Nanoscale optical integration is nowadays a strategic technological challenge and the ability of generating and manipulating nonlinear optical processes in sub-wavelength volumes is pivotal to realize efficient sensing probes and photonic sources for the next-generation communication technologies. Yet, confining nonlinear processes below the diffraction limit remains a challenging task because phase-matching is not a viable approach at the nanoscale. The localized fields associated to the resonant modes of plasmonic and dielectric nanoantennas offer a route to enhance and control nonlinear processes in highly confined volumes. In my talk I will discuss two nonlinear platforms based on plasmonic and dielectric nanostructures. The first relies on a broken symmetry antenna design, which brings about an efficient second harmonic generation (SHG). We recently applied this concept to an extended array of non-centrosymmetric nanoantennas for sensing applications. I will also show the evidence of a cascaded second-order process in Third Harmonic Generation (THG) in these nanoantennas.
Recently, dielectric nanoantennas emerged as an alternative to plasmonic nanostructures for nanophotonics applications, thanks to their sharp magnetic and electric Mie resonances along with the low ohmic losses in the visible/near-infrared region of the spectrum. I will present our most recent studies on the nonlinear properties of AlGaAs dielectric nanopillars. The strong localized modes along with the broken symmetry in the crystal structure of AlGaAs allow obtaining more than two orders of magnitude higher SHG efficiency with respect to plasmonic nanoantennas with similar spatial footprint and using the same pump power. I will also discuss a few key strategies we recently adopted to optically switch the SHG in these antennas even on the ultrafast time scale. Finally, I will show how to effectively engineer the sum frequency generation via the Mie resonances in these nanoantennas. These results draw a viable blueprint towards room-temperature all optical logic operation at the nanoscale.
All-dielectric optical metasurfaces consist of 2D arrangements of nanoresonators and are of great importance for shaping polarization, phase and amplitude of both linear and harmonic fields. Here, we demonstrate the generation of second harmonic (SH) with zero-order diffraction from nonlinear AlGaAs metasurfaces with spatial period comparable with a pump wavelength in the near-IR. Upon normal incidence of the pump, we demonstrate paraxial SH light into the zero order. SH polarization is effectively controlled via either the meta-atom shape or the pump polarization, with potential applications for on-axis imaging and free-space communication systems.
We demonstrate photon-pair generation via spontaneous parametric down-conversion (SPDC) from two types of metasurfaces composed by AlGaAs nanocylinders: 1) monolithically fabricated on a selectively oxidized layer of AlAs epitaxially grown on a GaAs wafer; 2) fabricated by reporting the AlGaAs nanostructures on a transparent wafer via wafer bonding. In these samples, we observed SPDC both in back- and forward-scattering configurations, under excitation with a CW pump around 775 nm and single-photon detection on the signal and idler channels. The Bragg modulation of Mie-resonances enables paraxial SPDC, which demonstrates the potential of all-dielectric metasurfaces for quantum applications like on-axis quantum imaging.
There are two physical effects that are exploited nowadays to implement flat metalenses requiring 2𝜋-phase excursion, either subwavelength guidance implementing varying propagation delays, or resonant confinement combining two resonances. We compare both approaches and identify possible limitations with the second approach.