Journal
ACS NANO
Volume 7, Issue 12, Pages 11138-11146Publisher
AMER CHEMICAL SOC
DOI: 10.1021/nn404869c
Keywords
near-field optical microscopy; symmetry; polarization independence; plasmonic oligomers
Categories
Funding
- EPSRC Active Plasmonics Programme
- Leverhulme Trust
- Australian Research Council [FT110100037]
- European Science Foundation [4370, 5214]
- EPSRC [EP/D063329/1, EP/G031819/1, EP/H000917/2] Funding Source: UKRI
- Engineering and Physical Sciences Research Council [GR/S80301/01, EP/G031819/1, EP/D063329/1, EP/H000917/2] Funding Source: researchfish
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Flexible control over the near- and far-field properties of plasmonic nanostructures is important for many potential applications, such as surface-enhanced Raman scattering and biosensing. Generally, any change in the polarization of the incident light leads to a change in the nanoparticle's near-field distribution and, consequently, in its far-field properties as well. Therefore, producing polarization-invariant optical responses in the far field from a changing near field remains a challenging issue. In this paper, we probe experimentally the optical properties of cruciform pentamer structures as an example of plasmonic oligomers-and demonstrate that they exhibit such behavior due to their symmetric geometrical arrangement. We demonstrate direct control over hot spot positions in sub-20 nm gaps, between disks of 145 nm diameter at a wavelength of 850 nm, by means of scattering scanning near-field optical microscopy. In addition, we employ the coupled dipole approximation method to define a qualitative model revealing the relationship between the near and far field in such structures. The near-field profiles depend on particular mode superpositions excited by the incident field and, thus, are expected to vary with the polarization. Consequently, we prove analytically that the far-field optical properties of pentamers have to be polarization-independent due to their rotational symmetry.
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