Interfaces Control Signal in Widefield Photothermal Heterodyne Imaging of Thin Films

Mid-infrared photothermal heterodyne imaging (PHI) is a label-free, bond-selective imaging technique that has demonstrated utility across multiple fields, including chemical identification and analysis, nanothermometry, and composition heterogeneity mapping. Widespread implementation is limited by a lack of understanding of signal origin in widefield PHI and the additional complexities of thin-film geometries. Here, we use a finite-difference model for widefield PHI of thin-film systems to identify the origins of signal generation and the theoretical approaches to describe the signal generation. Using mixed methylammonium-formamidinium lead iodide (MAxFA1–xPbI3) thin films as a model system, we demonstrate that signal strength is not proportional to target composition as previously assumed. Rather, signal generation is dominated by interfacial effects that dictate the magnitude, sign, and temporal evolution of the signal. Proximity to interfaces dictates the signal’s temporal evolution, which may be leveraged to extract information about the target vertical distribution. For thin specular films, Fabry–Pérot resonances have a significant impact, influencing the sign and enhancing the reflectivity signal by up to 4 orders of magnitude. Our results provide a computational and theoretical framework for informed widefield PHI optimization and suggest that interfacial effects can be leveraged to significantly enhance the signal.