A photopyroelectric method with air as the thermal coupling fluid

The photopyroelectric (PPE) technique is usually used as a contact photothermal method to characterize thin solid materials. We performed theoretical and experimental investigations of a non-contact PPE configuration to analyse the effects of an air gap between the thermal source and a PVDF sensor using a modulated laser beam as optical excitation. The detection of the thermal wave developed within the sample was possible across an air gap of up to 15 mm. The PPE signal was described well thanks to a two-dimensional (2D) model with axial symmetry, which could be reduced in some particular cases to a simpler 1D propagation model. Border effects can be artificially taken into account in the calculations by increasing the adiabatic lateral limits in the model, keeping the active sensor area constant. Despite there being strong attenuation with increasing modulation frequency, the PPE method can be used as well as can other non-contact photothermal techniques. New perspectives for the application of the PPE technique have been opened.