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We demonstrate that AlGaAs thin films and metasurfaces generate a distinct intrinsic nonlinear geometric phase in their second harmonic signals, differing significantly from previous studies on nonlinear dielectric, plasmonic, or hybrid metasurfaces. Unlike conventional observations, our study reveals that the second harmonic phase remains unaffected by the linear optical response at both pump and harmonic wavelengths, introducing a novel realm of achievable phase functions yet to be explored. Furthermore, we explore the interplay between this intrinsic nonlinear geometric phase and the geometric phase induced by rotations of nanoresonators within metasurface arrangements. Our findings extend the capabilities of nonlinear wavefront shaping metasurfaces, exploiting phase manipulation to uncover unique phenomena exclusive to the nonlinear regime.

Metasurfaces, composed of artificial meta-atoms of subwavelength size, can support strong light–matter interaction based on multipolar resonances and plasmonics, hence offering the great capability of empowering nonlinear generation. Recently, owing to their ability to manipulate the amplitude and phase of the nonlinear emission in the subwavelength scale, metasurfaces have been recognized as ultra-compact, flat optical components for a vast range of applications, including nonlinear imaging, quantum light sources, and ultrasensitive sensing. This review focuses on the recent progress on nonlinear metasurfaces for those applications. The principles and advances of metasurfaces-based techniques for image generation, including image encoding, holography, and metalens, are investigated and presented. Additionally, the overview and development of spontaneous photon pair generation from metasurfaces are demonstrated and discussed, focusing on the aspects of photon pair generation rate and entanglement of photon pairs. The recent blossoming of the nonlinear metasurfaces field has triggered growing interest to explore its ability to efficiently up-convert infrared images of arbitrary objects to visible images and achieve spontaneous parametric down-conversion. This recently emerged direction holds promising potential for the next-generation technology in night-vision, quantum computing, and biosensing fields.

In the framework of optical frequency conversion, metasurfaces have elevated the potential for effective interfacial nonlinear coefficients through various modes of field localization. For the generation of pulsed ultrafast terahertz (THz) signals, metasurfaces present a viable alternative in the domain of surface-scalable sources driven by low-power oscillators (using nJ pulses). However, recent innovations have predominantly relied on surface plasmons (metals) and, more broadly, on excitations within non-transparency windows—conditions that typically impose limitations on applications and the choice of platforms. Here, we demonstrate the utilization of a fully-dielectric, fully transparent semiconductor that exploits surface-nano-structure-mediated resonances alongside its inherent quadratic nonlinear response. Our system exhibits a remarkable 40-fold efficiency enhancement in comparison to the non-decorated substrate.

Optically resonant dielectric and semiconductor metasurfaces are an emerging and promising area of nanophotonics and light–matter interaction at the nanoscale. Recently, active tuning of the linear response and nonlinear effects of these components has received an increasing amount of interest. However, so far these research directions have remained separated with only few sporadic works that study their combination beginning to appear in the literature. The evolution of nonlinear metasurfaces based on dielectric and semiconductor materials toward reconfigurable and dynamic components could potentially answer the demand of integrated on-chip components that realize essential functionalities such as frequency conversion, active switching, optical isolation, and all-optical routing. This review provides an overview of recent investigations in this field, reviews the main physical phenomena enabling the dynamic control of the nonlinear response and compares the temporal dynamics of the diverse approaches that have been explored so far. Finally, future directions of dynamic nonlinear metasurfaces are outlined.

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 address the surface vs bulk origin of the second-order optical nonlinearity in AlGaAs nanocylinders through polarization-resolved measurements. By comparing numerical simulations accounting just for bulk second-order nonlinearity with experimental results, we show that the surface contribution to second-harmonic generation (SHG) cannot be neglected and depends on the resonant conditions of the nanocylinder. Additionally, our analysis suggests that bulk and surface SHG are competing effects, and that their interference might influence the overall efficiency.

In recent years a strong drive towards the miniaturization of nonlinear optics has been motivated by the functionalities it could empower in integrated devices. Among these, the upconversion of near-infrared photons to the visible and their manipulation is fundamental to downscale optical information. We propose a dual-beam scheme whereby a pulse at the telecom frequency ω (1550 nm wavelength) is mixed with its frequency-doubled replica at 2ω. When the two pump pulses are superimposed on a nonlinear, all-dielectric metasurface two coherent frequency-tripling pathways are excited: third-harmonic generation (THG, ω+ω+ω) and sum-frequency generation (SFG, ω+2ω). Their coherent superposition at 3ω produces interference, which we enable by filtering the k-space with the metasurface diffraction. The steering of the emission among diffraction orders, is sensitive to the relative phase between the two pumps. Therefore, by exploiting the phase as a tuning knob, the upconverted signal can be switched between diffraction orders with an efficiency >90%. The proposed approach can be envisioned as an all-optical method to reroute upconverted telecom photons.

We report the effect of the aluminum oxide substrate on the emission of monolithic AlGaAs-on-insulator nonlinear nanoantennas. By coupling nonlinear optical measurements with electron diffraction and microscopy observations, we find that the oxidation-induced stress causes negligible crystal deformation in the AlGaAs nanostructures and only plays a minor role in the polarization state of the harmonic field. This result highlights the reliability of the wet oxidation of thick AlGaAs optical substrates and further confirms the bulk χ(2) origin of second harmonic generation at 1.55 μm in these nanoantennas, paving the way for the development of AlGaAs-on-insulator monolithic metasurfaces.