communications www.MaterialsViews.com 4844 www.small-journal.com © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Controlling the Visible Electromagnetic Resonances of Si/SiO 2 Dielectric Core–Shell Nanoparticles by Thermal Oxidation Yuta Tsuchimoto, Taka-aki Yano,* Masaki Hada, Kazutaka G. Nakamura, Tomohiro Hayashi, and Masahiko Hara suppressing quenching. [2] These results are promising for the applications of dielectric nanostructures in field-enhanced spectroscopy and imaging. Dielectric nanoparticles have been also considered as a good magnetic resource for the metamaterials working in the visible spectral region. They are known to possess both elec- tric dipole (ED) and also magnetic dipole (MD) resonances in the visible region. [3–6] MDs are excited in the nanoparticles through circulating displacement currents generated inside the particles, and they scatter the “magnetic” light into far- field. Fundamental characteristics of the dielectric nanostruc- tures as a magnetic resource have been experimentally [3,5,7] and theoretically [8,9] studied. Using theoretical analysis based on Mie theory, Si nanoparticles have been shown to exhibit MD resonances in the visible [4] and infrared [8] regions, that are strongly dependent on the nanoparticle size. Strong MD responses of Si nanoparticles were experimentally observed in the visible region of the spectrum with the use of a dark- field optical microscope. [3,5] Along with the far-field studies, the near-field characteristics of localized magnetic fields of silicon dimmers have been investigated; in these, hot spots derived from MDs were visualized with the use of near-field scanning optical microscopy. [10] EDs and MDs excited in the dielectric nanostructures lead to attractive scattering properties enabling us to achieve unidirectional scattering. [11–15] In 1983, Kerker et al. theoreti- cally demonstrated that dielectric particles exhibit spatially anisotropic scattering patterns arising from interference of EDs and MDs inside the particles. [16] The theory predicted complete suppression of back scattering light from magne- todielectric particles that have recently been experimentally verified in the visible spectra region using Si and gallium arsenide nanoparticles. [11,12] Resonant wavelengths of electric and magnetic multipoles can be controlled by the shapes of dielectric nanostructures, leading to tunability of spectral distances of the optical reso- nances. [17,18] Laser printing of single Si nanoparticles, which enables control of particle size and arrangement of com- plex structures precisely, has been developed recently. [19] Among the dielectric nanostructures with various shapes, core–shell nanoparticles may open up a novel way of con- trolling the optical properties mentioned above because of their tunability of electromagnetic resonances and scattering directivity. Recently, several theoretical studies have been DOI: 10.1002/smll.201500884 Dielectrics Y. Tsuchimoto, Dr. T. Yano, Prof. T. Hayashi, Prof. M. Hara Department of Electronic Chemistry Tokyo Institute of Technology Yokohama, Kanagawa 226-8503, Japan E-mail: yano@echem.titech.ac.jp Dr. T. Yano, Prof. T. Hayashi, Prof. M. Hara RIKEN 2-1 Hirosawa Wako, Saitama 351-0198, Japan Dr. M. Hada, Prof. K. G. Nakamura Materials and Structures Laboratory Tokyo Institute of Technology Nagatsuta 4259, Yokohama 226-8503, Japan Dr. M. Hada PRESTO Japan Science and Technology Agency Kawaguchi 332-0012, Japan Prof. K. G. Nakamura CREST Japan Science and Technology Agency Kawaguchi 332-0012, Japan Prof. M. Hara Earth-Life Science Institute Tokyo Institute of Technology Meguro, Tokyo 152-8551, Japan Dielectric nanostructures with a high refractive index and a low optical loss have attracted considerable attention as an alternative to plasmonic nanostructures. Electromagnetic multipoles excited in the high-index dielectric nanostructures enable us to the manipulation of light beyond the diffraction limit and to offer high electromagnetic field enhancement comparable with that exhibited by the plasmonic nanostruc- tures. In addition, these nanostructures have the remarkable advantage of being able to suppress heat generation caused by energy losses and quenching. [1,2] Recently numerical cal- culations have demonstrated that dielectric nanoantennas made of gallium phosphide exhibit ultra-low energy losses with strongly enhanced electromagnetic fields in the visible region. [1] An analytical model based on dipole–dipole inter- actions has demonstrated that nanodimmers made of silicon (Si) produce strongly localized electromagnetic field and show a high quantum efficiency of localized emitters while small 2015, 11, No. 37, 4844–4849