Understanding and designing photothermal responses in complex layered systems

Abstract

Understanding heat transport and thermoelastic behavior in layered nanostructures is critical for designing advanced materials and devices. Here, we present a photothermal mirror-infrared (PTM-IR) spectroscopy approach that enables depth-sensitive, non-contact characterization of thermal dynamics in multilayer thin films. Using a trilayer polymer system composed of poly(methyl methacrylate) (PMMA) and SU-8 on a CaF₂ substrate, we extract layer-specific optical absorption coefficients and probe the time-resolved temperature and surface displacement evolution. We introduce a new one-dimensional (1D) Green's function framework that provides intuitive physical insight into the temporal evolution of photothermal signals in layered structures, revealing the roles of substrate interactions and interface effects. Experimental PTM-IR signals are in excellent agreement with both a two-dimensional (2D) axisymmetric space dimension finite element model and our analytical framework, validating our interpretation of the transient thermal and mechanical responses. We show that the thermal rise time is significantly shorter than the thermoelastic relaxation time and that both the temperature and surface displacement scale linearly with the absorption layer (SU-8) thickness. These results establish PTM-IR as a powerful tool for in situ analysis of multilayer systems, with applications ranging from thermal metrology to photonic and quantum materials.