Visualizing Mie Resonances in Low-Index Dielectric Nanoparticles

Resonant light scattering by metallic and high-index dielectric nanoparticles has received enormous attention and found many great applications. However, low-index dielectric nanoparticles typically do not show resonant scattering behaviors due to poor light confinement caused by small index contrast. This Letter describes a simple and effective approach to drastically enhance the resonance effect of the low-index particles by partial metal dressing. Mie resonances of low-index nanoparticles can now be easily visualized by scattered light. This scattering peak depends on sphere size and has a reasonable linewidth. A size difference as small as 8 nm was resolved by a peak shift or even by color change. The scattering peak is attributed to the enhanced ${\mathrm{TE}}_{11}$ Mie resonance of the low-index nanospheres. The metal dress not only provides a high-reflection boundary, but also functions as an antenna to couple the confined light power to the far field, leading to scattering maxima in the spectra. Additionally, the enhanced ${\mathrm{TE}}_{11}$ Mie resonance in low-index nanoparticles features a considerable magnetic response due to the strong circulating displacement currents induced by the intensified $E$ field despite of a low permittivity (hence low index) of the particles. The enhanced Mie resonances could be used to sense minute changes in size or refractive index of low-index nanoparticles and benefit a wide range of applications.

Figure 1

(a) SEM image of a metal-dressed ${\mathrm{SiO}}_2$ nanosphere. (b)–(d) Microscopic dark field transmission views of metal-dressed ${\mathrm{SiO}}_2$ nanospheres, ${\mathrm{SiO}}_2$ nanospheres on a fused silica substrate, and ${\mathrm{SiO}}_2$ nanospheres on a 30 nm Au film over a fused silica substrate. (e) Measured forward scattering spectrum of a 486 nm nanosphere (black) together with simulation (red), and that at a blank area without nanospheres (blue). The simulated spectrum is normalized to the incident power. The incident $E$ field is along the $x$ axis.

Figure 2

(a) SEM images of five selected metal-dressed ${\mathrm{SiO}}_2$ nanospheres with different sizes. (b) Background subtracted forward scattering spectra (black lines) of the five nanospheres and the corresponding simulations (dashed red lines). (c) Photopicture in the dark field transmission view for nanospheres including the five selected ones with size dependent colors. (d) SEM image of the same nanospheres in (c) with the same scope and the same magnification.

Figure 3

(a) Scattering efficiency ($Q_{\mathrm{sc}}$) spectrum at different core sizes of the complete-shell nanosphere (shell thickness 30 nm) in air. (b) $Q_{\mathrm{sc}}$ spectrum at the core size of 550 nm [indicated by the horizontal dashed line in (a)] and the decomposed contributions from the first three fundamental Mie modes (${\mathrm{TM}}_{11}$, ${\mathrm{TE}}_{11}$, and ${\mathrm{TM}}_{12}$). (c) Forward scattering spectrum of the complete-shell nanosphere (core size 550 nm) located on a Au film under dark field illumination. (d) Distributions of scattered electric field $x$ component $|E_{sc-x}|$ and scattered in-plane magnetic field vector ($H_{\mathrm{sc}-y}$, $H_{sc-z}$) at the ${\mathrm{TE}}_{11}$ resonance on the $y\text{−}z$ plane at $x=0$. (e) and (f) Forward scattering spectrum and field distribution of the metal-dressed nanosphere (486 nm) at the ${\mathrm{TE}}_{11}$ resonance.