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Register a new userNear-field Hyperspectral Imaging of Resonant Mie Modes in a Dielectric Island
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All-dielectric, sub-micrometric particles have been successfully exploited for light management in a plethora of applications at visible and near-infrared frequency. However, the investigation of the intricacies of the Mie resonances at the sub-wavelength scale has been hampered by the limitation of conventional near-field methods. Here we address spatial and spectral mapping of multi-polar modes of a Si island by hyper-spectral imaging. The simultaneous detection of several resonant modes allows to clarify the role of substrate and 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 are in agreement with simulations based on a Finite Difference Time Domain method. Our results are relevant to near-field imaging of dielectric structures, to the comprehension of the photophysics of resonant Mie structures, to beam steering and to the resonant coupling with light emitters. Our analysis paves the way for a novel approach to control the spatial overlap of a single emitter with localized electric field maxima.
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Future technologies underpinning high-performance optical communications, ultrafast computations and compact biosensing will rely on densely packed reconfigurable optical circuitry based on nanophotonics. For many years, plasmonics was considered as the only available platform for nanoscale optics, but the recently emerged novel field of Mie resonant metaphotonics provides more practical alternatives for nanoscale optics by employing resonances in high-index dielectric nanoparticles and structures. In this mini-review we highlight some recent trends in the physics of dielectric Mie-resonant nanostructures with high quality factor (Q factor) for efficient spatial and temporal control of light by employing multipolar resonances and the bound states in the continuum. We discuss a few applications of these concepts to nonlinear optics, nanolasers, subwavelength waveguiding, and sensing.
Cathodoluminescence (CL) imaging spectroscopy is an important technique to understand resonant behavior of optical nanoantennas. We report high-resolution CL spectroscopy of triangular gold nanoantennas designed with near-vacuum effective index and very small metal-substrate interface. This design helped in addressing issues related to background luminescence and shifting of dipole modes beyond visible spectrum. Spatial and spectral investigations of various plasmonic modes are reported. Out-of-plane dipole modes excited with vertically illuminated electron beam showed high-contrast tip illumination in panchromatic imaging. By tilting the nanostructures during fabrication, in-plane dipole modes of antennas were excited. Finite-difference time-domain simulations for electron and optical excitations of different modes showed excellent agreement with experimental results. Our approach of efficiently exciting antenna modes by using low index substrates is confirmed both with experiments and numerical simulations. This should provide further insights into better understanding of optical antennas for various applications.
We theoretically analyse the hybrid Mie-exciton optical modes arising from the strong coupling of excitons in organic dyes or transition-metal dichalcogenides with the Mie resonances of high-index dielectric nanoparticles. Detailed analytic calculations show that silicon--exciton core--shell nanoparticles are characterised by a richness of optical modes which can be tuned through nanoparticle dimensions to produce large anticrossings in the visible or near infrared, comparable to those obtained in plexcitonics. The complex magnetic-excitonic nature of these modes is understood through spectral decomposition into Mie-coefficient contributions, complemented by electric and magnetic near-field profiles. In the frequency range of interest, absorptive losses in silicon are sufficiently low to allow observation of several periods of Rabi oscillations in strongly coupled emitter-particle architectures, as confirmed here by discontinuous Galerkin time-domain calculations for the electromagnetic field beat patterns. These results suggest that Mie resonances in high-index dielectrics are promising alternatives for plasmons in strong-coupling applications in nanophotonics, while the coupling of magnetic and electric modes opens intriguing possibilities for external control.
A general strategy for the realization of electric and magnetic quasi-trapped modes located at the same spectral position is presented. This strategys application makes it possible to design metasurfaces allowing switching between the electric and magnetic quasi-trapped modes by changing the polarization of the incident light wave. The developed strategy is based on two stages: the application of the dipole approximation for determining the conditions required for the implementation of trapped modes and the creation of the energy channels for their excitation by introducing a weak bianisotropy in nanoparticles. Since excitation of trapped modes results in a concentration of electric and magnetic energies in the metasurface plane, the polarization switching provides possibilities to change and control the localization and distribution of optical energy at the sub-wavelength scale. We demonstrate a practical method for spectral tuning of quasi-trapped modes in metasurfaces composed of nanoparticles with a pre-selected shape. As an example, the optical properties of a metasurface composed of silicon triangular prisms are analyzed and discussed.
We study nonlinear response of a dimer composed of two identical Mie-resonant dielectric nanoparticles illuminated normally by a circularly polarized light. We develop a general theory describing hybridization of multipolar modes of the coupled nanoparticles, and reveal nonvanishing nonlinear circular dichroism (CD) in the second-harmonic generation (SHG) signal enhanced by the multipolar resonances in the dimer provided its axis is oriented under an angle to the crystalline lattice of the dielectric material. We present experimental results for this SHG-CD effect obtained for the AlGaAs dimers placed on an engineered substrate which confirm the basic prediction of our general multipolar hybridization theory.
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