Colloidal Moderate-Refractive-Index Cu2o Nanospheres as Visible-Region Nanoantennas with Electromagnetic Resonance and Directional Light-Scattering Properties
Update: 2016-02-02 14:07:59      Author: yangjuan@csrc.ac.cn

Magnetic resonance is required when producing negative-refractive-index metamaterials. However, nature lacks such materials that can support strong magnetic resonance in visible region. By properly designing plasmonic metal nanostructures, artificial magnetic resonance in visible region can be achieved, but the unavoidable electromagnetic energy loss due to Ohmic heating in metals reduces the performance of metamaterials made of metal nanostructures. Another type of materials that have been shown to support both electric and magnetic resonances is high-refractive-index dielectric nanoparticles, such as Si and Ge. In such nanoparticles, the electric resonances are derived from the oscillation of polarization charges, while the magnetic resonances are originated from the circular displacement currents that are excited inside the particle by incident light. However, the difficulty in the preparation of well-shaped and sized high-refractive-index dielectric nanostructures limits their practical use in materials.

Recently, the research group led by Prof. H. Q. Lin at Beijing Computational Science Research Center, together with Prof. J. F. Wang’s group at the Chinese University of Hong Kong demonstrated that comparing with high-refractive-index dielectric nanospheres, moderate-refractive-index nanospheres were more suitable as directional nanoantennas. Although both high-refractive-index dielectric nanospheres and moderate-refractive-index ones can support strong magnetic resonances, the magnetic dipole resonance is well separated spectrally from the electric dipole resonance in high-refractive-index nanoparticles, while in moderate-refractive-index nanoparticles the magnetic dipole resonance overlaps with the electric dipole resonance. The spectral overlap of electric and magnetic dipole resonances enables directional forward scattering to occur at the total scattering peak of nanospheres in visible region, and the appropriate refractive index for the occurrence of spectral overlap is identified to be in the range of ≈1.7–3.0. Colloidal Cu2O nanospheres synthesized chemically are chosen to demonstrate experimentally the directional forward scattering induced by spectral overlap of electric and magnetic dipole resonances in visible region, and the variation trend of measured forward-to-backward scattering ratios is in good agreement with those obtained from numerical simulations. This study opens a new direction for developing materials that can support magnetic resonances and for the design of dielectric nanoantennas and metamaterials.

Ref.: Zhang, S., Jiang, R., Xie, Y. M., Ruan, Q., Yang, B., Wang, J. & Lin, H.-Q. Colloidal moderate-refractive-index Cu2O nanospheres as visible-region nanoantennas with electromagnetic resonance and directional light-scattering properties. Adv. Mater. 27, 7432; DOI: 10.1002/adma.201502917 (2015).

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Fig. 1: Scattering properties of Cu2O nanospheres. a) Evolution of the scattering of Cu2O nanospheres as a function of the diameter. The orange and green lines indicate the spectral evolutions of the magnetic and electric dipole resonances, respectively. b) Total scattering spectra of a 200 nm Cu2O nanosphere and the contributions from the electric and magnetic dipole and quadrupole resonances. c) FDTD-simulated far-field scattering patterns of five representatively sized Cu2O nanospheres. In the lower row are the corresponding scattering patterns in a tilted view. The number above each scattering pattern is the diameter of the corresponding Cu2O nanosphere. d) Ratios of b1/a1 and forward-to-backward scattering. In calculating the forward-to-backward scattering ratios, the scattering intensities at 0° and 180° directions are taken as the forward and backward scattering intensities, respectively.

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Fig. 2: Directional forward scattering of the Au@Cu2O nanostructures. a) SEM image of the core@shell nanostructures with an overall diameter of 173 ± 9 nm and a gold core diameter of 60 ± 3 nm. b,c) Schematics of the setups for the backward and forward scattering measurements, respectively. d) Evolution of the ratio between the forward and backward scattering at the Cu2O scattering peak as a function of the diameter of the entire nanostructure. The theoretical values were obtained from the FDTD simulations. The lines are guides to eyes. e) Far-field scattering patterns of five representatively sized Au@Cu2O nanostructures at the Cu2O scattering peak. The diameter of the entire nanostructure is given below each scattering pattern. The Au cores used in the FDTD simulations are all 60 nm in diameter.


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