Figure 1

Optical detection of nano-objects (yellow) employing the enhanced near fields (a) in the feed gap between two antenna arms, (b) at the apex of a metal probe tip, and (c) in the gap formed between a probe tip and a highly reflecting substrate.Reuse & Permissions

Figure 2

(a) Schematics of the experimental setup. The tip is vibrating vertically with constant amplitude $\Delta z$, while the substrate covered with Au particles is scanned in the $x$, $y$, and $z$ directions. (b) Optical amplitude images ($s_3$) of 16 nm Au particles extracted from 3D data sets. Third row: Vertical slices showing the optical signal amplitude $s_3$ dependent on the gap width $d_0$ and the horizontal particle position. The black base thereby visualizes the topography, i.e., positions where the vibrating tip comes into contact with the surface. Second row: Upper parts of the vertical slices but displayed at enhanced image contrast. First row: Horizontal slices ($x\mathrm{\text{−}}y$) at distance $d_0=17\mathrm{nm}$ to the substrate, i.e., 1 nm above the particle (marked by the dashed line). (c) Normalized signal amplitudes along the dashed lines in (b): blue, $1080{\mathrm{cm}}^{-1}$; red, $927{\mathrm{cm}}^{-1}$.Reuse & Permissions

Figure 3

(a) Experimental (dots) and theoretical (solid lines) optical signal amplitudes $s_3$ and $s_3^{*}$ as a function of the gap width $d_0=d_{\mathrm{Au}}$. Each $s_3^{*}$ value (red symbols) represents an average obtained from more than 10 Au particles of diameter $d_{\mathrm{Au}}\pm 0.25\mathrm{nm}$. (b) Absolute optical particle contrast $\Delta s_3=s_3^{*}-s_3$ for different substrates. The experimental $\Delta s_3$ values (red symbols) for the Si substrate are taken from (a) and that for the 16 nm Au particles (green symbols) from Fig. 2c. The solid lines show dipole calculations applying the same parameters as in (a) but different dielectric values for the substrate. No further fitting was performed. All values are normalized to the $s_3$ value on the Si substrate at $d_0=0$. (c) Illustration of the dipole model.Reuse & Permissions

Figure 4

(a) Field distribution in the gap between a Pt cone (25 nm tip radius) and a Au surface at frequency $927{\mathrm{cm}}^{-1}$. (b) Au particle of 16 nm inside the gap. (c) Tenfold near-field enhancement in case of a phonon-polariton-resonant SiC substrate. (a)–(c) All values depict the electric field amplitudes normalized to the illuminating field. The 1 nm gap between particle and substrate accounts for the BSA layer in the experiment. As we operate our microscope in tapping mode, we assume that the smallest tip-particle distance during the tapping cycle is about 1–2 nm. Latter value is chosen for the calculations.Reuse & Permissions