Low-loss optical antennas based on high index dielectric nanostructures have been attracting much attention as an alternative to plasmonic antennas that often suffer from losses of noble metals. They have several characteristic features such as the enhancement of a magnetic field of incident electromagnetic wave and the directional scattering. These properties provide an opportunity to realize new types of metasurfaces and metamaterials. Among several high-index dielectrics, silicon (Si) has some advantages. It is abundant and the most commonly used material in semiconductor industry. Furthermore, most importantly, the imaginary part of the permittivity of crystalline Si is very small in the optical regime due to the indirect bandgap nature.
In this work, we focus on Si nanospheres 50 to 250 nm in diameter as a dielectric nanoantenna. In the size range, Si nanospheres exhibit the electric and magnetic Mie resonances in the visible range. So far, several techniques have been developed to fabricate submicron size Si particles. However, all the previously developed methods have problems in the control of the size, shape and crystallinity and in the scalability for mass production. In this work, we develop crystalline 50-250 nm Si nanospheres dispersible in alcohol by applying inorganic surface modification technique developed in our group. Thanks to the perfect dispersion of Si nanospheres, they can be placed on an arbitrary substrate, and the interaction between a single sphere and a substrate can be studied in detail. We show that forward and backward scattering spectra obtained from single Si nanospheres are very well reproduced by analytical Mie calculations, which guarantees high quality of our Si nanospheres. The important application of the developed Si nanosphere is an antenna for fluorescence enhancement of nearby materials. To demonstrate that, we placed a Si nanosphere on a thin dye layer (>1 nm in thickness) coated on a glass substrate. We observed up to 200 times enhancement of the dye fluorescence. Numerical simulations revealed that the observed fluorescence enhancement is mainly due to the enhancement of the incident electric field by the sphere. We also fabricated a hybrid nanoantenna structure composed of a Si nanospheres and a gold film separated by a very thin dielectric spacer. This structure can confine electric fields in the gap very effectively at the resonance wavelengths of the gap modes, and thus can further enhance the optical responses of a material in the gap. We demonstrate fluorescence enhancement by the hybrid nanoantenna by placing a monolayer of luminescent quantum dots in the gap. We also observed strong modification of the spectral shape due to the large Purcell enhancement.
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