We review the physics and some applications of photonic structures designed for the realization of strong nonlinear chiroptical response. We pay much attention to the recent strategy of utilizing different types of optical resonances in metallic and dielectric subwavelength structures and metasurfaces, including surface plasmon resonances, Mie resonances, lattice-guided modes, and bound states in the continuum. We summarize earlier results and discuss more recent developments for achieving large circular dichroism combined with the high efficiency of nonlinear harmonic generation.

Optical metasurfaces, i.e. arrays of nanoantennas with sub-wavelength size and separation, enable the manipulation of light-matter interactions in miniaturized optical components with no classical counterparts. Six decades after the first observation of the second harmonic generation (SHG) in bulk crystals, these devices are expected to break new ground in the field of nonlinear optics, shifting the focus from the phase matching approach achieved within long propagation distances to that of near-field resonances interplay in leaky nanocavities. Here we review the recent progress in SHG with all-dielectric metasurfaces. We discuss the most used technological platforms which underpinned such advances and analyze different SHG control approaches. We finally compare their performances with other well-established technologies, with the hope to delineate the current state-of-the-art and figure out a few scenarios in which these devices might soon offer unprecedented opportunities.Optical metasurfaces, i.e. arrays of nanoantennas with sub-wavelength size and separation, enable the manipulation of light-matter interactions in miniaturized optical components with no classical counterparts. Six decades after the first observation of the second harmonic generation (SHG) in bulk crystals, these devices are expected to break new ground in the field of nonlinear optics, shifting the focus from the phase matching approach achieved within long propagation distances to that of near-field resonances interplay in leaky nanocavities. Here we review the recent progress in SHG with all-dielectric metasurfaces. We discuss the most used technological platforms which underpinned such advances and analyze different SHG control approaches. We finally compare their performances with other well-established technologies, with the hope to delineate the current state-of-the-art and figure out a few scenarios in which these devices might soon offer unprecedented opportunities.

Dynamically controlling terahertz (THz) wavefronts in a designable fashion is highly desired in practice. However, available methods working at microwave frequencies do not work well in the THz regime due to lacking suitable tunable elements with submicrometer sizes. Here, instead of locally controlling individual meta-atoms in a THz metasurface, we show that rotating different layers (each exhibiting a particular phase profile) in a cascaded metadevice at different speeds can dynamically change the effective Jones-matrix property of the whole device, thus enabling extraordinary manipulations on the wavefront and polarization characteristics of a THz beam impinging on the device. After illustrating our strategy based on model calculations, we experimentally demonstrate two proof-of-concept metadevices, each consisting of two carefully designed all-silicon transmissive metasurfaces exhibiting different phase profiles. Rotating two metasurfaces inside the fabricated devices at different speeds, we experimentally demonstrate that the first metadevice can efficiently redirect a normally incident THz beam to scan over a wide solid-angle range, while the second one can dynamically manipulate both the wavefront and polarization of a THz beam. Our results pave the way to achieving dynamic control of THz beams, which is useful in many applications, such as THz radar, and bio- and chemical sensing and imaging.

Free from phase-matching constraints, plasmonic metasurfaces have contributed significantly to the control of optical nonlinearity and enhancement of nonlinear generation efficiency by engineering subwavelength meta-atoms. However, high dissipative losses and inevitable thermal heating limit their applicability in nonlinear nanophotonics. All-dielectric metasurfaces, supporting both electric and magnetic Mie-type resonances in their nanostructures, have appeared as a promising alternative to nonlinear plasmonics. High-index dielectric nanostructures, allowing additional magnetic resonances, can induce magnetic nonlinear effects, which, along with electric nonlinearities, increase the nonlinear conversion efficiency. In addition, low dissipative losses and high damage thresholds provide an extra degree of freedom for operating at high pump intensities, resulting in a considerable enhancement of the nonlinear processes. We discuss the current state of the art in the intensely developing area of all-dielectric nonlinear nanostructures and metasurfaces, including the role of Mie modes, Fano resonances, and anapole moments for harmonic generation, wave mixing, and ultrafast optical switching. Furthermore, we review the recent progress in the nonlinear phase and wavefront control using all-dielectric metasurfaces. We discuss techniques to realize all-dielectric metasurfaces for multifunctional applications and generation of second-order nonlinear processes from complementary metal–oxide–semiconductor-compatible materials.