Researchers have developed a new method to reconstruct the phase of signals obtained through nano-Fourier Transform Infrared (nano-FTIR) atomic force microscopy. This advancement allows for a more precise interpretation of light-matter interactions at the nanoscale, overcoming limitations of previous analyses that relied solely on signal amplitude. Phase reconstruction is crucial for understanding the optical and vibrational properties of complex materials, such as polymers, biomaterials, and semiconductors, where phase information encodes details about chemical composition and local structure.

The proposed method integrates the finite dipole model with Kramers-Kronig relations. The finite dipole model describes the interaction between an atomic force microscope (AFM) tip and the sample, while Kramers-Kronig relations allow deriving the phase from the amplitude of a signal, provided certain causality conditions are met. The combination of both theoretical frameworks provides a robust tool for extracting phase information from nano-FTIR measurements, which are inherently complex due to the near-field nature of the interaction.

This development is significant because the nano-FTIR technique has emerged as a powerful tool for spectroscopic characterization of materials at very small scales, overcoming the optical diffraction limit. However, a complete interpretation of its signals required a reliable way to obtain the phase. The ability to accurately reconstruct the phase opens new avenues for material analysis, enabling more conclusive identification of chemical species and a deeper understanding of their optical and electronic properties at the nanoscale. This method is expected to enhance the characterization of new materials with applications in optoelectronics, catalysis, and medicine.