Partially Metal-Coated Tips for Near-Field Nanospectroscopy

Scanning probes with functional optical responses are key components of scanning near-field optical microscopes. For nanospectroscopy performed at IR and terahertz (THz) frequencies, one major challenge is that the commonly used metal-coated silicon tips yield nonadjustable coupling efficiency across the spectrum, which greatly limits the signal-to-noise ratio. Here, we test the possibility of a generic design scheme for wavelength-selective tip enhancement via finite-element numerical modeling. We employ a $\mathrm{Si}$-based tip with various gold-coating lengths on the top, yielding a customizable near-field field strength at the tip apex. Calculations show a wavelength-dependent enhancement factor of the metal-coated tip due to the geometrical antenna resonances, which can be precisely tuned throughout a broad spectral range from visible to terahertz frequencies by adjusting the length of the metal coating. By changing the coating pattern into a chiral helical structure on an achiral tip, we also demonstrate the usefulness of coating-length effect in designing high-performance enantiomeric near-field scanning. Our methods and findings offer interesting perspectives for developing near-field optical probes, pushing the detection and resolution limits of tip-enhanced near-field detections, such as fluorescence, Raman, IR, and THz nanospectroscopies.

Figure 1

Numerical modeling of the resonant antenna probe based on a partially coated AFM tip for near-field nanospectroscopy. (a) Schematic of the numerical model to simulate the near-field enhancement at the tip apex for different $\mathrm{Au}$ lengths $(l)$ coated on a $\mathrm{Si}$ tip with a fixed total length ($L$). (b) The numerically calculated E_norm distribution around the apex of a half-coated 200-µm-long antenna tip under the illumination of THz light (wavelength of 200 µm). The upper image (i) shows the side view of the near-field distribution and the lower image (ii) shows the top view at 20 nm above half of the tip apex. The E_norm is normalized to the incident electric field (E_inc), which is linearly polarized at a 60° angle with respect to the tip axis. (c) The numerically calculated E_norm distribution around the apex of a fully coated pure $\mathrm{Au}$ tip [(i) and (iii)] and a partially coated $\mathrm{Au}/\mathrm{Si}$ tip [(ii) and (iv)]. The $\mathrm{Au}$ lengths $l$ of the two types of tips are the same, with 2 µm length in mid-IR range [(i) and (ii)] and 20 µm length in THz range [(iii) and (iv)]. (d) The E_norm of the two types of tips for illuminating light wavelength set to mid-IR range (top) and THz range (bottom).

Figure 2

Simulation of the CLE in near- and mid-IR range. (a) The near-field enhancement mapping for different length ratios $\mathrm{with}L$ fixed at 20 µm. The E_norm is calculated at 20 nm above the tip apex. (b) The horizontal line cuts of the mapping, showing the E_norm as a function of length ratio with different illuminating light wavelengths. (c) The vertical line cuts of the mapping, showing the E_norm as a function of illuminating light wavelength with different length ratios. The red dotted line marks the maximum E_norm point for each length ratio. (d) The wavelength and maximum E_norm of different length ratios corresponding to the red dotted line shown in (c), which is supplemented by a finer scan with smaller wavelength steps (Fig. S2 in the Supplemental Material [17]).

Figure 3

Simulation of the CLE in THz range. (a) The near-field enhancement mapping for different length ratios $\mathrm{with}L$ fixed at 200 µm. The E_norm is calculated at 20 nm above the tip apex. (b) The horizontal line cuts of the mapping, showing the E_norm as a function of length ratio with different illuminating light wavelengths. (c) The vertical line cuts of the mapping, showing the E_norm as a function of illuminating light wavelength with different length ratios. The red dotted line marks the maximum E_norm point for each length ratio. (d) The wavelength and maximum E_norm of different length ratios corresponding to the red dotted line shown in (c), which is supplemented by a finer scan with smaller wavelength steps (Fig. S2 in the Supplemental Material [17]).

Figure 4

Simulation of the near-field enhancement with different geometry. (a) Antenna geometries considered in the simulation. (i) only tip (default); (ii) antenna tip with $\mathrm{Au}$ substrate; (iii) antenna tip with $\mathrm{Au}$ substrate and $\mathrm{Si}$ cantilever. (b) Near-field intensity enhancement as a function of tip-substrate distance, for geometry (ii) depicted in (a). The monitoring point is fixed at 1 nm above the tip apex, shown as the red “x” marker. The illuminated light wavelength is fixed at 280 µm and the length ratio is fixed at 0.6 ($L$ = 200 µm). (c) The E_norm as a function of illuminating light wavelength for all geometries depicted in (a). The tip-substrate distance is 10 nm and the length ratio is fixed at 0.6 ($L$ = 200 µm). The spectra of geometry (i) is scaled by a factor of 10 for better visibility. (d) The scattered far-field distribution with different length ratios for geometry (ii) depicted in (a). The tip-substrate distance is 10 nm and the illuminated light wavelength is fixed at 100 µm.

Figure 5

Numerical modeling of the chiral probe based on partially coated AFM tip for near-field CPL enhancement and differentiation. (a) Schematic of the numerical model to simulate the chiral tip. $\mathrm{Au}$ with different coating length deposit on an achiral bare $\mathrm{Si}$ base in a right-handed chiral (R-chiral) helical pattern, pointing from bottom to the top. The period distance (total length, $L$) between the center of each lap is fixed at 2 µm, while the coating length ($l$) varies from 0.1 to 1.9 µm. The $\mathrm{Si}$ base is the same as the aforementioned mid-IR setup, with 20 µm tip length and apex geometry. CPL in mid-IR range with different chirality illuminates the tip at a 60° angle with respect to the tip axis. (b) The numerically calculated near-field enhancement mapping for different length ratios and different wavelengths. The E_norm is calculated at 20 nm above the R-chiral tip apex, which is illuminated by R-CPL. (c) The near-field enhancement mapping for the ratio of E_norm under R-CPL and L-CPL. (d) The horizontal line cuts of the ratio mapping, showing the E_R‐CPL and E_L‐CPL as a function of length ratio with different illuminating light wavelengths. (e) The vertical line cuts of the mapping, showing the E_R‐CPL and E_L‐CPL as a function of illuminating light wavelength with different length ratios.