Thermally thick/thin and optically opaque/transparent layer criteria in photothermal methodologies

This work explores the fundamental criteria for classifying material layers as thermally thick/thin and optically opaque/transparent, which are essential in photothermal methodologies. The analysis is based on the relationship between the sample thickness and the thermal diffusion length for thermal behavior, and the optical absorption length for optical behavior. Mathematical conditions are presented, such as 10 £ |Lj|/|µj| for thermally thick layers and |Lj|/|µj| £ 0.1 for thermally thin ones, along with their respective physical interpretations and implications for heat and light propagation equations. Additionally, the optothermal coupling coefficient, which quantifies the interaction between light absorption and heat diffusion, is discussed. The use of these classifications is important for interpreting experimental data and characterizing materials with unknown thermal properties, especially in multilayer models, where the complexity of the mathematical expressions demands a precise and robust interpretation.