Dual-band polarization-insensitive toroidal dipole quasi-bound states in the continuum in a permittivity-asymmetric all-dielectric meta-surface.

Ultra-high quality (Q) factor resonances derived from the bound states in the continuum (BICs) have drawn much attention in optics and photonics. Especially in meta-surfaces, they can enable ultrasensitive sensors, spectral filtering, and lasers because of their enhanced light-matter interactions and rare superiority of scalability. In this paper, we propose a permittivity-asymmetric all-dielectric meta-surface, comprising high-index cuboid tetramer clusters with symmetric structural parameters and configuring periodically on a glass substrate. Simulation results offer dual-band quasi-BICs with high Q values of 4447 and 11391, respectively. Multipolar decomposition in cartesian and electromagnetic distributions are engaged to analyze the physical mechanism of dual quasi-BIC modes, which reveals that they are both governed by magnetic quadrupole (MQ) and in-plane toroidal dipole (TD). The polarization-insensitive and scalable characteristics are also investigated. Additionally, we appraise the sensing performances of the proposed structure. As an example, our work supports an uncommon route to design dual-band polarization-insensitive TD quasi-BICs resonators and facilitates their applications in optic and photonics, such as low-threshold lasers and sensing.

Multiple Fano resonances excitation on all-dielectric nanohole arrays metasurfaces.

Both toroidal dipoles, electric dipoles and magnetic dipoles belong to one type of electromagnetic excitation. In this paper, we present an all-dielectric metasurface composed of an array of square nanoholes. It can simultaneously generate four resonance responses excited by TD, EQ and MD in the continuous near-infrared band. By introducing the in-plane symmetry breaking of the unit cell, asymmetric dielectric nanohole arrays are used to achieve two quasi-BIC resonance modes with high Q-factors excited by EQ and MD. The paper theoretically analyzes and demonstrates the relationship between structural asymmetry and the radiative Q-factor of two Fano resonances, that are governed by symmetry-protected BICs. And multipole decomposition and near-field analysis are performed to demonstrate the dominant role of various electromagnetic excitations in the four modes. The spectra response is also calculated for different incident polarization angles and medium refractive indices. The proposed metasurface is more feasible and practical compared to other complex nanostructures, which may open avenues for the development of applications such as biochemical sensing, optical switches and optical modulators, and provide a reference for the design of devices with polarization-independent properties.

Tuning Multiple Fano Resonances for On-Chip Sensors in a Plasmonic System.

This paper proposed a plasmonic resonator system, consisting of a metal-insulator-metal structure and two stubs, and a Fano resonance arose in its transmittance, which resulted from the coupling between the two stubs. On the basis of the proposed structure, a circle and a ring cavity are separately added above the stubs to create different coupled plasmonic structures, providing triple and quadruple Fano resonances, respectively. Additionally, by adjusting the geometric parameters of the system, multiple Fano Resonances obtained can be tuned. The proposed structure can be served as a high efficient refractive index sensor, yielding a sensitivity of 2000 nm/RIU and figure of merit (FOM) of 4.05 × 10 4 and performing better than most of the similar structures. It is believed that the proposed structure may support substantial applications for on-chip sensors, slow light and nonlinear devices in highly integrated photonic circuits.

[Study on characteristics of simultaneous removal of ammonium and phosphate from waste water by zeolitized fly ash].

Characteristics of simultaneous removal of ammonium and phosphate from wastewater by zeolite synthesized from fly ash was investigated. The amount of ammonium and phosphate removed by zeolitized fly ash changed with time, and approached to a constant value after adsorption time of 24h was reached. The amount of ammonium and phosphate removed also increased with the rise in the amount of zeolite added, but slackened above the solid/liquid ratio of 8 g x L(-1). The removal rate of ammonium by zeolitized fly ash could rise to the maximum value of 60% when pH was between 7 and 9, and declined out of the pH range. For phosphate, minimum removal rate of about 85% was obtained within pH 7-9, and the removal rate increased to about 100% out of the pH range. The adsorption of ammonium on zeolitized fly ash was an exothermic reaction; the removal rate of ammonium could decrease with the rise in temperature. On the contrary, the adsorption of phosphate was an endothermic reaction, and the rise in temperature favored the removal of phosphate by zeolitized fly ash. The adsorptive ability of ammonium on zeolitized fly ash was: Al-Z > Mg-Z > Ca-Z > Na-Z > Fe-Z. The order for phosphate was: Al-Z > Fe-Z > Ca-Z > Mg-Z > Na-Z. It was proposed that the mechanism for the removal of ammonium from wastewater by zeolitized fly ash was cation exchange process, while the mechanism for phosphate was not only the precipitation reaction of phosphate with cation in solution, but also adsorption mechanism.