Abstract
Density of nanoparticle (NP) arrays affects the hot spots distribution and strength in NP-metal film (NP-MF) geometry. In-depth understanding of the variation of electromagnetic (EM) field enhancement with NPs density is essential for wide applications of the NP-MF geometry such as surface-enhanced spectroscopies and enhanced efficiency of optoelectronic devices. Here, we show that the field distribution in the NP array on the metal film is greatly enhanced and confined at the NP-NP junctions for very small horizontal gap (g) between neighboring NPs, whereas the fields at the NP-MF junction are extremely small. When gradually increasing g, the field enhancement at the NP-NP junction decreases, along with the gradually enhanced fields at the NP-MF junction. We show that there is an optimal value of horizontal gap (∼75 nm for 80 nm Au NP array on Ag film with 532 nm normal incidence), indicating that the average field enhancement in NP-MF geometry can be optimized by adjusting the horizontal gap. More importantly, it is found that the EM field enhancement is greatly decreased when g fulfills the requirement to couple the 532 nm incident light into SPPs, because of the interference between the LSPR and the SPPs, which leads to a Fano dip at the incident wavelength of 532 nm.
Highlights
Noble metal nanoparticles (NPs) have attracted considerable interest in the fields of surface-enhanced spectroscopies, refractive index sensing, photo-thermal therapy and imaging, photocatalysis, and water splitting because of their ability to generate huge electromagnetic (EM) field enhancement arising from the localized surface plasmon resonances (LSPRs).[1,2,3,4,5,6] LSPR is the coherent oscillation of free electrons at the NP surface.[1]
We show that there is an optimal value of horizontal gap (∼75 nm for 80 nm Au NP array on Ag film with 532 nm normal incidence), indicating that the average field enhancement in NP-metal film (NP-MF) geometry can be optimized by adjusting the horizontal gap
It is found that the EM field enhancement is greatly decreased when g fulfills the requirement to couple the 532 nm incident light into surface plasmon polaritons (SPPs), because of the interference between the LSPR and the SPPs, which leads to a Fano dip at the incident wavelength of 532 nm
Summary
Noble metal nanoparticles (NPs) have attracted considerable interest in the fields of surface-enhanced spectroscopies, refractive index sensing, photo-thermal therapy and imaging, photocatalysis, and water splitting because of their ability to generate huge electromagnetic (EM) field enhancement arising from the localized surface plasmon resonances (LSPRs).[1,2,3,4,5,6] LSPR is the coherent oscillation of free electrons at the NP surface.[1]. We show that the field distribution in the NP array on the metal film is greatly enhanced and confined at the NP–NP junctions for very small horizontal gap (g) between neighboring NPs, whereas the fields at the NP-MF junction are extremely small.
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