Extreme Field Localization and Enhancement in Multi-Scale Plasmonic Nanostructures and their SERS Application

Zhendong Zhu Department of Precision Instrument, Tsinghua University, Beijing, China Benfeng Bai Department of Precision Instrument, Tsinghua University, Beijing, China Oubo You Department of Precision Instrument, Tsinghua University, Beijing, China Qunqing Li Department of Precision Instrument, Tsinghua University, Beijing, China Shoushan Fan Department of Precision Instrument, Tsinghua University, Beijing, China

The importance of photon-electron interactions lies in its ability to concentrate the light energy in nanoscale volumes in the metallic nanostructures and thereafter boost the intensity of the optical near field by several orders of magnitude. Owing to these properties, the localized SPs can facilitate many applications based on the enhanced light-matter interaction, such as the detection of trace element chemical varieties in the vicinity of metallic nanostructures by surface-enhanced Raman spectroscopy (SERS). The surface plasmon enhanced optical near field is typically and predominantly localized in some very small geometric features of metallic nanostructures. Alternatively, one may realize the extreme field localization and enhancement via a so-called cascaded field enhancement (CFE) mechanism [1], which firstly proposed by Stockman and co-authors with a self-similar chain of gold nanoparticles. Here, we propose and demonstrate a multiscale M-shaped nanograting, in which strong CFE can be achieved by the intercoupling between the localized SPs in two adjacent V-grooves, leading to pronounced enhancement of the SERS sensitivity. [2] Another effective way of tuning Fano resonance in a multiscale plasmonic system is through mode hybridization, which results from the interference between two or more plasmonic modes. We also demonstrate the nanoparticle-in-cavity (PIC) nanoantenna array, a multiscale plasmonic nanostructures where individual tiny metal NPs are coupled to some extended metallic structures are ideal systems for generating both the CFE and mode hybridization. A reliable nanofabrication technique based on room-temperature nanoimprinting lithography guarantees the generation of only one gold NP at the bottom of each nanocavity, which is crucial for the generation of the expected CFE. [3]

Reference:
[1] Li K., Stockman M. I., Bergman D. J. Phys. Rev. Lett. 91, 227402 (2003).
[2] Zhu Z., Bai B., Duan H., Zhang H., Zhang M., You O., Li Q., Tan Q., Wang J., Fan S., Jin G. Small 10, 1603-1611 (2014).
[3] Zhu Z., Bai B., You O., Li Q., and Fan S. Nature-Light: Science & Applications (Submitted).

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