Origins and consequences of asymmetric nano-FTIR interferograms

Infrared spectroscopy is essential in understanding the optical properties of materials and is a critical analytical technique for identifying and determining the properties of materials. One of its major shortcomings, the relatively low spatial resolution, was overcome by combining atomic force microscopes and infrared light sources in near-field IR microscopes. Scattering-type near-field optical microscopy, s-SNOM, has been a flagship technique in the infrared community for its ability to provide information on optical properties well below the far-field diffraction limit. When combined with broad-band light sources, the technique gives rise to the nano-FTIR technique, which enables IR spectroscopy at the nanoscale. While s-SNOM and nano-FTIR data analysis are understood, fundamental discussion of experimental data, especially in the context of previously developed frameworks is lacking. Here, we unambiguously describe the origins of asymmetric interferograms recorded with s-SNOM instruments, give detailed analysis of potential artifacts and recommendations on optimal instrument settings as well as data analysis parameters.