Second harmonic generation in AlGaAs nanodisk dimers

All-dielectric nonlinear meta-optics is attracting a great deal of interest thanks to the feasibility of high refractive-index contrast nanostructures available with semiconductor lithography. While c(3) effects have been... [ view full abstract ]

All-dielectric nonlinear meta-optics is attracting a great deal of interest thanks to the feasibility of high refractive-index contrast nanostructures available with semiconductor lithography. While c⁽³⁾ effects have been reported in silicon-on-insulator nanoantennas [1], the AlGaAs-on-insulator platform has recently enabled the demonstration of second harmonic generation (SHG) in c⁽²⁾ nanoantennas [2]. When one excites them with a plane wave at normal incidence, they exhibit efficient SHG driven by a magnetic-dipole resonance at the pump frequency in the optical telecom range and a polarisation behaviour dominated by a high-order multipole resonance at the second harmonic [3].

Here we focus on SHG from monolithic dimers of Al_0.18Ga_0.82As-on-AlOx nanoantennas, where AlOx is obtained from selective wet etching of micrometer-thick aluminium-rich AlGaAs epitaxial layer. Such a low refractive index substrate allows to effectively decouple the nanoantenna modes from the underlying GaAs (100) wafer. As a prototype example, we report in the figure the case of a set of coupled nanodisks with height h = 400 nm, gap g = 30 nm and radii ranging from 175 nm to 225 nm, which we excite with a pump beam linearly polarized along the dimer axis. As in the case of a single nanodisk, the SHG selection rules are mainly set by the properties of bulk AlGaAs χ⁽²⁾ tensor: in a good agreement with finite-element calculations, we clearly observe that the main resonance associated to the single pillar splits in two distinct features, owing to near-field coupling between the two pillars. The measured peak conversion efficiency, exceeding 10^-5 for a 1.6 GW/cm² pump intensity, paves the way to the engineering of coupled nanoantennas and nonlinear metasurfaces for molding SHG properties at the nanoscale.

REFERENCES
[1] M. R. Shcherbakov et al., “Enhanced third-harmonic generation in silicon nanoparticles driven by magnetic response”, Nano Lett. 14, 6488 (2014).
[2] V. F. Gili et al., “Monolithic AlGaAs second-harmonic nanoantennas”, Opt. Expr. 24, 15965 (2016).
[3] L. Ghirardini et al., “Polarization properties of second-harmonic generation in AlGaAs optical nanoantennas”, Opt. Letters 42, 559 (2017).