Subgroup Decomposition of Complex Plasmon Resonances in Metal Nanoparticle and Nanohole Clusters for Ultrasensitive Biosensing

Plasmonic Fano resonances arising from electromagnetic interaction in metallic nanostructures exhibit spectral characteristics analogous to those from the electron waves in molecular oligomers. Though a great deal of research interest has been attracted to metal nanoparticle clusters, the plasmonic response of their complementary structures – nanohole clusters – remains largely unexplored. In the first part of this talk, we show that a nanohole quadrumer can sustain two Fano resonances when the incident electric field is oriented along the long-axis of the quadrumer system [1]. The underlying physical mechanism responsible for the Fano resonance formation is revealed explicitly by spectrally deconstructing the Fano line-shape, spatially decomposing the structure configuration and mapping the electric field profile and charge distribution [2]. We further show that the spectral profile including resonance linewidth and spectral contrast can be engineered flexibly by adjusting the geometrical parameters such as nanohole diameter, film thickness and inter-hole distance. With an optimized and realistic geometrical configuration, the nanohole quadrumer system exhibits a sensing figure of merit up to 14.25, far surpassing the value reported for nanoparticle clusters. In the second part, we show our recent experimental investigation on the sensitivities of nanoparticle clusters to the adsorption of self-assembled nanometer-thick alkanethiol monolayers [3]. The monolayer sensitivity of such nanoclusters is found to be significantly higher than that of single plasmonic nanoparticles and depends on the nanocluster arrangement, constituent nanoparticle shape, and the plasmon resonance wavelength. Together with full-wave numerical simulation results and the electromagnetic perturbation theory, we unveil a direct correlation between the sensitivity and the near-field intensity enhancement and spatial localization in the plasmonic hot spots generated in each nanocluster.

References:

[1] Y. Zhan, D. Y. Lei, X. Li & S. A. Maier, Nanoscale 6, 4705-4715 (2014).

[2] M. Rahmani et al, Nano Lett. 12, 2101-2106 (2012).

[3] M. Konig et al, ACS Nano 8, 9188-9198 (2014).

dylei@polyu.edu.hk