Tip-sample interactions in terahertz scattering scanning near-field optical microscopy and its influences

Abstract

Terahertz scattering scanning near-field optical microscopy (s-SNOM), as an important means to break through the limits of conventional optical diffraction, can achieve super-resolution imaging on a nanoscale and has a wide range of applications in biological nano-imaging, terahertz nano-spectroscopy, nanomaterials imaging, and the study of polarized excitations. As an important component of the terahertz s-SNOM, the atomic force microscope tip plays a key role in implementing the near-field excitation, detection, and enhancement. However, the tip-sample interaction can greatly affect the results. In this paper, the effects of tip-sample interaction on near-field excitation, near-field detection, and terahertz near-field spectrum in terahertz s-SNOM are revealed through simulations and experiments. First, the wave vector coupling weight of the near field excited by the tip is investigated, and it is found that the wave vector is concentrated mainly on the order of 10<sup>5</sup> cm<sup>–1</sup>, which differs from that of the general terahertz excitations by 2 to 3 orders of magnitude, indicating that the terahertz near field is difficult to excite terahertz excitations. Secondly, through theoretical and experimental studies, it is found that the metal tip interferes with the surface near-field of the graphene disk structure, which indicates the limitations of the terahertz s-SNOM in probing the near-field distribution of the structure. Finally, the influence of the tip on the near-field spectrum is studied. It is found that the tip length and cantilever length are important parameters affecting the near-field spectrum, and the influence of the tip on the near-field spectrum can be reduced by increasing the tip length or cantilever length.