Yuanqing Yang

Direct Amplitude-Phase Near-Field Observation of Higher-Order Anapole States

Anapole states associated with the resonant suppression of electric-dipole scattering exhibit minimized extinction and maximized storage of electromagnetic energy inside a particle. Using numerical simulations, optical extinction spectroscopy, and amplitude-phase near-field mapping of silicon dielectric disks, we demonstrate high-order anapole states in the near-infrared wavelength range (900-1700 nm). We develop the procedure for unambiguously identifying anapole states by monitoring the normal component of the electric near-field and experimentally detect the first two anapole states as verified by far-field extinction spectroscopy and confirmed with the numerical simulations. We demonstrate that higher-order anapole states possess stronger energy concentration and narrower resonances, a remarkable feature that is advantageous for their applications in metasurfaces and nanophotonics components, such as nonlinear higher-harmonic generators and nanoscale lasers.

A review of gap-surface plasmon metasurfaces: fundamentals and applications

Abstract Plasmonic metasurfaces, which can be considered as the two-dimensional analog of metal-based metamaterials, have attracted progressively increasing attention in recent years because of the ease of fabrication and unprecedented control over the reflected or transmitted light while featuring relatively low losses even at optical wavelengths. Among all the different design approaches, gap-surface plasmon metasurfaces – a specific branch of plasmonic metasurfaces – which consist of a subwavelength thin dielectric spacer sandwiched between an optically thick metal film and arrays of metal subwavelength elements arranged in a strictly or quasi-periodic fashion, have gained awareness from researchers working at practically any frequency regime as its realization only requires a single lithographic step, yet with the possibility to fully control the amplitude, phase, and polarization of the reflected light. In this paper, we review the fundamentals, recent developments, and opportunities of gap-surface plasmon metasurfaces. Starting with introducing the concept of gap-surface plasmon metasurfaces, we present three typical gap-surface plasmon resonators, introduce generalized Snell’s law, and explain the concept of Pancharatnam-Berry phase. We then overview the main applications of gap-surface plasmon metasurfaces, including beam-steerers, flat lenses, holograms, absorbers, color printing, polarization control, surface wave couplers, and dynamically reconfigurable metasurfaces. The review is ended with a short summary and outlook on possible future developments.

Polarization conversion within ultra-compact on-chip all-plasmonic nanocircuits

Plasmonic nanocircuits have the potential to open up new routes in manipulating optical information beyond the diffraction limit and future quantum information processing technologies. We present on-chip polarization conversion based on interaction between two interfering surface plasmon modes supported by metal-insulator-metal (MIM) waveguides. The functional device is realized by all-plasmonic Mach-Zehnder Interferometers (MZI) equipped with impedance-matched Yagi-Uda style nanoantennas for highly efficient far-field coupling. By controlling the relative phase difference between guided MIM gap plasmons propagating in a MZI, namely by precisely differentiating the individual path length, a rotation of the mode optical axes is observed. Depending on the phase difference the resulting mode at the junction point, where two branching channels merge into one channel, can be symmetric or antisymmetric. Accordingly, two cases are investigated in ultra-compact (< 40 μm2) high-definition circuits, where the antisymmetric and symmetric mode are distinguished by observing the origin position and the polarization of the scattering signals. A high conversion efficiency can be realized, encouraging potential application in functional plasmonic nanocircuits.

Anapole-Assisted Strong Field Enhancement in Individual All-Dielectric Nanostructures

High-index dielectric nanostructures have recently become prominent forefront alternatives for manipulating light at the nanoscale. Their electric and magnetic resonances with intriguing characteristics endow them with a unique ability to strongly enhance near-field effects with minimal absorption. Similar to their metallic counterparts, dielectric oligomers consisting of two or more coupled particles are generally employed to create localized optical fields. Here we show that individual all-dielectric nanostructures, with rational designs, can produce strong electric fields with intensity enhancements exceeding 3 orders of magnitude. Such a striking effect is demonstrated within a Si nanodisk by fully exploiting anapole generation and simultaneously introducing a slot area with high-contrast interfaces. By performing finite-difference time-domain simulations and multipole decomposition analysis, we systematically investigate both far-field and near-field properties of the slotted disk and reveal a subtle...