Implementing nonlinear optical components in nanoscale photonic devices is challenged by phase matching conditions requiring thickness in the order of hundreds of wavelengths and disadvantaged by the short optical interaction depth of nanometer-scale materials and weak photon-photon interactions. Here we report that ferroelectric NbOI2 nanosheets exhibit giant SHG with conversion efficiencies that are orders of magnitude higher than commonly reported nonlinear crystals. The nonlinear response scales with layer thickness and is strain-and electrical-tunable; a record >0.2 % absolute SHG conversion efficiency and an effective NL susceptibility 𝜒 !"" ($) in the order of 10 −9 m V -1 are demonstrated at average pump intensity of 8 kW/cm 2 . Due to the interplay between anisotropic polarization and excitonic resonance in NbOI2, the spatial profile of the polarized SHG response can be tuned by the excitation wavelength. Our results represent a new paradigm for ultrathin, efficient NL optical components.

High–refractive index nanostructured dielectrics have the ability to locally enhance electromagnetic fields with low losses while presenting high third-order nonlinearities. In this work, we exploit these characteristics to achieve efficient ultrafast all-optical modulation in a crystalline gallium phosphide (GaP) nanoantenna through the optical Kerr effect (OKE) and two-photon absorption (TPA) in the visible/near-infrared range. We show that an individual GaP nanodisk can yield differential reflectivity modulations of up to ~40%, with characteristic modulation times between 14 and 66 fs, when probed at the anapole excitation (AE). Numerical simulations reveal that the AE represents a unique condition where both the OKE and TPA contribute with the same modulation sign, maximizing the response. These findings highly outperform previous reports on sub–100-fs all-optical switching from resonant nanoscale dielectrics, which have demonstrated modulation depths no larger than 0.5%, placing GaP nanoantennas as a promising choice for ultrafast all-optical modulation at the nanometer scale.

A system of amorphous gallium phosphide nanopatches is shown to be a flexible, cheap and efficient platform for ultrafast and nonlinear nanophotonics.

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Since their experimental discovery in 2015, Weyl semimetals have generated a large amount of attention due their intriguing physical properties that arise from their linear electron dispersion relation and topological surface states. In particular, in the field of nonlinear (NL) optics and light harvesting, Weyl semimetals have shown outstanding performances and achieved record NL conversion coefficients. In this context, the first steps toward Weyl semimetal nanophotonics are performed here by thoroughly characterizing the linear and NL optical behavior of epitaxially grown niobium phosphide (NbP) thin films, covering the visible to the near‐infrared regime of the electromagnetic spectrum. Despite the measured high linear absorption, third‐harmonic generation studies demonstrate high conversion efficiencies up to 10−4% that can be attributed to the topological electron states at the surface of the material. Furthermore, nondegenerate pump–probe measurements with sub‐10 fs pulses reveal a maximum modulation depth of ≈1%, completely decaying within 100 fs and therefore suggesting the possibility of developing all‐optical switching devices based on NbP. Altogether, this work reveals the promising NL optical properties of Weyl semimetal thin films, which outperform bulk crystals of the same material, laying the grounds for nanoscale applications, enabled by top‐down nanostructuring, such as light‐harvesting, on‐chip frequency conversion, and all‐optical processing.

amplitude, [3,4] directionality of light scattering, [5,6] spin, [7,8] and orbital angular momentum [9,10] without the limitations of intrinsic material losses as for metal-based approaches. In particular, applications driven by near-field enhancement, such as biomolecular sensing, rely on high resonance quality (Q) factors (defined as resonance wavelength divided by line width), and hence high electromagnetic near-field intensities to achieve maximum specimen sensitivity. [11,12] The inherent correlation between the resonance quality factor and the resonator refractive index [13] known from, for example, Mie theory, thus led to the advance of all-dielectric nanophotonics based on high-refractive-index material systems, such as silicon, [14,15] germanium, [16,17] or gallium phosphide. [18,19] Although these materials offer great properties for high-Q resonances in the near-infrared (NIR) and infrared (IR) spectral region, they are accompanied by high material-intrinsic interband absorption losses throughout the visible spectral range due to their intermediate bandgap energies. Owing to these fundamental material limitations, lossless high-index materials throughout the complete visible spectral range are lacking. [20][21][22][23] In particular, for the visible wavelength range, there exists a competition between a large bandgap for lossless All-dielectric optical metasurfaces with high quality (Q) factors have been hampered by the lack of simultaneously lossless and high-refractive-index materials over the full visible spectrum. In fact, the use of low-refractiveindex materials is unavoidable for extending the spectral coverage due to the inverse correlation between the bandgap energy (and therefore the optical losses) and the refractive index (n). However, for Mie resonant photonics, smaller refractive indices are associated with reduced Q factors and low mode volume confinement. Here, symmetry-broken quasi bound states in the continuum (qBICs) are leveraged to efficiently suppress radiation losses from the low-index (n ≈ 2) van der Waals material hexagonal boron nitride (hBN), realizing metasurfaces with high-Q resonances over the complete visible spectrum. The rational use of low-and high-refractive-index materials as resonator components is analyzed and the insights are harnessed to experimentally demonstrate sharp qBIC resonances with Q factors above 300, spanning wavelengths between 400 and 1000 nm from a single hBN flake. Moreover, the enhanced electric near fields are utilized to demonstrate second-harmonic generation with enhancement factors above 10 2 . These results provide a theoretical and experimental framework for the implementation of lowrefractive-index materials as photonic media for metaoptics.

An alternative approach that can address the limitations of IR detectors is to upconvert the IR photons into the ultraviolet (UV)/visible (vis) domain, where room temperature-operating photon detectors are far more efficient. [2] The generation of high-frequency light quanta is also of great interest for a variety of applications such as coherent light sources, [3] photo-therapy, [4] time-domain fluorescence spectroscopy, [5] and nanolithography. [6] Frequency upconversion through a nonlinear parametric sum frequency generation (SFG) has been widely utilized to upconvert IR and vis light into blue and UV light. [7] SFG is a three-wave mixing process in which two incident photons of frequencies ω 1 and ω 2 are converted into an SFG photon at their sum frequency ω 3 (ω 3 = ω 1 + ω 2 ). [8] A special case of SFG is the second-harmonic generation (SHG), which involves two input photons of equal frequencies and one output SHG photon at the doubled frequency (ω 1 = ω 2 = ω, Parametric infrared (IR) upconversion is a process in which low-frequency IR photons are upconverted into high-frequency ultraviolet/visible photons through a nonlinear optical process. It is of paramount importance for a wide range of security, material science, and healthcare applications. However, in general, the efficiencies of upconversion processes are typically extremely low for nanometer-scale materials due to the short penetration depth of the excitation fields. Here, parametric IR upconversion processes, including frequency doubling and sum-frequency generation, are studied in layered van der Waals NbOCl 2 . An upconversion efficiency of up to 0.004% is attained for the NbOCl 2 nanosheets, orders of magnitude higher than previously reported values for nonlinear layered materials. The upconverted signal is sensitive to layer numbers, crystal orientation, excitation wavelength, and temperature, and it can be utilized as an optical cross-correlator for ultrashort pulse characterization.