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

Dipole Determination by Polarimetric Spectroscopy Yielding the Orientation of Gold Nanorods

Plasmonic nanorods and other colloidal nanoparticles are widely used probes for enhanced spectromicroscopies. Precise knowledge of the angular orientation of individual nanostructures, respectively of their resonant modes, is required for purposes such as chiroptical spectroscopy or metasurfaces. However, noninvasive measurements of structures below the diffraction limit prove to be challenging. In this article, dark-field spectroscopy requiring only four spectra in a simple microscope setup with a polarizer and an analyzer is employed in combination with an analytical dipole model to extract the orientation of the longitudinal dipolar mode of exemplary gold nanorods. This mode coincides with the azimuthal orientation of the colloid, and for irregularly shaped particles it can help to determine their dominant axis. The technique is demonstrated on multiple nanorods with varying aspect ratios. The spectroscopically determined orientations are compared to orientations extracted from electron microscopy images, resulting in standard deviations as low as ±2.4°. The method can be generalized to nanostructures with 3D orientations or multiple resonances.

Surface-enhanced spectroscopy technology based on metamaterials

Surface-enhanced spectroscopy technology based on metamaterials has flourished in recent years, and the use of artificially designed subwavelength structures can effectively regulate light waves and electromagnetic fields, making it a valuable platform for sensing applications. With the continuous improvement of theory, several effective universal modes of metamaterials have gradually formed, including localized surface plasmon resonance (LSPR), Mie resonance, bound states in the continuum (BIC), and Fano resonance. This review begins by summarizing these core resonance mechanisms, followed by a comprehensive overview of six main surface-enhanced spectroscopy techniques across the electromagnetic spectrum: surface-enhanced fluorescence (SEF), surface-enhanced Raman scattering (SERS), surface-enhanced infrared absorption (SEIRA), terahertz (THz) sensing, refractive index (RI) sensing, and chiral sensing. These techniques cover a wide spectral range and address various optical characteristics, enabling the detection of molecular fingerprints, structural chirality, and refractive index changes. Additionally, this review summarized the combined use of different enhanced spectra, the integration with other advanced technologies, and the status of miniaturized metamaterial systems. Finally, we assess current challenges and future directions. Looking to the future, we anticipate that metamaterial-based surface-enhanced spectroscopy will play a transformative role in real-time, on-site detection across scientific, environmental, and biomedical fields.

Azimuth-dependent chiroptical response in dielectric achiral metasurfaces

Dielectric chiral metasurfaces have attracted tremendous attention for their potential applications on next-generation planar photonic and biophotonic devices. However, most chiral metasurfaces are generally composed of complex chiral meta-atom structures, and few design schemes are developed to dynamically tune the chiroptical response. Here, we experimentally demonstrated an effective strategy to design achiral metasurfaces based on the extrinsic chirality of all-dielectric nanodisks arranged in a square array. We found there are two characteristic lattice modes corresponding to the coupling of nanodisk meta-atoms, and the azimuth-induced asymmetric distribution of electromagnetic fields leads to an obvious CD signal. Intriguingly, the azimuth-dependent CD signals exhibit fourfold rotational symmetry that is strongly dependent on the lattice symmetry. Meanwhile, the CD response can be modulated by simply changing azimuth of the nanodisks without fabricating two opposite chiral structures. Our work provides a simple design strategy for chiral metasurfaces based on the geometrically simplest 2D planar achiral meta-atoms and highlights the generation mechanism as well as adjustable ability of the chirality, which may promise various practical applications on on-chip azimuth and horizontal sensors.

Observation of Photonic Chiral Flatbands

Distinct selectivity to the spin angular momenta of photons has garnered significant attention in recent years, for its relevance in basic science and for imaging and sensing applications. While nonlocal metasurfaces with strong chiral responses to the incident light have been reported, these responses are typically limited to a narrow range of incident angles. In this study, we demonstrate a nonlocal metasurface that showcases strong chirality, characterized by circular dichroism (∼0.6), over a wide range of incident angles ±5°. Its quality factor, circular dichroism and resonant frequency can be optimized by design. These findings pave the way to further advance the development of valley-selective optical cavities and augmented reality applications.

Strong Chiro-Optical Activity of Plasmonic Metasurfaces with Inverted Pyramid Arrays

Chiral plasmonics has emerged as a powerful tool for manipulating light at the nanoscale with unprecedented control over light polarization. The advances in nanofabrication have led to the creation of nanostructures that support strong chiroptical responses. However, the complexity of the fabrication and the associated high costs remain major challenges in upscaling these architectures. Here, we report on the development of chiral plasmonic metasurfaces composed of inverted pyramid arrays with mismatched directions with respect to the lattice vectors of the array. These metasurfaces are fabricated using a combination of soft lithography and anisotropic etching, resulting in cost-effective and reproducible chiral nanostructures without the need for expensive equipment. The fabricated metasurfaces exhibit high differential transmittance values in the visible spectrum, which are among the highest reported for plasmonic films. Theoretical modeling corroborates the experimental results, demonstrating the significant influence of the mismatch angle on the chiral behavior. Complete polarimetric characterization reveals exceptional chiro-optical activity with circular birefringence exceeding 375°/μm and Kuhn's dissymmetry factors (g-factors) approaching unity.

Chiral Dichroism in Resonant Metasurfaces with Monoclinic Lattices

We demonstrate that chiral response can be achieved in resonant metasurfaces with a monoclinic lattice symmetry (the so-called Bravais oblique lattices) where the mirror symmetry is broken by the lattice asymmetry and also by a substrate, whereas each individual meta-atom remains fully achiral. We describe the underlying physics by introducing a mode chirality parameter as a quantitative measure of the lattice chiral eigenmodes. We confirm experimentally selective linear and nonlinear chiral interaction of resonant silicon metasurfaces with circularly polarized light.

Hybrid Anapole Induced Chirality in Metasurfaces

The interaction between light and matter, particularly chirality, plays a pivotal role in modern science and technology. Typically, metasurfaces achieve chiro-optical effects by coupling electric and magnetic dipoles in specific orientations. In this work, the design and optimization of an asymmetric H-shaped metasurface is explored to induce hybrid anapole (HA) for optical activity. When the symmetry of the metasurface structure is disrupted, the design can simultaneously excite first-order and pseudo high-order HA under illumination with a specific circular polarization, both occurring within the same spectral regime. This results in high reflection for one circular polarization and a significant reduction in reflection for the orthogonal polarization, thereby exhibiting exceptional chiro-optical activity. Moreover, the HA-based chiral metasurface demonstrates strong polarization control capabilities, as verified by Stokes parameter analysis, revealing high birefringence and a pronounced dependence on the incident polarization angle. These results provide valuable insights for the design and optimization of HA metasurfaces for advanced optical applications and polarization control, paving the way for new developments in chiral nanophotonics.

Thin-Film-Assisted Photothermal Deformation of Gold Nanoparticles: A Facile and In-Situ Strategy for Single-Plate-Based Devices

Optical-induced shape transformation of single nanoparticles on substrates has shown benefits of simplicity and regularity for single-particle device fabrication and on-chip integration. However, most of the existing strategies are based on wet chemical growth and etching, which could lead to surface contamination with limited local selectivity and device compatibility. Shape deformation via the photothermal effect can overcome these issues but has limited versatility and tunability largely due to the high surface tension of the molten droplet. Here we show gold nanoparticles (Au NPs) can drastically transform into nanoplates under the irradiation of a continuous wave laser (446 nm). We reveal the dielectric thin film underneath the molten Au is critical in deforming the NP into faceted nanoplate under the drive of photothermophoretic forces, which is sufficient to counteract the surface tension of the molten droplet. Both experimental evidence and simulations support this thin-film-assisted photothermal deformation mechanism, which is local selective and generally applicable to differently shaped Au NPs. It provides a facile and robust strategy for single-plate-based device applications.