Nanoscale-resolved chemical identification of thin organic films using infrared near-field spectroscopy and standard Fourier transform infrared references

S. Mastel, A. A. Govyadinov, T. V. A. G. de Oliveira, I. Amenabar, R. Hillenbrand

Applied Physics Letters 106, 023113 (2015)
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.