Influence of the tip in near-field imaging of nanoparticle plasmonic modes: Weak and strong coupling regimes

We identify weak and strong coupling regimes between a near-field probing tip and a plasmonic sample by imaging plasmon-resonant gold nanodisks with scattering-type scanning near-field optical microscopy (s-SNOM). By means of rigorous electrodynamical calculations based on a model system, we find that in the weak coupling regime, s-SNOM can be applied for direct mapping of plasmonic nanoantenna modes, while in the strong coupling regime, the near-field probe allows for high-precision opto-mechanical control of the antenna response.

Figure 1

(Color online) (a) Schematics of the near-field imaging process. A tip located at position $\left(D_x,D_y,D_z\right)$ scans the nanoantenna under study (gold nanodisk). We define $D_x=0$, $D_y=0$, and $D_z=0$ to be at the center of the disk on the top surface. Incident light $E_{\text{inc}}$ illuminates the tip-antenna system and backscattered far-field amplitude $s$ is recorded. (b) Calculated near-field amplitude on top of a gold nanodisk in absence of the tip.

Figure 2

(Color online) [(a) and (b)] Topography and [(c) and (d)] near-field images of a $1\times 1$ micrometer size sample area showing several gold nanodisks. The images on the [(a) and (c)] left-hand side are topography and near field of disks taken with a carbon nanotube tip, whereas in the images of the [(b) and (d)] right-hand side the topography and near field are obtained with a Pt-coated Si tip. Signals from the single disks marked by the white squares are represented in more detail in Fig. 3.

Figure 3

(Color online) [(a) and (c)] Topography and near-field amplitude signal $s_3$ of a gold nanodisk obtained with use of a CNT tip. [(b) and (d)] Topography and near-field amplitude signal $s_3$ obtained with use of a Pt-coated Si tip.

Figure 4

(Color online) Calculated optical images of a 90-nm-wide, 20-nm-high gold nanodisk at a wavelength of $\lambda =633\text{nm}$. [(a) and (b)] Images showing the backscattered far-field amplitude $s$ normalized to the incident field $E_{\text{inc}}$ and the radius of the disk $a$, when the disk is scanned with (a) a weak dielectric probe and (b) a metallic probe. [(c) and (d)] Images showing the near-field signal $s_3$ when the disk is scanned with (c) a weak dielectric probe and (d) a metallic probe. The contours of the disks are marked by white dashed circles.

Figure 5

(Color online) [(a) and (b)] Spectra of the backscattered far-field amplitude $s$ calculated for the probe-disk system normalized to the incident field $E_{\text{inc}}$ and the radius of the disk $a$. (a) Weak dielectric probe, (b) metallic probe. The vertical distance between probe and gold nanodisk is $D_z=21\text{nm}$. [(c) and (d)] Spectra of the near-field signal $s_3$ calculated for (c) a weak dielectric probe and for (d) a metallic probe. Three different horizontal probe positions are considered in all calculations: far edge $D_x=+40\text{nm}$ in red, center $D_x=0\text{nm}$ in blue, and near edge $D_x=-40\text{nm}$ in green.

Figure 6

(Color online) Schematics of the probe-disk coupling as a response to the incident field $E_{\text{inc}}$ in [(a) and (b)] weak and [(c) and (d)] strong probe-disk coupling regimes. When a weak probe dipole is located at the far edge (a) or the near edge (b) of a nanodisk, the total response is basically given by the disk by itself. (c) When a strong probe dipole is located at the far edge of a nanodisk, longitudinal probe-disk coupling is obtained. (d) When the strong probe dipole is located at the near edge of a nanodisk, transversal probe-disk coupling is generated. A schematics of the corresponding polarization pattern in terms of plasmonic particles is displayed close to each diagram.

Figure 7

(Color online) Fourth power of the calculated local field enhancement normalized to the incoming field on top of a gold nanodisk for four different positions of a strongly interacting dipolar probe: (a) far edge at 41 nm on top of the disk, (b) far edge at 21 nm on top of the disk, (c) center at 21 nm on top of the disk, and (d) near edge at 21 nm on top of the disk.