ABSTRACT:Boosting nonlinear frequency conversion in extremely confined volumes remains a key challenge in nano-optics, nanomedicine, photocatalysis, and background-free biosensing. To this aim, field enhancements in plasmonic nanostructures are often exploited to effectively compensate for the lack of phase-matching at the nanoscale. Second harmonic generation (SHG) is, however, strongly quenched by the high degree of symmetry in plasmonic materials at the atomic scale and in nanoantenna designs.Here, we devise a plasmonic nanoantenna lacking axial symmetry, which exhibits spatial and frequency mode overlap at both the excitation and the SHG wavelengths. The effective combination of these features in a single device allows obtaining unprecedented SHG conversion efficiency. Our results shed new light on the optimization of SHG at the nanoscale, paving the way to new classes of nanoscale coherent light sources and molecular sensing devices based on nonlinear plasmonic platforms.

The quest for nanoscale light sources with designer radiation patterns and polarization has motivated the development of nanoantennas that interact strongly with the incoming light and are able to transform its frequency, radiation and polarization patterns. Here, we demonstrate dielectric AlGaAs nanoantennas for efficient second harmonic generation, enabling the control of both directionality and polarization of 1

We demonstrate monolithic aluminum gallium arsenide (AlGaAs) optical anoantennas.Using a selective oxidation technique, we fabricate such epitaxial semiconductor nanoparticles on an aluminum oxide substrate. Second harmonic generation from an AlGaAs nanocylinder of height h=400 nm and varying radius pumped with femtosecond pulses delivered at 1554-nm wavelength has been measured, revealing a peak conversion efficiency exceeding 10 -5 for nanocylinders with an otpimized geometry.

Being motivated by the recent prediction of high-Q modes in subwavelength dielectric resonators inspired by bound states in the continuum (BIC), we study the second-harmonic generation from isolated subwavelength AlGaAs nanoantennas. We reveal that nonlinear effects at the nanoscale can be enhanced dramatically provided the resonator parameters are tuned to the BIC regime. We predict a record-high conversion efficiency for nanoscale resonators that exceeds by 2 orders of magnitude the conversion efficiency observed at the magnetic dipole Mie resonance, thus opening the way for highly efficient nonlinear metasurfaces and metadevices.

We consider experimentally three-wave resonant nonlinear interactions of fields propagating in nonlinear media. We investigate the spatial dynamics of two diffractionless beams at frequency omega1, omega2 which mix to generate a field at the sum frequency omega3. If the generated field at omega3 can sustain a soliton, it decays into solitons at omega1, omega2. We report the experimental evidence of the transition from steady frequency wave generation to solitonic decay in nonlinear optics.

Abstract:We designed AlGaAs-on-aluminium-oxide all-dielectric nanoantennas with magnetic dipole resonance at near-infrared wavelengths. These devices, shaped as cylinders of 400nm height and different radii, offer a few crucial advantages with respect to the silicon-on-insulator platform for operation around 1.55μm wavelength: absence of two-photon absorption, high χ (2) nonlinearity, and the perspective of a monolithic integration with a laser. We analyzed volume χ (2) nonlinear effects associated to a magnetic dipole resonance in these nanoantennas, and we predict second-harmonic generation exceeding 10 −3 efficiency with 1GW/cm 2 of pump intensity.

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We study the manipulation of the radiation pattern of second harmonic generation from AlGaAs all-dielectric nanoantennas exhibiting electric and magnetic resonances. We show the importance of the interference of different higher-order multipoles in the nonlinear response of dielectric antennas for shaping of their second harmonic radiation pattern. In particular, we show how such multipolar interference can be engineered in AlGaAs nanodisks by manipulating the pump beam polarization, incidence angle and disk geometry. In this way we demonstrate the shaping of the radiation pattern in order to increase the second harmonic power by a factor of 8 when measured in experiment through a finite numerical aperture microscope objective.