Engineering of Active and Passive Loss in High-Quality-Factor Vanadium Dioxide-Based BIC Metasurfaces.

Active functionalities of metasurfaces are of growing interest in nanophotonics. The main strategy employed to date is spectral resonance tuning affecting predominantly the far-field response. However, this barely influences other essential resonance properties like near-field enhancement, signal modulation, quality factor, and absorbance, which are all vital for numerous applications.

Continuous spectral and coupling-strength encoding with dual-gradient metasurfaces.

To control and enhance light-matter interactions at the nanoscale, two parameters are central: the spectral overlap between an optical cavity mode and the material's spectral features (for example, excitonic or molecular absorption lines), and the quality factor of the cavity. Controlling both parameters simultaneously would enable the investigation of systems with complex spectral features, such as multicomponent molecular mixtures or heterogeneous solid-state materials. So far, it has been possible only to sample a limited set of data points within this two-dimensional parameter space.

Revealing Mode Formation in Quasi-Bound States in the Continuum Metasurfaces via Near-Field Optical Microscopy.

Photonic metasurfaces offer exceptional control over light at the nanoscale, facilitating applications spanning from biosensing, and nonlinear optics to photocatalysis. Many metasurfaces, especially resonant ones, rely on periodicity for the collective mode to form, which makes them subject to the influences of finite size effects, defects, and edge effects, which have considerable negative impact at the application level. These aspects are especially important for quasi-bound state in the continuum (BIC) metasurfaces, for which the collective mode is highly sensitive to perturbations due to high-quality factors and strong near-field enhancement.

Two-Dimensional Chiral Metasurfaces Obtained by Geometrically Simple Meta-atom Rotations.

Two-dimensional chiral metasurfaces seem to contradict Lord Kelvin's geometric definition of chirality since they can be made to coincide by performing rotational operations. Nevertheless, most planar chiral metasurface designs often use complex meta-atom shapes to create flat versions of three-dimensional helices, although the visual appearance does not improve their chiroptical response but complicates their optimization and fabrication due to the resulting large parameter space. Here we present one of the geometrically simplest two-dimensional chiral metasurface platforms consisting of achiral dielectric rods arranged in a square lattice.

Permittivity-Asymmetric -Bound States in the Continuum.

Breaking the in-plane geometric symmetry of dielectric metasurfaces allows us to access a set of electromagnetic states termed symmetry-protected -bound states in the continuum (BICs). Here we demonstrate that BICs can also be accessed by a symmetry breaking in the permittivity of the comprising materials. While the physical size of atoms imposes a limit on the lowest achievable geometrical asymmetry, weak permittivity modulations due to carrier doping, and electro-optical Pockels and Kerr effects, usually considered insignificant, open the possibility of infinitesimal permittivity asymmetries for on-demand, dynamically tunable resonances of extremely high quality factors.

Plasmonic bound states in the continuum to tailor light-matter coupling.

Plasmon resonances play a pivotal role in enhancing light-matter interactions in nanophotonics, but their low-quality factors have hindered applications demanding high spectral selectivity. Here, we demonstrate the design and 3D laser nanoprinting of plasmonic nanofin metasurfaces, which support symmetry-protected bound states in the continuum up to the fourth order. By breaking the nanofins' out-of-plane symmetry in parameter space, we achieve high-quality factor (up to 180) modes under normal incidence.

An achromatic metafiber for focusing and imaging across the entire telecommunication range.

Dispersion engineering is essential to the performance of most modern optical systems including fiber-optic devices. Even though the chromatic dispersion of a meter-scale single-mode fiber used for endoscopic applications is negligible, optical lenses located on the fiber end face for optical focusing and imaging suffer from strong chromatic aberration. Here we present the design and nanoprinting of a 3D achromatic diffractive metalens on the end face of a single-mode fiber, capable of performing achromatic and polarization-insensitive focusing across the entire near-infrared telecommunication wavelength band ranging from 1.

Nanophotonics shines light on hyperbolic metamaterials.

Hyperbolic metamaterials with a unique hyperbolic dispersion relation allow propagating waves with infinitely large wavevectors and a high density of states. Researchers from Korea and Singapore provide a comprehensive review of hyperbolic metamaterials, including artificially structured hyperbolic media and natural hyperbolic materials. They explain key nanophotonic concepts and describe a range of applications for these versatile materials.