Dielectric nanostructures for control of electromagnetic waves

• Main Author: Shibanuma, Toshihiko
• Other Authors: Maier, Stefan A. ; Albella, Pablo
• Published: Imperial College London 2017
• Subjects: 530
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.754704

High refractive index dielectric nanoantennas have emerged as a promising unit for improving optical nanodevices by compensating the drawbacks of plasmonic nanoantennas, which have played a key role in nanophotonics to date. The features of high refractive index dielectric nanoantennas, such as low energy losses, excitation of strong magnetic resonances and enhancement of electric field inside and outside the particle, are expected to provide novel methods to manipulate electromagnetic waves in the nanometer scale. In this thesis, we theoretically explore and experimentally demonstrate a variety of nanostructures based on high refractive index dielectric nanoantennas to aim at the efficient and tuneable control of electromagnetic waves in linear and nonlinear manners. Firstly, asymmetric Si dimers are investigated to achieve unidirectional forward scattering with high efficiency. An electric or magnetic dipole mode is excited in each particle constituting the asymmetric dimer at the same wavelength. The interference between these two dipolar modes can direct the scattered field selectively into the forward direction with high scattering efficiency. Secondly, we investigate metasurfaces built of array of Si nanodimers to obtain switching from high transmission to reflection depending on the incident polarization. The different linear polarization direction of the incident light can alter the hybridization modes of the constituent Si dimers and, hence, the effective permittivity and permeability of the metasurface. The resulted overlap and separation of the electric and magnetic dipolar resonances facilitates the control over the switching between high transmission and reflection. Thirdly, asymmetric Si dimers are explored to obtain tuneable control of directional scattering either in the left or right direction from the incident axis. Our theoretical analysis reveals that the electric or magnetic dipoles excited perpendicular to the dimer axis are mainly responsible for the tuneable scattering. Experimental demonstration of the scattering tuneability is carried out along the substrate by using back focal plane techniques combined with a prism coupling setup. Fourthly, we show that the third harmonic generation from a high refractive index dielectric nanoantenna can be significantly improved by adding a metallic component to build a metal-dielectric hybrid nanostructure. In this way, the plasmonic resonance of a Au nanoring can boost the anapole mode excited in a Si nanodisk, strongly enhancing the electric field inside the Si nanodisk. As a result, high third harmonic intensity and conversion efficiency can be achieved even in nanometer scale. Our findings on how we can attain the efficient and tuneable control of electromagnetic waves using high refractive index dielectric nanostructures will contribute to opening the new paths towards the realization of novel optical nanodevices.