Abstract
Plasmonic nanostructures presenting either structural asymmetry or metal-dielectric-metal (M-D-M) architecture are commonly used structures to increase the quality factor and the near-field confinement in plasmonic materials. This characteristic can be leveraged for example to increase the sensitivity of IR spectroscopy, via the surface enhanced IR absorption (SEIRA) effect. In this work, we combine structural asymmetry with the M-D-M architecture to realize Ag-Ag(2)O-Ag asymmetric ring resonators where two Ag layers sandwich a native silver oxide (Ag(2)O) layer. Their IR response is compared with the one of fully metallic (Ag) resonators of the same size and shape. The photothermal induced resonance technique (PTIR) is used to obtain near-field SEIRA absorption maps and spectra with nanoscale resolution. Although the native Ag(2)O layer is only 1 nm to 2 nm thick, it increases the quality factor of the resonators' dark-mode by ≈27% and the SEIRA enhancement by ≈44% with respect to entirely Ag structures.
Highlights
Metamaterials are artificially engineered materials that exhibit peculiar electromagnetic properties such as negative refraction [1], artificial magnetism [1] and symmetry breaking absorption [2]
The atomic force microscopy (AFM) tip was moved to the surface-enhanced infrared absorption (SEIRA) hot-spot locations and photothermal induced resonance technique (PTIR) spectra were obtained by sweeping the laser wavelength
To take into account sample-to-sample differences in the PMMA thickness, the bare PMMA spectra from asymmetric split ring resonators (ASRRs)-Ag was used as a reference and the bare PMMA spectrum from the ASRR-Ag-Ag2O-Ag sample was normalized in its respect
Summary
Metamaterials are artificially engineered materials that exhibit peculiar electromagnetic properties such as negative refraction [1], artificial magnetism [1] and symmetry breaking absorption [2]. The strong coupling between light and the SPRs in the visible and infrared (IR) regions give rise to significant radiation losses in the resonators, [13, 25] that combined with the metamaterials absorption losses, de facto limit the quality factors in plasmonic materials [13, 25]. Despite these limitations, the strong mode confinement in the resonators’ near-field compensates the low Q and typically results in better detection sensitivity than alternative technology such as photonic crystals and resonators [24]. Because the resonant wavelength of the resonators analyzed here is ≈ 8.6 μm and the thickness of the native silver oxide is in the order of 1 nm to 2 nm [39, 40], the deeply subwavelength (between ≈ λ/4300 and ≈ λ/8600) dielectric layer in these structures has an pronounced effect on the field localization and enhancement
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