Scattering-type scanning near-field optical microscopy (s-SNOM) enables super-resolution spectral imaging of the surfaces of a wide range of materials and nanostructures. However, although s-SNOM offers high lateral spatial resolution, the near-field response characteristics perpendicular to the sample surface have rarely been investigated.

Current s-SNOM methods face challenges in characterizing vertical interactions, which indirectly stems from their large far-field scattering background — that is, reflection or scattering from the tip shaft and cantilever, as well as reflection or scattering caused by sample roughness, in the absence of short-range tip-sample interactions. In s-SNOM, to remove this large background, the probe tip oscillates sinusoidally above the sample in tapping mode, and the near-field signal is extracted at harmonics of the tip oscillation frequency using a lock-in amplifier. However, approach curves do not directly describe the vertical dependence of the near-field signal. They are obtained through a complex signal generation mechanism that depends on the tip oscillation amplitude and the order of lock-in demodulation. As a result, approach curves lack a direct and explicit relationship with the vertical characteristics of the near-field interaction between tip and sample.

The paper introduced in this article proposes an accurate and fast reconstruction method to obtain the vertical characteristics of near-field interactions. The authors also investigated the bound electromagnetic field components of surface phonon polaritons on boron nitride nanotubes, finding that they decay within a range of 20 nm, with significant phase changes in the near-field signal.

Figure 1. a) Schematic of the experimental setup and working principle. The inset on the right shows the lock-in demodulated amplitude at the 18th harmonic, measured experimentally with a platinum-coated probe and a gold substrate.

b) Reconstructed raw s-SNOM waveform.

c) s-SNOM interaction curve. The dashed line shows the linear trend of the background, which is characteristic of the far-field contribution.

Figure 1a shows the experimental setup and working principle, with which the authors obtained the experimentally measured 18th-harmonic signal. The signal waveform was then reproduced through a Fourier series (Equation 1), as shown in Figure 1b. Equation 2 describes the position change of the probe tip. Figure 1c shows the s-SNOM interaction curve obtained from Equation 3, where the dashed part is the fitted far-field contribution. It can be seen that when the tip-sample distance is within 50 nm, a nonlinear growth of the signal is observed, i.e., the near-field signal dominates. At larger tip-sample distances, the far-field signal dominates.

For the AC far-field signal, the sinusoidally oscillating probe scatters the incident infrared field over a nanoscale uniform region. Therefore, the far-field contribution to the AC signal is sinusoidal and appears only in the first-harmonic demodulation. This yields the following equation:

If the value of η can be obtained in some way, the near-field signal can be fully determined. The authors argue that when the tip-sample distance D is large, the near-field signal is insensitive to changes in D, which can be used to solve for η — corresponding to minimizing the slope in the following equation:

To verify the accuracy of the results, the authors compared the experimentally obtained results with the point-dipole model (Figure 2a). Figure 2b shows the near-field time-domain signal obtained after solving for η. Figure 2c shows the s-SNOM interaction curve obtained from Equation 5, where the dashed line is the interaction curve of the point-dipole model. Figure 2d shows the normalized approach curves of the second to fifth harmonics of the interaction curve; it can be seen that the third-harmonic normalization agrees almost perfectly with the experimentally obtained third-harmonic approach curve.

Figure 2. a) Point-dipole model simulation of the relationship between the effective polarizability amplitude and the tip-sample distance.

b) Near-field waveform reconstructed from the previous data. The first-harmonic contribution was found to be 0.262.

c) s-SNOM interaction curve. The dashed curve is the result plotted from the point-dipole model. d) Normalized approach curves derived from the second to fifth harmonics of the interaction curve. The inset shows a comparison between the derived third-harmonic approach curve and the experimentally obtained one.

Finally, the authors applied the derived interaction equations to observe the vertical response of surface phonon polaritons on boron nitride nanotubes, as shown in Figure 3.

Figure 3. a) AFM topography image of a boron nitride nanotube.

b) Scattering near-field optical microscopy image at 1400 cm⁻¹.

c) Scattering near-field optical microscopy image at 1420 cm⁻¹.

d) Scattering near-field optical microscopy image at 1360 cm⁻¹.

e) Interaction curves at position 1 (red) and position 2 (blue) at 1400 cm⁻¹.

f) Interaction curves at position 3 (red) and position 4 (blue) at 1420 cm⁻¹.

Because the surface phonon polaritons of boron nitride have field components bound to the surface, additional enhancement of the near-field scattering occurs when the tip is very close. Owing to the presence of the short-range surface-bound components, the measured decay length of boron nitride at the phonon-polariton-active frequencies is shortened compared with the case without polaritons. The trend of the decay length decreasing with frequency (Figure 3f) indicates that at higher infrared frequencies, the field components of the surface phonon polaritons are more strongly bound to the boron nitride surface.

In summary, the paper introduced in this article developed an experimental method for reconstructing vertical near-field interactions in scattering-type scanning near-field optical microscopy (s-SNOM), which is promising for characterizing the vertical field distributions of a wide range of nano-optical materials and structures.

Reference

Wang, L., Xu, X. Scattering-type scanning near-field optical microscopy with reconstruction of vertical interaction. Nat Commun 6, 8973 (2015). https://doi.org/10.1038/ncomms9973.

Originally published in Chinese on the WeChat official account "见微而知著-觅几科技" (Miji Technology). English translation by Chengdu Miji Technology Co., Ltd. Source: WeChat article