Direct Observation of Infrared Plasmonic Fano Antiresonances by a Nanoscale Electron Probe

In this Letter, we exploit recent breakthroughs in monochromated aberration-corrected scanning transmission electron microscopy (STEM) to resolve infrared plasmonic Fano antiresonances in individual nanofabricated disk-rod dimers. Using a combination of electron energy-loss spectroscopy and theoretical modeling, we investigate and characterize a subspace of the weak coupling regime between quasidiscrete and quasicontinuum localized surface plasmon resonances where infrared plasmonic Fano antiresonances appear. This work illustrates the capability of STEM instrumentation to experimentally observe nanoscale plasmonic responses that were previously the domain only of higher-resolution infrared spectroscopies.

Figure 1

(a) Schematic of a gold disk-rod dimer indicating the relevant system parameters and electron-beam location where spectra are acquired (green $\times$). (b) Experimental EEL spectrum of a dimer consisting of an 800-nm-diameter gold disk and a $5\text{−}\mu \mathrm{m}$-long gold rod separated by a 50 nm gap (green curve). Blue and red curves show the monomer spectra for a near-identical disk and rod, respectively. The dimer spectrum is not a simple sum of the two monomer spectra but instead exhibits a narrow dip at the spectral location of each rod mode. A typical example of the EEL spectrum acquired at the rod end may be found in Supplemental Material [52].

Figure 2

EEL point spectrum of a gold disk-rod dimer composed of a 650-nm-diameter disk and a $5\mu \mathrm{m}$ rod separated by a 50 nm gap. The spectrum exhibits a progression of infrared Fano antiresonances due to the interaction between the broad disk dipole plasmon resonance and the spectrally narrow plasmon modes of the rod. The upper panels display the (a) experimental (green curve) and fit (black curve) EEL spectrum and (b) reduced EEL spectrum of the dimer collected at the disk end. The lower panel in (a) shows the experimental monomer spectra of an isolated disk (blue curve) and rod (red curve). As an independent check of the fitting procedure, the theoretical monomer spectra are reconstructed from the dimer fit parameters (black curves), showing excellent agreement. The lower panel in (b) displays the decomposition of each antiresonance in the reduced spectrum into a product of Fano line shapes ${\mathcal{F}}_j(q_j,{\epsilon }_j)$ as described in Eq. (5) with the corresponding value of the real part of the asymmetry parameter $q_{r,j}=\mathrm{Re}{q}_j({\omega }_j)$ indicated above each feature.

Figure 3

Graphical summary of the interaction between individual rod resonances and the disk dipole plasmon in a collection of disk-rod dimers. Each mode pair is represented by a distinct symbol and is characterized by its relative coupling strength $\mathrm{Re}{g}_j/{\gamma }_0({\omega }_j)\sqrt{{\omega }_0{\omega }_j}$ and dissipation rate ${\gamma }_j/{\gamma }_0({\omega }_j)$. Dimers denoted by $R1$ ($R2$) consist of $2.5\text{−}\mu \mathrm{m}$- ($5\text{−}\mu \mathrm{m}$-) long rods, while those denoted by $D1$ ($D2$) consist of 650-nm- (800-nm-) diameter disks. In all dimers, the disk and rod are separated by a gap of 50 nm, and, since $\mathrm{Re}{g}_j/{\gamma }_0({\omega }_j)\sqrt{{\omega }_0{\omega }_j}=1$ denotes the boundary between weak and strong coupling, all dimers are in the weak coupling regime. The gray triangle symbols indicate specific Fano antiresonances shown explicitly in Fig. 2.