Non-contact multilayer thickness measurements with reflection-mode terahertz time-domain spectroscopy

The global increase of plastic production has led to an increase in coating materials available for plastics, as many plastics need to be protected from environmental factors such as light, solvents and other chemicals. For this reason the coating of plastics is a useful example for the coating thickness measurement on non-metal surfaces. Analogies can also be found in many other areas such as the wood and building industry. Terahertz (THz) waves penetrate nonconductive coatings and are reflected at the boundary layers between different refraction indices. This creates new potential for quality control when added to the already established non destructive testing techniques such as ultrasonic or x-ray. On the basis of practical relevance we have chosen the perpendicular reflection geometry for the investigation of the coating systems. The terahertz pulses were generated using the photo-Dember effect on an InAs surface emitter using a modelocked Ti:sapphire laser with approx. 100 fs short pulses at a center wavelength of about 800 nm (xu et al., 1992). The generated THz pulses are sampled using delayed laser pulses. The individual delay and the corresponding field strength are recorded via a PC-based data acquisition unit.