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Metasurfaces have recently gained extensive interest because of their extraordinary optical behavior as artificial material interfaces with ultrahigh compactness. In this framework, dielectric platforms have newly become very promising for nonlinear nanophotonics, providing opportunities, especially for ultrafast optical switching, and high harmonic generation, opening the research field of nonlinear metaoptics. Up to now, nonlinear metaoptics have been mostly explored using single metasurfaces. However, in a long-term vision, the stacking of optical metasurfaces, very challenging in terms of fabrication, is one key goal of this research field. Here, we demonstrate a three-layer metasurface in the AlGaAs-on-insulator platform, which improves the second harmonic generation efficiency by more than one order of magnitude with respect to its one-layer counterpart. Our achievement paves the way toward phase-shaping multilayer and multifunctional all-dielectric metasurfaces.

We demonstrate optically tunable control of second-harmonic generation in all dielectric nano-antennas: by using a control beam which is absorbed by the nano-resonator, we thermo-optically change the refractive index of the radiating element to modulate amplitude and phase of the second-harmonic signal. For a moderate temperature increase, sizeable modulation is demonstrated; the large tunability of the single meta-atom response paves the way to exciting avenues for reconfigurable homogeneous and heterogeneous metasurfaces.

We experimentally demonstrate phase encoding in SHG with transparent all-dielectric metasurfaces. While a similar task was previously achieved with plasmonic metasurfaces for THG beam shaping, here we obtain three-order-of-magnitude higher generation efficiency without thermal dissipation.

High refractive index dielectric nanostructures can be exploited to enhance nonlinear optical processes via the strong light confinement by their resonant modes. The sensitive dependence of these m...

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.

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.

We demonstrate an AlGaAs metasurface that implements second harmonic generation with engineered phase-front. Nonlinear holography is obtained, with three-order-of-magnitude higher conversion efficiency than in χ(3) plasmonic metasurfaces.

We propose a robust design for both second harmonic generation and its wavefront control in an AlGaAs-on-sapphire metasurface. In analogy with optical phased arrays, we achieve 0-2π phase modulation on the second harmonic.

Metal-less nanophotonics offers new opportunities for non-linear optics with respect to optical waveguides and microresonators, taking advantage of the progress within nanofabrication to boost the development of subwavelength Mie-type structures. Here, we review recent results on second harmonic generation with semiconductor nanoresonators, focusing on their scattering features in terms of efficiency and control over radiation patterns. First, two theoretical models are comparatively discussed with a view to possible improvements in analysis and design. Then, some relevant experiments are reported, and the origin of the χ(2) generation is discussed, outlining the main open topics to investigate in the near future and the advantages offered by these nanostructures to the development of novel photonic devices.

Optical dielectric metasurfaces composed of arrayed nanostructures are expected to enable arbitrary spatial control of incident wavefronts with subwavelength spatial resolution. For phase modulation, one often resorts to two physical effects to implement a 2\pi-phase excursion. The first effect relies on guidance by tall nanoscale pillars and the second one exploits resonant confinement by nanoresonators with two degenerate Mie-resonances. The first approach requires high aspect ratios, while the second one, known as Huygens metasurfaces, is much flatter, and thus easier to manufacture. We compare the two approaches, more focusing on conceptual rather than technological issues, and identify fundamental limitations with the latter. We explain the origin of the limitations based on general arguments, such as reciprocity, multimode/monomode operation and symmetry breaking., Comment: 6 figures

All-dielectric nanophotonics has recently raised an increasing interest because the optical response of high-permittivity dielectric nanoparticles exhibits negligible dissipative losses and strong magnetic multipole resonances [1–4] in the visible and near-IR. We recently reported a second harmonic generation (SHG) conversion efficiency higher than 10−5 at 2 = 1554 nm (pump intensity ≈ 1.6 GW/cm2; pulse width ≈ 150 fs; repetition rate = 80 MHz) in all-dielectric Al 0.18 Ga 0.82 As-on-AlOx nanodisks [5–7]. Such a remarkable efficiency is achieved thanks to the exploitation of magnetic and electric resonances emerging at coincidence with both the pump and SHG wavelength. In addition, the choice of both the pump and, consequently, the SHG (≈ 777 nm) wavelength, set below the energy of the material bandgap, allow minimizing the two-photon absorption of the pump and the re-absorption of the SHG.

We demonstrate optically tunable control of second-harmonic generation in all-dielectric nanoantennas: by using a control beam that is absorbed by the nanoresonator, we thermo-optically change the refractive index of the radiating element to modulate the amplitude of the second-harmonic signal. For a moderate temperature increase of roughly 40 K, modulation of the efficiency up to 60% is demonstrated; this large tunability of the single meta-atom response paves the way to exciting avenues for reconfigurable homogeneous and heterogeneous metasurfaces.