Wave interference is a fundamental manifestation of the superposition principle with numerous applications. While in conventional optics interference occurs between waves undergoing different phase advances during propagation, we show that the vectorial structure of the near-field of an emitter is essential for controlling its radiation as it interferes with itself on interaction with a mediating object. We demonstrate that the near field interference of a circularly polarized dipole results in the unidirectional excitation of guided electromagnetic modes in the near-field, with no preferred far-field radiation direction. By mimicking the dipole with a single illuminated slit in a gold film, we measured unidirectional surface-plasmon excitation in a spatially symmetric structure. The surface wave direction is switchable with the polarization.Interference is the cornerstone of various phenomena in nature enabling numerous applications. In optics, it is intensively used in microscopy, stellar measurements, spectroscopy, and communication technologies, among many others, and is the basis behind the concepts of reflection, refraction and light bending (1, 2). Typically, interference occurs due to the relative phase lag of different propagating waves. On the other hand, nanophotonics -the branch of optics studying the interaction of light with subwavelength nanoscale structures-deals inherently with phenomena that occur via near-field interactions before appreciable phase lags can be accumulated (3). A radiationless form of interference in the near field (4) is behind new exciting applications such as the focusing of evanescent components to achieve subwavelength resolution in imaging (5-8). Near field interference achieved through the full coherent control of the phase and amplitude of excitation light allows asymmetric spatial field localization (9, 10) and selection of propagation paths at intersections of waveguides (11).We demonstrate near field interference by considering a single source of radiation coupled to a mode with a vectorial structure of electromagnetic field. Using an additional degree of freedom provided by the vectorial character of the field, control over the near-field interference can be achieved. We show that an elliptically polarized dipole can produce destructive or constructive interference of different evanescent components in its near field, and as a result, excite electromagnetic modes in neighbouring material structures, such as dielectric and plasmonic waveguides and diffraction gratings, with a controlled directionality of propagation.

Optical nanoantennas have shown a great capacity for efficient extraction of photons from the near to the far-field, enabling directional emission from nanoscale single-photon sources. However, their potential for the generation and extraction of multi-photon quantum states remains unexplored. Here we demonstrate experimentally the nanoscale generation of two-photon quantum states at telecommunication wavelengths based on spontaneous parametric down-conversion in an optical nanoantenna. The antenna is a crystalline Al-GaAs nanocylinder, possessing Mie-type resonances at both the pump and the bi-photon wavelengths and when excited by a pump beam generates photonpairs with a rate of 35 Hz. Normalized to the pump energy stored by the nanoantenna, this rate corresponds to 1.4 GHz/Wm, being one order of magnitude higher than conventional on-chip or bulk photon-pair sources. Our experiments open the way for multiplexing several antennas for coherent generation of multi-photon quantum states with complex spatial-mode entanglement and applications in free-space quantum communications and sensing.

We review recent achievements in the field of nanoscale nonlinear AlGaAs photonics based on all-dielectric optical antennas. After discussing the motivation and main technological challenges for the development of an AlGaAs monolithic platform for χ nonlinear nanophotonics, we present numerical and experimental investigations of the second-order nonlinear response and physical reasons for high efficiency of second-order nonlinear interactions in the AlGaAs nano-antennas. In particular, we emphasize the role of the dipolar resonances at the fundamental frequency and the multipolar resonances at the second harmonic wavelength. We also discuss second-harmonic generation directionality and show possible strategies to engineer the radiation pattern of nonlinear antennas.

Based on quasinormal-mode theory, we propose a novel approach enabling a deep analytical insight into the multi-parameter design and optimization of nonlinear photonic structures at subwavelength scale. A key distinction of our method from previous formulations relying on multipolar Miescattering expansions is that it directly exploits the natural resonant modes of the nanostructures, which provide the field enhancement to achieve significant nonlinear efficiency. Thanks to closedform expression for the nonlinear overlap integral between the interacting modes, we illustrate the potential of our method with a two-order-of-magnitude boost of second harmonic generation in a semiconductor nanostructure, by engineering both the sign of χ (2) at subwavelength scale and the structure of the pump beam.

All-dielectric metasurfaces consist of two-dimensional arrangements of nanoresonators and are of paramount importance for shaping polarization, phase, and amplitude of both fundamental and harmonic optical waves. To date, their reported nonlinear optical properties have been dominated by local features of the individual nanoresonators. However, collective responses typical of either Mie-resonant metamaterials or photonic crystals can potentially boost the control over such optical properties. In this work we demonstrate the generation of a second harmonic optical wave with zero-order diffraction, from a metasurface made out of AlGaAs-on-AlOx nanocylinders arranged with spatial period comparable to the pump telecom wavelength. Upon normal incidence of the pump beam, the modulation of Mie resonances via Bragg scattering at both fundamental and second harmonic frequencies enables paraxial second harmonic light generation by diffraction into the zero order, with a 50-fold increase in detected power within a solid angle of 5°. Exquisite control of a higher harmonic wavefront can be thus achieved in all-dielectric nonlinear metasurfaces, with potential applications for on-axis optical systems.

Background: Dielectric nanoantennas have recently emerged as an alternative solution to plasmonics for nonlinear light manipulation at the nanoscale, thanks to the magnetic and electric resonances, the strong nonlinearities, and the low ohmic losses characterizing high refractive-index materials in the visible/near-infrared (NIR) region of the spectrum. In this frame, AlGaAs nanoantennas demonstrated to be extremely efficient sources of second harmonic radiation. In particular, the nonlinear polarization of an optical system pumped at the anapole mode can be potentially boosted, due to both the strong dip in the scattering spectrum and the near-field enhancement, which are characteristic of this mode. Plasmonic nanostructures, on the other hand, remain the most promising solution to achieve strong local field confinement, especially in the NIR, where metals such as gold display relatively low losses. Results: We present a nonlinear hybrid antenna based on an AlGaAs nanopillar surrounded by a gold ring, which merges in a single platform the strong field confinement typically produced by plasmonic antennas with the high nonlinearity and low loss characteristics of dielectric nanoantennas. This platform allows enhancing the coupling of light to the nanopillar at coincidence with the anapole mode, hence boosting both second- and third-harmonic generation conversion efficiencies. More than one order of magnitude enhancement factors are measured for both processes with respect to the isolated structure. Conclusion: The present results reveal the possibility to achieve tuneable metamixers and higher resolution in nonlinear sensing and spectroscopy, by means of improved both pump coupling and emission efficiency due to the excitation of the anapole mode enhanced by the plasmonic nanoantenna.

Hyperbolic plasmonic metamaterials provide numerous opportunities for designing unusual linear and nonlinear optical properties. In this work, second‐harmonic generation in a hyperbolic metamaterial due to a free‐electron nonlinear response of a plasmonic component of the metamaterial is studied. It is shown that owing to a rich modal structure of an anisotropic plasmonic metamaterial slab, the overlap of fundamental and second‐harmonic modes results in the broadband enhancement of radiated second‐harmonic intensity by up to 2 orders of magnitude for TM‐ and TE‐polarized fundamental light, compared to a smooth Au film under TM‐polarised illumination. Compared to the radiated second‐harmonic intensity from a bulk LiNbO3 nonlinear crystal of the same thickness, the SHG intensity from a metamaterial slab may be up to 2 orders of magnitude higher at the certain metamaterial resonances. The results open up possibilities to design tuneable frequency‐doubling integratable metamaterial with the goal to overcome limitations associated with classical phase matching conditions in thick nonlinear crystals.

High-permittivity semiconductor nanoresonators have shown a great potential for enhanced nonlinear light-matter interactions at the nanoscale due to the availability of a rich variety of resonances combined with low optical losses and a strong bulk nonlinearity. Second harmonic generation in AlGaAs nanoantennas can be extremely efficient and exhibits a complex radiation