Artefacts in apertureless SNOM

We have implemented an optical homodyne interferometer to measure the tip oscillation amplitude in apertureless near-field optical microscopy. The setup is fully embedded in the microscope’s design, avoiding the presence of external arms. Our method is based on the synchronous detection of the interference between the fields reflected by the tip and a glass sample surface, while scanning the tip–sample distance over a few wavelengths. With the help of a simple model, we show how the different interference terms arising at frequencies multiple of the tip oscillation can be exploited to easily achieve sub-Ångstrom resolution

In apertureless near-field optical microscopy the vertical dithering of the tip, associated with demodulation at higher harmonics (n>1), allows us to suppress the far-field background, providing artifact free elastic scattering images. This work analyzes, both theoretically and experimentally, the physical origin of the background signal at the different harmonics and the mechanisms underlying its rejection for the general case of propagative-field illumination. We show that Fourier components of the background must be expected at every harmonic, evidencing why demodulation at higher harmonics is not an inherently background-free technique, and assessing the experimental conditions in which it becomes like that. In particular, we put forward the fundamental roles of both the harmonic order and the tip oscillation amplitude in the background suppression mechanisms. Furthermore, we outline how the lock-in detection of the signals amplitude can enhance the nonlinear dependence of the background on the tip-sample distance. Such effect provides a more subtle source of topography artifacts since the optical maps become qualitatively uncorrelated from the topographic counterpart, requiring an upgrade of the criteria to assess the absence of artifacts from the optical maps.