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Results in Physics

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TTopological insulators, Plasmons (Physics), Nanoparticles -- Optical properties


Topological insulators (TI) are quantum states of (2D/3D) matter with an insulating interior but conducting edge/surface states, with these boundary conducting states being protected topologically by time-reversal symmetry. Composite materials of heavy atoms such as Bi2Te3 can be fabricated to show TI properties due to the strong intrinsic spin-orbit coupling of the electrons in these materials. Among the so many intriguing physical properties of these materials, their topological magneto-electric (TME) response is unique and has been studied intensively in the literature, leading to intriguing optical effects such as Faraday and Kerr rotations of incident polarized beams at THz frequencies, as recently observed in a series of ingenious experiments. In addition, nontrivial modifications from this TME in the optical reciprocity of propagating EM waves have also been studied in recent time.

Aside from such TME effect on traveling electromagnetic (EM) waves, the corresponding effect on confined evanescent waves has also been studied in the literature. In particular, excitation of surface plasmons at an interface of a TI and a metal has been investigated and the effect has been found to be small depending on the square of the fine-structure constant. In a recent study, we have extended such plasmonic excitation to the geometry of a spherical metallic shell with a TI core, and have observed such TME effect to be relatively more significant for the low frequency bonding modes of the metal shell – leading to manifested red-shifts of these modes. Moreover, this previous study was limited to an incident far-field source and only the dipolar response of the metal-coated TI core has been considered.

The goal of this paper is to extend our previous work, to the study of the TME on the near fields of the TI-metal core-shell system. We thus here consider a localized dipole emitter in the vicinity of such a metal-coated TI sphere and study how the emitting characteristics of the dipole are modified by the excitation of all orders of multipoles of the coated sphere. This problem with a dipole interacting with a TI sphere has been actively studied in the recent literature, but only limited to “bare” TI particles. Realistic experiments can be designed via fluorescence studies of molecules adsorbed to these coated particles similar to those performed with bare metallic nanoparticles, with an eye on further manifestation of the TME effects from the TI, as well as the possibility of employing such effects for the control of the emission properties of the molecules.

Since we shall consider both the molecule-sphere distance and the size of the coated spheres are going to be small compared to the emission wavelengths of the molecule, we shall adopt a long-wavelength formulation of the problem and all the multipolar responses of the coated sphere can be accounted for from calculating the various polarizabilities of the system. In the previous study with a far-field source, scattering experiments were referred to which require all the four (electric-electric; electric-magnetic; magnetic-electric; and magnetic-magnetic) polarizabilities to be calculated. However, in our present study of near-field source in the long wavelength approximation, we will only need the electric-electric polarizability since the other three responses will only lead to higher order contributions when we study how the induced fields from the coated TI sphere will affect the source dipole.


© 2021Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license ( licenses/by-nc-nd/4.0/).


Originally published in Results in Physics, vol. 23, 2021, published by Elsevier B.V.



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