Distinguishing local photoinduced forces from global photoacoustic interactions in homodyne infrared photoinduced force microscopy

The rise of tip-based optical force microscopy has transformed nanoscale optical and chemical imaging, enabling sub-10 nm spatial resolution by integrating optical excitation with atomic force microscopy. Photoinduced force microscopy (PiFM) stands out as a key technique, with applications in single-molecule imaging, Raman spectroscopy, and near-field electromagnetic characterization. However, the contrast mechanisms underlying homodyne PiFM, particularly in the infrared regime, remain underexplored. This study provides novel insights into the interpretation of PiF signals, focusing on homodyne IR-PiFM. Contrary to previous assumptions, we demonstrate that the linear dependence of the PiF signal on sample thickness in homodyne mode originates from global photoacoustic forces rather than localized photothermal effects. By minimizing global interactions through power reduction and tip-sample proximity, we achieve a spatial resolution of 11 nm, comparable to heterodyne PiFM. Our findings reveal that both homodyne and heterodyne modes are fundamentally sensitive to tip-enhanced near-field optical intensity, with similar dependencies on sample thickness. This work advances the understanding of the contrast mechanism of PiFM and demonstrates that both homodyne and heterodyne modes can achieve high-resolution imaging, paving the way for broader applications in nanoscale optical and chemical analysis.