The High‐Order Toroidal Moments and Anapole States in All‐Dielectric Photonics

Abstract

All‐dielectric nanophotonics attracts ever increasing attention nowadays due to the possibility of controlling and configuring light scattering on high‐index semiconductor nanoparticles. It opens a room of opportunities for designing novel types of nanoscale elements and devices, and paves the way for advanced technologies of light energy manipulation. One of the exciting and promising prospects is associated with utilizing the so‐called toroidal moment, being the result of poloidal currents excitation, and anapole states, corresponding to the interference of dipole and toroidal electric moments. Here, higher‐order toroidal moments of both types (up to the electric octupole toroidal moment) are presented and investigated in detail via the direct Cartesian multipole decomposition allowing new near‐ and far‐field configurations to be obtained. Poloidal currents can be associated with vortex‐like distributions of the displacement currents inside nanoparticles, revealing the physical meaning of the high‐order toroidal moments and the convenience of the Cartesian multipoles as an auxiliary tool for analysis. High‐order nonradiating anapole states accompanied by the excitation of intense near‐fields are demonstrated. It is believed that the results are of high importance for both the fundamental understanding of light scattering by high‐index particles and a variety of nanophotonics applications and light governing on nanoscale.

Fano‐Enhanced Circular Dichroism in Deformable Stereo Metasurfaces

“…Such a difference in handedness‐dependent excitations of the EQ moments results in the differential lineshapes of Fano resonances. It should be pointed out that we also consider the recently proposed high‐order toroidal moments in the contributions of electromagnetic multiple excitations, which validate that the EQ moment still dominates the resonant light–matter interaction. Further optimization of the proposed deformable stereo metasurface would benefit from considering these interesting high‐order toroidal multipole moments .…”

“…It should be pointed out that we also consider the recently proposed high‐order toroidal moments in the contributions of electromagnetic multiple excitations, which validate that the EQ moment still dominates the resonant light–matter interaction. Further optimization of the proposed deformable stereo metasurface would benefit from considering these interesting high‐order toroidal multipole moments . And with the increase of the stereo height, the two Fano profiles evolve into dramatically different lineshapes, i.e., one with a transmission peak and the other with a transmission dip at certain wavelength (Figure c), which consequently induces a much enhanced CD compared with the 2D structures (Figure d).…”

“…The refractive index of the PDMS mediated polymer was set to 1.4. The decompositions of scattering power were calculated using the induced current in the unit cell of the stereo metsurface …”

“…Further optimization of the proposed deformable stereo metasurface would benefit from considering these interesting high-order toroidal multipole moments. [54] And with the increase of the stereo height, the two Fano profiles evolve into dramatically different lineshapes, i.e., one with a transmission peak and the other with a transmission dip at certain wavelength (Figure 2c), which consequently induces a much enhanced CD compared with the 2D structures (Figure 2d). It should be noted that there is an optimized stereo height where CD reaches a maximum at 0.58, as shown in the inset of Figure 2d.…”

Reconfigurable Near‐field Enhancement with Hybrid Metal‐Dielectric Oligomers

“…Moreover, one of the interesting optical properties which can be realized in our nanostructures is a toroidal response. Toroidal multipoles are known to be fundamental electromagnetic excitations different from the generated electric and magnetic multipoles . In this case, the response is mainly determined by the electromagnetic coupling between the particles inside the cluster (the so‐called intra‐cluster coupling) rather than by the individual disks.…”