Revealing Buried Thermal Responses in Polymer Multilayers Using Photothermal Mirror Infrared Spectroscopy

Depth-resolved characterization of buried layers is crucial for understanding energy transport and optical response in multilayer materials. Photothermal signal generation in such systems is commonly viewed in terms of heat loss and signal attenuation, with amplitude and sensitivity decreasing rapidly as overlayer thickness increases. Here, we demonstrate that the photothermal mirror infrared (PTM-IR) technique exhibits the opposite behavior: its transient signal increases with overlayer thickness, driven by enhanced surface displacement resulting from improved thermal confinement within the overlayer. A combined finite-element analysis (FEA) and one-dimensional analytical model based on Green's function formalism quantitatively describe the temperature and surface displacement dynamics in multilayer materials. Experiments on poly(methyl methacrylate)-polystyrene (PMMA-PS) bilayers on CaF₂ substrates validate the model, showing excellent agreement between theory and measurement. The results establish PTM-IR as a non-destructive infrared technique capable of probing buried absorbers and determining both absorber and overlayer thicknesses, extending the reach of depth-restricted photothermal techniques toward deeper, non-contact subsurface characterization.