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.

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 use numerical simulations to demonstrate third-harmonic generation with near-unity nonlinear circular dichroism (CD) and high conversion efficiency $\left({10}^{-2}{\text{W}}^{-2}\right)$ in asymmetric Si-on-$\mathrm{Si}{\mathrm{O}}_2$ metasurfaces. The working principle relies on the spin-selective excitation of a quasibound state in the continuum, characterized by a very high $(>{10}^5)$ quality factor. By tuning multimode interference with the variation of the metasurface geometrical parameters, we show the possibility to control both linear CD and nonlinear CD. Our results pave the way for the development of all-dielectric metasurfaces for nonlinear chiro-optical devices with high conversion efficiency.

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 ⁽²⁾ 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.

Dielectric nanocavities are emerging as a versatile and powerful tool for the linear and nonlinear manipulation of light at the nanoscale. In this work, we exploit the effective coupling of electric and toroidal modes in AlGaAs nanodimers to locally enhance both electric and magnetic fields while minimizing the optical scattering, thereby optimizing their second-harmonic generation efficiency with respect to the case of a single isolated nanodisk. We also demonstrate that proper near-field coupling can provide further degrees of freedom to control the polarization state and the radiation diagram of the second-harmonic field.