Although Raman spectroscopy has been commercialized, low-cost and large-area surface enhanced Raman spectroscopy (SERS) substrates with localized enhanced field are heavily required. However, currently dominant manufacturing techniques are expensive and complicated for large-area fabrication. Furthermore, most SERS substrates can only be used for individual excitation wavelengths. In this work, we will report an ultra-broadband super absorbing metasurface to enhance SERS signals in a broadband region (i.e. from 450 nm to 1000 nm). The design consisting of an Ag ground plate, a SiO2 spacer, and a layer of Ag nanoparticles was fabricated using simple film deposition and thermal annealing techniques. A broadband absorption over 80% from 414 nm to 956 nm was obtained, resulting in localized field enhancement between adjacent nanoparticles. We employed this metasurface to test its broadband SERS signal by adsorbing 1,2-Bis(4-pyridyl)-ethylene (BPE) molecules on top of it. We employed 5 laser lines (i.e., 514, 532, 633, 671 and 785 nm) to excite the sample and observed fingerprint signature of BPE molecules under all 5 excitation wavelengths with the average enhancement factor up to 5.3×107. Therefore, the designed SERS substrate can work for almost “all” available excitation wavelengths over a broadband, which is particularly useful for sensing a broad spectrum of chemicals on the same chip.
Potential solar energy applications of metamaterial absorbers require spectrally tunable resonance to ensure the
overlap with intrinsic absorption profiles of active materials. Although those resonance peaks of metamaterial
absorbers can be tuned precisely by lithography-fabricated nanopatterns with different lateral dimensions, they are
too expensive for practical large-area applications. In this work, we will report another freedom to tune the spectral
position of the super absorbing resonance, i.e. the spacer thickness. The structure was fabricated by evaporating an
optically opaque metallic ground plate, a dielectric spacer layer, and a top metallic thin film followed by thermal
annealing processes to form discrete nanoparticles. As the spacer thickness increases from 10-90 nm, two distinct
shifts of the absorption peak can be observed [i.e. a blue-shift for thinner (10-30 nm) and a red-shift for thicker
spacer layers (30-90 nm)]. To understand the physical mechanism, we characterized effective optical constants of
top nanopattern layer and loaded them into numerical simulation models. A good agreement with experimental data
was only observed in the thick spacer region (i.e. 30-90 nm). The optical behavior for thinner spacers cannot be
explained by effective medium theory and interference mechanism. Therefore, a microscopic study has to be
performed to reveal strongly coupled modes under metallic nanopatterns, which can be interpreted as separate
antennas strongly coupled with the ground plate. Since the resonant position is sensitive to the spacer thickness, a
tunable super absorbing metasurface is realizable by introducing spatial tunable materials like stretchable chemical/
biomolecules.
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