Photothermal Surface Deformation Technique—a Goal for Nondestructive Evaluation in Thin-film Optics

Using a localized excitation source such as an appropriately focused laser beam to illuminate a sample under investigation, the photohermoelastic material response leads, necessarily, to a localized surface deformation. A suitable theoretical calculation has been performed showing the characteristic dependence of this absorption-induced surface deformation on optical and thermophysical parameters as well as on experimental conditions. For frequently existing, experimentally realized limiting cases, useful analytical expressions have been derived interpreting both depth and laterally resolved photothermal measurements of material parameters of thin-film-on-bulk optics. Thus, the photothermal surface deformation (PTD) technique is suggested as an alternative, contactless method in the field of nondestructive material evaluation (NDE).