Theoretical Description of Infrared Near-Field Spectroscopy of In- and Out-of-Plane Molecular Vibrations in Thin Layers

Infrared nanospectroscopy, based on scattering-type scanning near-field optical microscopy (s-SNOM), allows for chemical nanoidentification of organic composite layers by local probing of their molecular vibrations. However, the conditions for probing in-plane and out-of-plane molecular vibrations by this technique remain largely unexplored. Here, we perform a systematic theoretical study of the local infrared near-field response of isotropic and anisotropic thin layers using electrostatic numerical calculations, complemented by analytical electrodynamic point-dipole model calculations. Specifically, we study uniaxial thin layers exhibiting molecular vibrations with different orientations on highly and weakly reflecting substrates. We find that both in-plane and out-of-plane molecular vibrations can be probed, with the sensitivity to in-plane vibrations being reduced in samples where the near fields are vertically oriented, such as in thin layers on highly reflecting substrates. We finally show that fast calculations of near-field spectra of uniaxial thin layers can be done with a perturbative finite-dipole model, achieving reasonable quantitative accuracy compared to electrostatic numerical results.