Dynamical tuning of the nonlinear optical wavefront allows for a specific spectral response of predefined profiles, enabling various applications of nonlinear nanophotonics. This study experimentally demonstrates the dynamical switching of images generated by an ultrathin silicon nonlinear metasurface supporting a high‐quality leaky mode, which is formed by partially breaking a bound‐state‐in‐the‐continuum (BIC) generated by the collective magnetic dipole (MD) resonance excited in the subdiffractive periodic systems. Such a quasi‐BIC MD state can be excited directly under normal plane wave incidence and leads to a strong near‐field enhancement to further boost the nonlinear process, resulting in a 500‐fold enhancement of the third‐harmonic emission experimentally. Due to sharp spectral features and asymmetry of the unit cell, it allows for effective tailoring of the nonlinear emissions over spectral or polarization responses. Dynamical nonlinear image tuning is experimentally demonstarted via polarization and wavelength control. The results pave the way for nanophotonics applications such as tunable displays, nonlinear holograms, tunable nanolaser, and ultrathin nonlinear nanodevices with various functionalities.

In this review, the recent progress in the electrochemical sensing of dopamine with various graphene and their nanocomposite materials modified electrodes are presented.

We demonstrate that a dielectric anapole resonator on a metallic mirror can enhance the third harmonic emission by two orders of magnitude compared to a typical anapole resonator on an insulator substrate. By employing a gold mirror under a silicon nanodisk, we introduce a novel characteristic of the anapole mode through the spatial overlap of resonantly excited Cartesian electric and toroidal dipole modes. This is a remarkable improvement on the early demonstrations of the anapole mode in which the electric and toroidal modes interfere off-resonantly. Therefore, our system produces a significant near-field enhancement, facilitating the nonlinear process. Moreover, the mirror surface boosts the nonlinear emission via the free-charge oscillations within the interface, equivalent to producing a mirror image of the nonlinear source and the pump beneath the interface. We found that these improvements result in an extremely high experimentally obtained efficiency of 0.01%.

Nonlinear light sources are central to a myriad of applications, driving a quest for their miniaturisation down to the nanoscale. In this quest, nonlinear metasurfaces hold a great promise, as they enhance nonlinear effects through their resonant photonic environment and high refractive index, such as in high-index dielectric metasurfaces. However, despite the sub-diffractive operation of dielectric metasurfaces at the fundamental wave, this condition is not fulfilled for the nonlinearly generated harmonic waves, thereby all nonlinear metasurfaces to date emit multiple diffractive beams. Here, we demonstrate the enhanced single-beam second- and third-harmonic generation in a metasurface of crystalline transition-metal-dichalcogenide material, offering the highest refractive index. We show that the interplay between the resonances of the metasurface allows for tuning of the unidirectional second-harmonic radiation in forward or backward direction, not possible in any bulk nonlinear crystal. Our results open new opportunities for metasurface-based nonlinear light-sources, including nonlinear mirrors and entangled-photon generation.

Metasurfaces are thin and flat arrays of subwavelength nanoparticles, enabling control over the polarization, phase, amplitude, and dispersion of light.¹ They can be used for light emission, detection, modulation, control, and/or amplification at the nanoscale. In recent years, metasurfaces have been a subject of ongoing intense studies as their optical properties can be adapted to a diverse set of applications, including superlenses, tunable images, holograms, etc.^{1 – 5} High-refractive-index dielectric metasurfaces provide a powerful platform for controlling light that can go beyond plasmonics as they cause negligible losses as compared with plasmonic metasurfaces. Dielectric materials offer the unique ability to efficiently manipulate light at the nanoscale based on the simultaneous excitation and control over the optically induced electric and magnetic Mie-type resonances. Resonant dielectric metasurfaces with high-quality factor (high-$Q$) resonances, in particular, are of significant interest due to their ability to strongly enhance the electromagnetic near-fields and boost the light–matter interactions at the nanoscale. In other words, they allow for enhancing the response of metasurfaces to an external electromagnetic field at a particular frequency. Moreover, high-$Q$ metasurfaces can increase the storage time of photons and, thus, light–matter interactions within the subwavelength resonators. This will facilitate various nanophotonics applications, such as enhanced nonlinear photon generation, optical sensing, optoacoustic vibrations, and narrowband filtering. In the last decade, high-$Q$ metasurfaces were mainly associated with Fano resonances (FRs) featuring asymmetric spectral line profiles.^{6 – 10} In FRs, the asymmetry originates from a close interaction of a discrete (localized) state with a continuum of propagation modes.^{11 – 13} Some examples are trapped modes in arrays of dielectric nanodisks with asymmetric holes,^{14 – 16} ring and disk cavities,¹⁷ Dolmen structures,^18,19 and aggregated nanoparticles.^{20 – 23} Recently, different approaches for generating high-$Q$ resonances have been based on the bound-state-in-the-continuum (BIC), i.e., a localized state with zero linewidth that is embedded in the continuum.^{15,24 – 32} Indeed, optical BICs provide a unique opportunity to manipulate the light–matter interaction within the radiative continuum because of their ultrahigh-$Q$ origin and associated giant enhancement of the electromagnetic near-field.³³