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Strong nonlinearities, like high harmonic generation in optical systems, can lead to interesting applications in photonics. Here the authors fabricate a thin resonant gallium phosphide metasurface capable of avoiding the laser-induced damage and demonstrate efficient even and odd high harmonic generation from it when driven by mid-infrared laser pulses.

Generating intense bursts of high-energy radiation usually requires the construction of large and expensive facilities, such as free-electron lasers, which are based on conventional particle accelerators. Laser-driven accelerators offer a cheaper and smaller alternative, and they are now capable of generating intense bursts of gamma-rays.

Laser machining can modify and reshape materials on the scale comparable to light’s wavelength. Here, authors use tailored microstructures to push the limit of laser machining to a scale that is almost 100 times smaller than a wavelength of light.

In a major departure from their humble origins as ultrathin monolayers, optical metamaterials have now advanced to three-dimensional bulk media exhibiting both electric and magnetic activity.

A photonic crystal can realize an analogue of a valley Hall insulator, promising more flexibility than in condensed-matter systems to explore these exotic topological states.

A three-dimensional spectroscopic sensor is demonstrated that uses a polarization-resolved scattering technique to determine the ___location and orientation of small particles. It exploits strong Fano resonance between a pair of particles, one barely visible and another that is relatively bright, to obtain nanoscale orientation information.

A photonic topological edge state, achieved by employing hexagonal boron nitride and patterned gold films, confines light four orders of magnitude below the diffraction limit while preserving a high quality factor.

Exploiting optical multimodal confinement, the deep-subwavelength confinement of hyperbolic phonon polaritons is demonstrated in isotopically pure hexagonal boron nitride, enabling nanoscale polariton manipulation.

Photon acceleration, which can be used to generate tunable high harmonic radiation, typically requires high-intensity lasers and long propagation distances. Here, Shcherbakov et al. show efficient photon acceleration at low power input power from a semiconductor metasurface, less than a micron thin.

Non-trivial topological phases can allow for one-way spin-polarized transport along the interfaces of topological insulators but they are relatively uncommon in the condensed state of matter. By arranging judiciously designed metamaterials into two-dimensional superlattices, a photonic topological insulator has now been demonstrated theoretically, enabling unidirectional spin-polarized photon propagation without the application of external magnetic fields or breaking of time-reversal symmetry.

Despite their two-dimensional nature, metasurfaces offer flexible and efficient control over the properties of light passing through them. Here, the authors realize chiral silicon-based metasurfaces for applications as polarizers or as emitters of polarized thermal radiation.

Plasmonic nanostructures are known to be an attractive platform for highly sensitive molecular sensors, although they often lack specificity. A plasmonic device with a sharp optical resonance tuned to biomolecules selectively captured on the surface of the device now offers a versatile yet highly specific platform for molecular sensing.

Topological photonic structures offer unique features such as reflection-free and non-reciprocal devices. This Review highlights the experimental progress in the relatively new field of photonic topology.