Field Manipulation of Infrared Absorption Properties in Thin Films

Abstract

Infrared (IR) spectroscopy is a sensitive technique for fast and comprehensive analyses of organic and hybrid organic–inorganic materials, surfaces, and thin films. For interpretation of reflection and absorption IR spectra of thin films, vibrational bands in unpolarized IR spectra are often directly inspected. This approach can lead to misinterpretations. Herein, the influence of anisotropy and oscillator strength on the frequency and shape of observed vibrational bands in IR spectra of thin films is investigated. IR ellipsometric, polarization‐dependent reflectance spectra and polarization‐dependent atomic force microscope–IR spectroscopy (AFM–IR) nanopolarimetric absorption measurements of an about 84 nm thick anisotropic polyimide film (PI‐2611) are compared. The s‐ and p‐polarized spectra are used for separation of in‐plane and out‐of‐plane absorption components. The far‐field spectra are interpreted by analytical modeling. In detail, the frequency range of bands related to antiphase and in‐phase C═O stretching vibrations of the imide rings is investigated. For the antiphase C═O vibration as a relatively strong oscillator, the manipulation of absorption by the incident radiation‐induced electric fields in direction normal to the surface can be revealed in p‐polarized reflectance spectra and is identifiable in the p‐polarized IR nanopolarimetric absorption spectra.