We present an infrared perfect absorber model composed of gold nanobars and a photonic microcavity. The inevitable losses in metamaterials are taken as an advantage for high absorbance efficiency. By adjusting the structural geometry, the device can be used for refractive index sensing. In our calculation with a spacer thickness H = 90 nm it can yield more than 99% absorbance in the near-infrared frequency region. The full-width at half-maximum can be realized up to an extremely narrow value of 40.8 nm and the figure of merit can be obtained as high as 357. For sensing applications with a perfect absorber, our work can serve as a model of coupling between the localized surface plasmon within nanoparticles and the propagating surface plasmon along the planar metal layer. The novel concept has great potential to maintain its performance of localized surface plasmon in practical applications.

High-efficiency and polarization-independent focused vortex beam generators and detectors are realized and demonstrated with all dielectric metasurfaces in near-infrared regime.

Recent advances in the physics and technology of light generation via free-electron proximity and impact interactions with nanostructures (gratings, photonic crystals, nano-undulators, metamaterials and antenna arrays) have enabled the development of nanoscale-resolution techniques for such applications as mapping plasmons, studying nanoparticle structural transformations and characterizing luminescent materials (including time-resolved measurements). Here, we introduce a universal approach allowing generation of light with prescribed wavelength, direction, divergence and topological charge via point-excitation of holographic plasmonic metasurfaces. It is illustrated using medium-energy free-electron injection to generate highly-directional visible to near-infrared light beams, at selected wavelengths in prescribed azimuthal and polar directions, with brightness two orders of magnitude higher than that from an unstructured surface, and vortex beams with topological charge up to ten. Such emitters, with micron-scale dimensions and the freedom to fully control radiation parameters, offer novel applications in nano-spectroscopy, nano-chemistry and sensing.

Metasurfaces provide a compact, flexible, and efficient platform to manipulate the electromagnetic waves. However, chromatic aberration imposes severe restrictions on their applications in broadband polarization control. Here, we propose a broadband achromatic methodology to implement polarization-controlled multifunctional metadevices in mid-wavelength infrared with birefringent meta-atoms. We demonstrate the generation of polarization-controlled and achromatically on-axis focused optical vortex beams with diffraction-limited focal spots and switchable topological charge (L∥ = 0 and L⊥ = 2). Besides, we further implement broadband achromatic polarization beamsplitter with high polarization isolation (extinction ratio up to 21). The adoption of all-silicon configuration not only facilitates the integration with CMOS technology but also endows the polarization multiplexing meta-atoms with broad phase dispersion coverage, ensuring the large size and high performance of the metadevices. Compared with the state-of-the-art chromatic aberration-restricted polarization-controlled metadevices, our work represents a substantial advance and a step toward practical applications.

Tunable and high-sensitivity sensing based on Fano resonance is analytically and numerically investigated in coupled plasmonic cavities structure. To analyze and manipulate the Fano line shape, the coupled cavities are taken as a composite cavity that supports at least two resonance modes. A theoretical model is newly-established, and its results agree well with the finite difference time domain (FDTD) simulations for the plasmonic stub-pair structure. The detection sensitivity factor in coupled cavities approaches 6.541 × 107 m−1, which is an order of magnitude larger than single stub case. In addition, the wavelengths of resonant modes in the plasmonic stub-pair structure can be adjusted independently, which paves a new way for improving detection sensitivity. These discoveries hold potential applications for realizing tunable and highly integrated photonic devices.

Metasurface provides a powerful platform to correct the chromatic aberration of conventional lenses in a flexible, integratable, and ultra-compact manner. Mid-wavelength infrared has promised many exciting applications ranged from molecular fingerprint detection to low-light-level night vision. Developing broadband achromatic metalens in mid-wavelength infrared becomes necessary to meet the increasingly urgent demands on high performance photonic devices. Here, we demonstrate the broadband achromatic metalenses from 3.5 to 5 µm with all-silicon metasurfaces. Large phase dispersion control range is achieved to realize metalenses with both large numerical apertures and sample sizes while maintaining high focusing efficiency. The experimental results verify the diffraction-limited achromatic focusing and imaging of the metalenses in mid-wavelength infrared. Additionally, we also demonstrate the versatility by successfully implementing the generation of the broadband achromatic focusing optical vortex (L = 2). This work represents a solid step toward practical implementation of metalens and may find applications in mid-wavelength infrared imaging and detections.

It is well established that for symmetry-protected bound states in the continuum (BICs), introducing the broken geometry symmetry in a dielectric metasurface transforms such a BIC into a quasi-BIC (QBIC) with high-quality factor (Q-factor). Typically, the smaller the asymmetry parameter, the larger the Q-factor. However, it is very challenging to fabricate such nanostructures with an ultra-small asymmetry parameter, thus limiting the measured Q-factor of QBIC. In this work, the authors demonstrated that BICs can be sustained at 𝚪-point in an asymmetric dielectric metasurface, whose unit cell is composed of a dielectric cuboid with an off-centre hole inside it. Multipole decompositions and near-field distributions indicate that the toroidal dipole dominates the nature of such a QBIC. Furthermore, the authors found that such a BIC is robust against the shape of the hole. Besides, two BICs at different wavelengths can be achieved by applying either a rectangular hole or a rectangular lattice. Finally, the authors presented experimental verifications of BIC types by fabricating asymmetric silicon metasurfaces. Measurement results show that the Q-factor of QBIC can reach almost 5,000. The results may enrich the library of BICs and find exciting applications in developing high-performance photonics devices, such as nanolasers, biosensors and enhanced nonlinear harmonic generation.

We propose a novel approach to designing an ultrathin polarization-independent metalens (PIM) by utilizing antennas without rotational invariance. Two arrays of nanoblocks are elaborately designed to form the super cell of the PIM, which are capable of focusing right-handed circularly polarized and left-handed circularly polarized lights. With such a strategy, the PIM is able to achieve polarization-independent focusing, since the light with any polarization can be treated as a combination of the two orthogonal ones. A theoretical analysis based on the Jones vector is proposed to detailedly explore the underlying physics. The polarization-independent characteristic of the designed PIM is also demonstrated by utilizing finite difference time domain simulations. Moreover, polarization-independent focusing can be achieved within a wavelength range of 400 nm. These results can deepen our understanding of polarization-independent focusing and provide a new method for designing ultrathin polarization-independent devices.