CD Laboratory for Photoacoustic Imaging and Laser Ultrasonics

The focus is on fundamental research in the field of photoacoustic imaging and laser ultrasound. Building on this, non-contact methods for the non-destructive testing of materials using optically excited sound waves are being developed.

From biomedicine to the steel industry, imaging techniques are needed to visualise the inner structure of materials without destroying them. Sound waves can be used to test a material in this way. This is done using two different methods that are being investigated and further developed as part of the research work.

If laser ultrasound is used, the emitted sound waves are absorbed directly at the surface as soon as they hit the material sample - swallowed, so to speak. At this point, the sample suddenly heats up and thermally expands, creating an ultrasonic pulse. This pulse propagates inside the sample and is reflected by the internal structures. This reflection can be detected on the surface with an optical interferometer and provide information about the interior of the sample. If the samples to be analysed are semi-transparent, the light from the pulsed laser is only absorbed inside the sample. The resulting sound wave can in turn be detected on the sample surface and used in a further step to reconstruct the absorbed energy density. This is the method of photoacoustic imaging.

This method has so far been successfully used for biomedical applications, but has some shortcomings in the field of non-destructive testing, which are being eliminated thanks to the research work. Also, when using conventional detectors, the previously necessary coupling of the detector with the sample through a water bath, for example, hinders the use of the photoacoustic method in industrial quality control. Furthermore, it is not always possible to assume a constant speed of sound during image reconstruction. This is not the case with composites - i.e. materials consisting of steel and plastic, for example - which can lead to distorted results.

For this reason, optical detection methods are being used to develop a new method, namely non-contact photoacoustic tomography for non-destructive testing. Combining the method with laser ultrasound in a common setup makes it possible to determine the localised distribution of the speed of sound in the sample. Acoustic inhomogeneities can be taken into account in the photoacoustic reconstruction.

The investigations will produce significant results for the use of non-destructive testing methods in wide areas of industry.