Abstract
All-dielectric sub-micrometric particles have been successfully exploited for light management in a plethora of applications at visible and near-infrared frequencies. However, the investigation of the intricacies of the Mie resonances at the sub-wavelength scale has been hampered by the limitations of conventional near-field methods. In this paper, we address the spatial and spectral mapping of multipolar modes of a Si island by hyper-spectral imaging. The simultaneous detection of several resonant modes allows us to clarify the role of the substrate and the incidence angle of the impinging light, highlighting spectral splitting of the quadrupolar mode and resulting in different spatial features of the field intensity. We explore theoretically and experimentally such spatial features. Details as small as 200 nm can be detected and agree with simulations based on the finite difference time domain method. Our results are relevant to near-field imaging of dielectric structures, the comprehension of the resonant features of sub-micrometric Mie antennas, beam steering, and the resonant coupling with light emitters. Our analysis suggests a novel approach to control the absorption of a single emitter in the framework of surface enhanced absorption or stimulated emission applications.
Highlights
All-dielectric sub-wavelength sized Mie resonators have emerged in the last decade as promising building blocks for optoelectronic devices, since they provide the possibility to efficiently redirect and concentrate light with low absorption losses.1 The optical modes in high-index dielectric nanoparticles originate from the excitation of optically induced displacement currents and can be both magnetic and electric in nature
Finite difference time domain (FDTD) numerical calculations represent a powerful tool to systematically study the resonant properties of high-index dielectric scitation.org/journal/app particles
The direct experimental characterization of these peculiarities has almost exclusively dealt with far-field measurements12,13 that do not provide access to the spatial distribution of the resonant modes
Summary
All-dielectric sub-wavelength sized Mie resonators have emerged in the last decade as promising building blocks for optoelectronic devices, since they provide the possibility to efficiently redirect and concentrate light with low absorption losses. The optical modes in high-index dielectric nanoparticles originate from the excitation of optically induced displacement currents and can be both magnetic and electric in nature. A similar approach based on scattering-type near-field microscopy was efficiently exploited for mapping the near-field features of plasmonic resonances formed in gold-based rods, gap antennas, and meta-molecules, allowing us to retrieve the phase of the scattered field These methods are either limited to the collection of the signal in dielectric regions, i.e., only in the small area covered by the sub-wavelength sized Mie resonators, or do not provide information on more than one optical mode at a time owing to the use of a laser at a fixed frequency, disregarding the essential aspects of the compresence of spectral and spatial features of the investigated structures. The ability to identify in Si islands the resonances with magnetic nature can be exploited to promote the magnetic radiative decay in quantum emitters with forbidden dipole transitions, such as trivalent lanthanide ions that undergo magnetic spontaneous emission. Mie resonators characterized by a relatively high-quality factor and negligible Ohmic losses are a very attractive approach to promote magnetic emission in quantum emitters, which represents an important step toward the realization of bright magnetic emitters at optical frequencies.
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