Plasmonic mode interferences and Fano resonances in Metal-Insulator-Metal nanostructured interface OPEN

International audienceMetal-insulator-metal systems exhibit a rich underlying physics leading to a high degree of tunability of their spectral properties. We performed a systematic study on a metal-insulator-nanostructured metal system with a thin 6 nm dielectric spacer and showed how the nanoparticle sizes and excitation conditions lead to the tunability and coupling/decoupling of localized and delocalized plasmonic modes. We also experimentally evidenced a tunable Fano resonance in a broad spectral window 600 to 800 nm resulting from the interference of gap modes with white light broad band transmitted waves at the interface playing the role of the continuum. By varying the incident illumination angle shifts in the resonances give the possibility to couple or decouple the localized and delocalized modes and to induce a strong change of the asymmetric Fano profile. All these results were confirmed with a crossed comparison between experimental and theoretical measurements, confirming the nature of different modes. The high degree of control and tunability of this plasmonically rich system paves the way for designing and engineering of similar systems with numerous applications. In particular, sensing measurements were performed and a figure of merit of 3.8 was recorded ranking this sensor among the highest sensitive in this wavelength range. Surface plasmon polariton (SPP) and Localized surface plasmon (LSP) have attracted numerous researchers due to their high technological potential. SPP's are surface waves confined near a metal dielectric interface that can propagate over large distances 1 , making them appealing for applications in biosens-ing 2,3. On the other hand LSP resonances can be defined as the localized resonance condition that massively enhances the electromagnetic field in the vicinity of a metal nanoparticle (NP), when the NP have dimensions much smaller than the excitation wavelength 4. LSP resonance is very sensitive to changes in the NP's dimensions, the dielectric constant of the surrounding media and the nature of the substrate. Because of intense local electrical field enhancements and sharp resonance excitation peaks, metallic NPs are of great interest for applications in surface enhanced Raman spectroscopy (SERS) 5 , chemical and biological sensors 3,6 , cancer treatment 7 and light harvesting 8–10. Recently, strong attention was paid to the potentials of SPP and LSP combinations by investigating metallic NPs on top of metallic thin films. Several studies on such systems have indeed shown the coupling and hybridization between localized and delocalized modes, and the effect of the thickness of the dielectric spacer. Those works have revealed that such coupled systems exhibit enhanced optical properties and larger tunability of their spectral properties compared to uncoupled systems 1,4,11–2