Substrate‐Modulated Electromagnetic Resonances in Colloidal Cu₂O Nanospheres

Abstract

Dielectric nanoparticles are expected to complement or even replace plasmonic nanoparticles in many optical and optoelectronic applications, because they exhibit small absorption losses and support strong electric and magnetic resonances simultaneously. Dielectric nanoparticles need to be deposited on various substrates in many applications. Understanding the substrate effect on the electromagnetic resonances of dielectric nanoparticles is of great importance for engineering their resonance properties and designing optical devices. In this study, moderate-refractive-index cuprous oxide nanospheres with uniform sizes and shapes are synthesized. The scattering spectra and images of the nanospheres deposited on three types of substrates are analyzed experimentally and theoretically. When supported on indium tin oxide-coated glass slides and Si wafers, the color of the nanospheres varies from blue, cyan, green, yellow, orange and red, covering almost the entire visible region. When deposited on gold films, the electromagnetic resonances of the nanospheres redshift intensively and a new effective magnetic resonance mode appears. The enhanced Raman scattering reveals that large electromagnetic field enhancements are produced in the gap region between the nanosphere and the substrate. The results shed light on the manipulation of the electromagnetic responses of dielectric nanoparticles and the design of dielectric metamaterials in the presence of various substrates. Part. Part. Syst. Charact. 2020, 37, 2000106 2000106 (2 of 12) www.advancedsciencenews.com www.particle-journal.com of many optical and optoelectronic applications, such as optical antennas and surface-enhanced spectroscopies. There have been only a few reports on the study of the substrate effect on the resonance properties of dielectric nanoparticles so far. [23][24][25][26][27][28][29][30][31][32][33] Among them, the resonance properties of dielectric nanoparticles placed on different substrates have mainly been explored through numerical simulations in several works. [23][24][25][26] The coupling between dielectric nanoparticles and metallic substrates has also been reported. [27][28][29][30][31][32][33] All of these previous works about the substrate effect on dielectric nanoparticles, however, have focused on high-refractive-index nanoparticles, especially Si. Compared to high-refractive-index dielectric nanoparticles, such as Si, dielectric nanoparticles with moderate refractive indexes possess several advantages. First, the optical properties of highrefractive-index nanoparticles are quite different from those of moderate-refractive-index ones. The electric dipole and magnetic dipole resonances of dielectric nanoparticles with high refractive indexes (above 3.0), such as Si, are spectrally separated from each other and form two distinct peaks. [11,12,[34][35][36] In contrast, the electric and magnetic resonances of dielectric nanoparticles with moderate refractive indexes (≈1.7-3.0) are close to each other spectrally in the visible region, giving ...

Colloidal Mie Resonators for All‐Dielectric Metaoptics

“…The process to grow moderate refractive index ( n = 2.5–3.5) nanoparticles is more mature than that to grow Si nanoparticles and very high‐quality materials have been developed. Titanium dioxide (TiO 2 ), [ 42,43 ] copper oxide (Cu 2 O), [ 44,45 ] Selenium (Se), [ 46,47 ] and boron (B) [ 48 ] have been proposed as moderate refractive index dielectrics. Figure a,b shows SEM images of submicrometer TiO 2 nanoparticles synthesized via solvothermal nonaqueous technique.…”

“…Wang and coworkers developed single‐crystalline Cu 2 O nanoparticles with uniform size and shape by a one‐step wet‐chemical method. [ 44,45 ] The average diameters are controlled from 126 to 340 nm and the size distributions are 6–9% (Figure 4c). The Cu 2 O nanoparticles have Mie resonances in the visible region (Figure 4d).…”

“…[ 59 ] Similar coupling effects appear in moderate refractive index nanoparticles such as Cu 2 O (Figure 6c). [ 45 ] The structure composed of a nanoparticle and a metal film can confine electromagnetic field into a very small gap, which enhances the light–matter interaction significantly.…”

Directional Control of Light with Nanoantennas

“…In contrast, the ED and MD resonances of moderate-refractive-index (≈1.7-3.0) dielectric nanospheres are spectrally close to each other in the visible region, giving rise to only one peak in the final spectrum. [9,137] The overlap of their resonance frequencies allows for efficient interaction between the ED and MD resonances, which is highly desired for the realization of directional The plane-wave incident light is scattered into the leftward and rightward directions diagonally at different wavelengths. The 3D scattering patterns are obtained from FDTD simulations.…”

“…In contrast, the ED and MD resonances of moderate‐refractive‐index (≈1.7–3.0) dielectric nanospheres are spectrally close to each other in the visible region, giving rise to only one peak in the final spectrum. [ 9,137 ] The overlap of their resonance frequencies allows for efficient interaction between the ED and MD resonances, which is highly desired for the realization of directional scattering. In addition, moderate‐refractive‐index dielectric materials can function as building blocks in the construction of metal@dielectric core@shell nanostructures for realizing zero backward scattering.…”