We demonstrate monolithic aluminum gallium arsenide (AlGaAs) optical nanoantennas. Using a selective oxidation technique, we fabricated epitaxial semiconductor nanocylinders on an aluminum oxide substrate. Second harmonic generation from AlGaAs nanocylinders of 400 nm height 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 optimized geometry.

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 experimentally demonstrate 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 AlGaAs nanocylinder, possessing Mie-type resonances at both the pump and the bi-photon wavelengths, and when excited by a pump beam it generates photon pairs with a rate of 35 Hz. Normalized to the pump energy stored by the nanoantenna, this rate corresponds to 1.4 GHz/Wm, being 1 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.

Optical metasurfaces, i.e. arrays of nanoantennas with sub-wavelength size and separation, enable the manipulation of light-matter interactions in miniaturized optical components with no classical counterparts. Six decades after the first observation of the second harmonic generation (SHG) in bulk crystals, these devices are expected to break new ground in the field of nonlinear optics, shifting the focus from the phase matching approach achieved within long propagation distances to that of near-field resonances interplay in leaky nanocavities. Here we review the recent progress in SHG with all-dielectric metasurfaces. We discuss the most used technological platforms which underpinned such advances and analyze different SHG control approaches. We finally compare their performances with other well-established technologies, with the hope to delineate the current state-of-the-art and figure out a few scenarios in which these devices might soon offer unprecedented opportunities.

Infrared (IR) spectroscopy and imaging are growing in demand due to the increasing number of applications in this spectral region, including optical tomography,¹ process monitoring,² food and agriculture quality control,³ and night vision devices,⁴ as well as LIDAR and remote sensing.^5,6 Commercial IR imaging detectors rely on the absorption of incident photons in narrow bandgap semiconductor materials and the release of electrons that are electrically detected. However, due to the low IR photon energy, such IR detection schemes require low-temperature and even cryogenic cooling. As a result, IR cameras are generally bulky, containing several components for photon–electron conversion. An alternative scheme, which can potentially overcome the limitations of photoconductive detectors, is the use of nonlinear optical processes for upconversion of the photon energy. In this approach, the IR image is not directly detected; instead, a parametric nonlinear optical process is employed to convert the image to higher frequencies and detect it using regular cameras in a process known as upconversion IR imaging. In 1968, Midwinter first demonstrated IR upconversion imaging by converting the IR signal to the visible spectrum using a nonlinear crystal and the aid of a pump beam.⁷ The spatial information of a short-wave IR (SWIR) image ($\lambda =1.6\mu \mathrm{m}$) was coherently transferred to the visible domain ($\lambda =0.484\mu \mathrm{m}$) using a parametric second-order nonlinear process known as sum-frequency generation (SFG). In the SFG process, two incident waves with frequencies ${\omega }_1$ and ${\omega }_2$ interact inside a second-order nonlinear material leading to sum-frequency emission with frequency ${\omega }_{\mathrm{SFG}}={\omega }_1+{\omega }_2$, as shown in Fig. 1(a). The results obtained by Midwinter showed the possibility of detecting IR images with relatively high sensitivities using standard, fast, and uncooled Si-based detectors. A decade of intensive research followed the first demonstration of upconversion IR imaging, where the performance and resolution of the imaging systems were studied in different arrangements, including various incidence angles,⁸ nonlinear bulk crystals,⁹ optical configurations,¹⁰ bandwidth of the IR radiation,¹¹ and pump beams.¹² However, the low signal-to-noise ratio of CCD detectors and poor quality of nonlinear crystals at the time prevented practical developments of upconversion imaging systems. Recently, the interest in such imaging systems has been renewed, driven by the availability of periodically poled nonlinear crystals,^{13 – 15} new laser sources,^{16 – 18} and the use of intracavity configurations,^14,19 which can improve the performance of the systems. The main difficulty in the realization of these IR imaging systems is the phase-matching condition, which not only restricts the conversion efficiency of the upconversion process but also limits the spectral bandwidth, resolution, and field of view of the IR image. Another approach in the detection of IR radiation is the use of quantum imaging techniques, such as quantum ghost imaging.²⁰ However, in addition to the phase-matching condition, quantum imaging approaches require the entanglement of the illuminating photons, the use of single-photon detectors, and possibly the use of interferometry, therefore increasing the complexity of these imaging techniques.

We report on second-harmonic generation in whispering-gallery-mode AlGaAs microcavities suspended on a GaAs pedestal. Frequency doubling of a 1.58 μm pump is observed with $7\times {10}^{-4}{\mathrm{W}}^{-1}$ conversion efficiency. This device can be integrated in a monolithic photonic chip for classical and quantum applications in the telecom band.