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The toroidal dipole is a localized electromagnetic excitation independent from the familiar magnetic and electric dipoles. It corresponds to currents flowing along minor loops of a torus. Interference of radiating induced toroidal and electric dipoles leads to anapole, a nonradiating charge-current configuration. Interactions of induced toroidal dipoles with electromagnetic waves have recently been observed in artificial media at microwave, terahertz, and optical frequencies. Here, we demonstrate a quasi-planar plasmonic metamaterial, a combination of dumbbell aperture and vertical split-ring resonator, that exhibits transverse toroidal moment and resonant anapole behavior in the optical part of the spectrum upon excitation with a normally incident electromagnetic wave. Our results prove experimentally that toroidal modes and anapole modes can provide distinct and physically significant contributions to the absorption and dispersion of slabs of matter in the optical part of the spectrum in conventional transmission and reflection experiments.

Measuring the polarization state of light and determining the optical properties of chiral materials are inherently complex issues because of the requirement of consequential measurements between different orthogonal states of polarization. Here, we introduce an on-chip polarimetry based on the visible metasurfaces for addressing the issue of polarization analysis with compact components. We demonstrate integrated metasurface chips can effectively determine a set of Stokes parameters covering a broad wave-band at visible light. For the proof of concept, the optical properties of chiral materials are measured using our proposed device, while experimental verifications are convincing by comparing with the data obtained from commercial ellipsometry.

Metasurfaces comprising 3D chiral structures have shown great potential in chiroptical applications such as chiral optical components and sensing. So far, the main challenges lie in the nanofabrication and the limited operational bandwidth. Homogeneous and localized broadband near‐field optical chirality enhancement has not been achieved. Here, an effective nanofabrication method to create a 3D chiral metasurface with far‐ and near‐field broadband chiroptical properties is demonstrated. A focused ion beam is used to cut and stretch nanowires into 3D Archimedean spirals from stacked films. The 3D Archimedean spiral is a self‐similar chiral fractal structure sensitive to the chirality of light. The spiral exhibits far‐ and near‐field broadband chiroptical responses from 2 to 8 µm. With circularly polarized light (CPL), the spiral shows superior far‐field transmission dissymmetry and handedness‐dependent near‐field localization. With linearly polarized excitation, homogeneous and highly enhanced broadband near‐field optical chirality is generated at a stably localized position inside the spiral. The effective yet straightforward fabrication strategy allows easy fabrication of 3D chiral structures with superior broadband far‐field chiroptical response as well as strongly enhanced and stably localized broadband near‐field optical chirality. The reported method and chiral metasurface may find applications in broadband chiral optics and chiral sensing.

inducing electromagnetic fields. [9,10] In general, SHG conversion efficiency relies on several factors: inversion asymmetry of the hosting media, spatial overlap of the fundamental and second-harmonic (SH) modes, quality factor (Q-factor) of the involved modes, and the field intensity of the fundamental resonant mode. [11][12][13][14][15][16] Among these factors, plasmonic nanostructures [17][18][19][20][21][22][23] have a clear advantage of being particularly effective in inducing strong electromagnetic field that can increase the SHG conversion efficiency. For instance, the large field confinement associated with the magnetic resonances of split-ring resonators (SRRs) has led to a significant SHG enhancement. [23][24][25][26][27][28] While these prior results have been a major step forward in demonstrating SHG in plasmonic structures, further improvement is needed if they were to become a competitive alternative. Take SRR metasurface as an example, most of fabricated SRRs are of planar subwavelength structures, in which they lay flat on a high index dielectric substrate as illustrated in the inset of Figure 1a. While such planar SRRs (PSRRs) are relatively easy to fabricate, the strong fields present in PSRR gaps are inevitably exposed to the underlying substrate, resulting in leakage of the electromagnetic energy into the substrate, reducing the exposure of enhancedThe second harmonic generation (SHG) of vertical and planar split-ring resonators (SRRs) that are broken centro-symmetry configurations at the interface of metal surface and air is investigated. Strong interactions, better electromagnetic field confinements, and less leakage into the substrate for vertical SRRs are found. Experimental results show a 2.6-fold enhancement of SHG nonlinearity, which is in good agreement with simulations and calculations. Demonstrations of 3D metastructures and vertical SRRs with strong SHG nonlinearity majorly result from magnetic dipole and electric quadrupole clearly provides potential applications for photonics and sensing. StereometamaterialsSince its discovery soon after the invention of the laser, the second harmonic generation (SHG) as a prominent nonlinear optical effect has played an important role in various photonic applications ranging from light source, [1] high-resolution imaging, to spectroscopy. [2][3][4] Fundamentally, SHG is prohibited in bulk noble metals such as gold and silver occasionally used in metamaterials because of their centrosymmetric crystal lattices. Recently, the phenomenon of SHG has been widely studied in various metallic nanostructures, such as nanorods, nanoparticles, multiresonant, nanoantennas, etc. [5][6][7][8] In plasmonic systems, nonlinearity was mainly attributed to either the broken centro-symmetry at the metal surface or to the high degree of the asymmetric spatial variation of the