We show that the pump and SH fields are co-(cross-)polarized in the MD (ED) case [5], and we explain this with the contribution of bulk χ(2).
These results show the huge potential of AlGaAs nanoantennas for nonlinear nanophotonics, in terms of both high conversion efficiency and rich polarization behavior of the SH field. The latter promise intriguing potential applications to e.g. background-free wave-shaping metasurfaces.

[1] A. B. Evlyukhin, et al., Nano Lett. 12, 3749 (2012).
[2] S. Person et al., Nano Lett. 13, 1806 (2013).
[3] L. Carletti et al., Opt. Express 23, 26544 (2015).
[4] V. F. Gili et al., Opt. Express 24, 15965 (2016).
[5] S. Liu et al., Nano Lett. (in press).