ACS Photonics 11, 3131

Cultural heritage artifacts present a distinct set of challenges when it comes to characterizing the material composition of extracted samples. Whatever samples are removed must be small to preserve the integrity of the artwork, and ideally, applied techniques should be nondestructive to allow samples to be saved for future use. Unfortunately, most nondestructive optical detection schemes are constrained to spatial resolutions set by the diffraction limit of light. Since many material-specific properties used for identification are dependent upon molecular vibrations and conductivity, this implies the need for spectral analysis in the infrared. The long wavelengths associated with infrared light can make it difficult to definitively identify materials when the sample composition varies at the length scales of nanometers. Nano-FTIR provides a unique solution whereby scattering-type scanning near-field optical microscopy is combined with a broadband illumination source for FTIR-like detection with subdiffraction spatial resolution (∼20 nm). The nondestructive nature of the technique makes it optimally suited to characterize art conservation samples while minimizing sample extraction by allowing individual samples to be characterized by multiple complementary techniques. In this work, the capabilities of nano-FTIR are evaluated in the study of surface coatings sampled from African wood sculptures, where the submicrometer scale of some of the layers makes the composition of the cross sections difficult to interpret using other established techniques. We show that the functional groups corresponding to different classes of organic coatings and inorganic pigments can be assigned based on point spectra, indicating that nano-FTIR holds great promise for the field of art conservation.