Treatment of Thermal Gratings in the Time and Frequency Domain

Abstract

In this contribution we present the range of applicability on solids and surfaces of the photothermal displacement technique refined by using laser-induced thermal gratings. A suitable theoretical calculation has been performed showing the characteristic dependence of the absorption-induced surface deformation on optical and thermophysical parameters as well as on pulsed and amplitude-modulated laser excitation. The potential of the grating technique for thermal diffusivity measurements and for the study of heat diffusion in a preferential direction is clearly demonstrated. Moreover equivalence and convertibility of both dynamic approaches for thermal diffusivity measurements is found. However, in relation to other applications the selected operation form (pulsed or c.w.-modulated) presents specific advantages which justify its choice. The equivalent and complementary information obtained in the time and frequency domain is discussed using for this purpose experimentally limiting cases which are commonly applied. Useful analytical expressions have been derived interpreting both depth and laterally resolved photothermal measurements of homogeneous material parameters. Thus, the photothermal displacement grating technique is suggested as an alternative, contactless method, sensitive to surfaces, in the field of non-destructive evaluation.