Spatially and temporally modulated surface acoustic waves

The proposed project deals with generation and detection of surface acoustic waves (SAWs) on coated and uncoated microstructures and thin films. The aim of the project is the contactless, optical characterization and non- destructive testing (NDT) of coatings, thin films and bulk materials; in particular the development and investigation of new experimental and numerical techniques for spatially and temporally modulated optical generation of SAWs. Rayleigh waves at the surface of an isotropic material are non-dispersive. This behavior, however, changes when a film with different acoustic properties is deposited onto the material. The wavelength and the penetration depth of the wave depend on the frequency. Hence, the ratios of the wave propagating in the coating and in the bulk material change for different frequencies, resulting in a dispersive behavior. Using the dispersion relation of these SAWs the thickness and/or the elastic parameters of the film can be evaluated. In Laser Ultrasonics (LUS) usually short laser pulses - ranging from picoseconds to nanoseconds - with high power densities are used to excite broadband ultrasonic waves. Classical single-pulse generated laser ultrasound for evaluation of SAWs has two major drawbacks. First, to increase the signal to noise ratio higher optical powers are needed which can damage the material. Second, due to dispersion the broadband acoustic pulse changes its shape as it propagates and, thus, different frequencies arrive at different times. While this behavior is welcome, as the evaluation of the material properties relies on it, the interpretation of the obtained waveforms is difficult. Especially in the presence of noise, unwrapping of the detected waveform is difficult, which reduces the accuracy of this method. Both problems can be overcome by pulse modulation techniques spreading the laser energy in time and/or space. By pulse modulation the power densities can be reduced or, when holding the power density constant, the SNR can be increased. Furthermore, the evaluation of the detected wave form is simplified as only one spatial or temporal frequency is excited. Currently various different schemes for temporal or spatial pulse modulation exist, each with different benefits and limitations. The general objective of the proposed project is the generation and detection of surface acoustic waves which are modulated temporally and spatially. By combining temporal and spatial modulated ultrasound, a high accuracy and highly reliable device for evaluation of material parameters of thin films can be realized. In addition to the experimental work, fundamental numerical / theoretical investigation of the low-power laser ultrasonic generation of acoustic waves will be carried out. The problem can be addressed by the simultaneous solution of the wave equation and the heat conduction equation. Simulation of the excited temporally and spatially modulated patterns will be performed in a finite element code.