Nonlinear optics in nanostructured materials has emerged as a central theme in modern photonics, driven by the quest for compact, efficient, and ultrafast platforms capable of manipulating light beyond the limits of bulk media. In nanometric volumes or atomic length scales interaction lengths dramatically decrease potentially hindering the efficiency of nonlinear optical processes. Yet, optical resonances as well as the strong confinement of electronic motion can dramatically enhance light–matter interactions, giving rise to nonlinear responses that are orders of magnitude stronger than those found in conventional crystals. This Research Topic, Nonlinear Optical Processes in Emerging Nanomaterials, brings together contributions that address this opportunity from complementary perspectives, spanning material characterisation, fundamental nonlinear dynamics, nanoscale morphology, and photonic integration.

Low-dimensional and nanoscale systems—ranging from two-dimensional (2D) materials to plasmonic nanoparticles—provide an ideal playground for exploring nonlinear optical phenomena. Their reduced dimensionality, strong Coulomb interactions, and pronounced surface or interface effects enable efficient harmonic generation, frequency mixing, and strong-field phenomena on ultrafast timescales. At the same time, these properties pose challenges in terms of characterisation, control, and integration, which the articles in this Research Topic address in a coherent and progressive manner.

A crucial prerequisite for exploiting nonlinear effects in emerging nanomaterials is the ability to characterise their structural and electronic properties in a rapid and non-invasive way. Nonlinear optical techniques are uniquely suited for this task, as they are inherently sensitive to symmetry, interfaces, and electronic structure. In this context, the reviewed work by Sousa et al. on nonlinear optical imaging highlights how second-harmonic generation (SHG), third-harmonic generation (THG), and four-wave mixing (FWM) can be employed to map crystallographic orientation, defects, and stacking configurations in 2D materials. In non-centrosymmetric monolayers, SHG provides direct sensitivity to crystal symmetry and domain structure, while in centrosymmetric systems such as graphene, higher-order nonlinear processes enable access to otherwise hidden structural information. Importantly, these approaches enable the determination of twist angles in vertically stacked heterostructures and, in suitable materials, access valley-selective responses relevant to valleytronic concepts. Together, they establish nonlinear optics not only as a functional tool, but also as a powerful diagnostic technique for emerging nanomaterials.

Beyond characterisation, the Research Topic addresses extreme nonlinear regimes where light drives electronic dynamics far from equilibrium. Gadermaier presents a focused review of high-harmonic generation (HHG) in two-dimensional semiconductors, with particular emphasis on transition-metal dichalcogenides. In these atomically thin systems, in contrast to bulk crystals where propagation and phase-matching effects dominate, HHG occurs in a strongly non-perturbative regime and is profoundly influenced by many-body interactions, Berry curvature, and crystal symmetry. The review highlights how HHG spectra encode detailed information about band structure and ultrafast carrier dynamics on attosecond timescales, while benefiting from the reduced propagation and reabsorption effects inherent to monolayers. Importantly, the work discusses all-optical control of HHG via photodoping, where a weak resonant control pulse can strongly modulate the harmonic yield with high contrast. This mechanism demonstrates how nonlinear optical processes in 2D materials can be actively controlled, pointing toward ultrafast optoelectronic functionalities operating at unprecedented speeds.

While 2D materials offer an extended and tunable platform, nanoscale confinement in zero-dimensional systems provides an alternative route to enhancing nonlinear responses. Russier-Antoine et al. investigate, using hyper-Rayleigh scattering, the nonlinear optical properties of gold–silver nanoparticles, focusing on the role of morphology in determining efficiency. By directly comparing Au@Ag core–shell nanoparticles with their laser-annealed alloyed counterparts, the authors demonstrate that the core–shell architecture exhibits a first hyperpolarizability approximately three orders of magnitude larger than that of the alloyed particles. This striking enhancement is attributed to the presence of two closely spaced nonlinear interfaces—the surrounding medium–silver interface and the internal silver–gold metal–metal interface—which contribute constructively to the nonlinear signal. The study clearly illustrates how nanoscale morphological engineering can be used to tailor and amplify nonlinear optical responses, with direct relevance for sensing, imaging, and plasmon-enhanced nonlinear spectroscopy.

The final thematic element of the Research Topic concerns the translation of strong nonlinearities into integrated photonic architectures, addressed by Rojas Yanez et al. Achieving high nonlinear efficiency in practical devices requires not only suitable materials, but also photonic structures capable of confining light to nanoscale volumes. The investigated metal–dielectric–metal slot waveguide platforms exemplify this approach, combining extreme field confinement with the unique properties of epsilon-near-zero materials. By exploiting slow-light effects and uniform energy confinement, these architectures offer a route to boosting nonlinear conversion efficiencies in ultra-compact geometries, relevant for on-chip nonlinear and quantum photonics, including the generation of squeezed light and correlated photon pairs.

Taken together, the contributions to this Research Topic highlight the transformative potential of emerging nanomaterials for nonlinear optics. From nonlinear optical imaging and strong-field physics in 2D semiconductors, to morphology-driven enhancement in plasmonic nanoparticles and pathways toward integrated photonic platforms, the collected works demonstrate how nanoscale control enables access to regimes unattainable in bulk media. As research continues to bridge fundamental nonlinear phenomena with scalable photonic integration, the insights presented here provide a solid foundation for the development of future ultrafast, efficient, and multifunctional nonlinear optical technologies.