We establish a solid basis for the interpretation of infrared near-field spectra of thin organic films on highly reflective substrates and provide guidelines for their straightforward comparison to standard far-field Fourier transform infrared (FTIR) spectra. Particularly, we study the spectral behavior of near-field absorption and near-field phase, both quantities signifying the presence of a molecular resonance. We demonstrate that the near-field phase spectra only weakly depend on the film thickness and can be used for an approximate comparison with grazing incidence FTIR (GI-FTIR) spectra. In contrast, the near-field absorption spectra can be compared more precisely with far-field spectra: for ultrathin films they match well GI-FTIR spectra, while for thick films a good agreement with standard transmission FTIR spectra is found. Our results are based on experimental data obtained by nanoscale FTIR (nano-FTIR) spectroscopy and supported by a comprehensive theoretical analysis.
References
In contrast to standard FTIR, the sample is located in one of the interferometer arms.
While the sample is scanned, the tip-scattered radiation is superimposed at the detector with a reference wave with a linear-in-time changing phase. Recording of the detector signal pixel-by-pixel yields a synthetic hologram, in which the near-field amplitude and phase are encoded. The near-field amplitude and phase images are reconstructed by standard digital holography processing.
Previously used in Ref. 7.
