Imaging of Surface Vibrations Using Heterodyne Interferometry

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto.fi Author Kimmo Kokkonen Name of the doctoral dissertation Imaging of Surface Vibrations Using Heterodyne Interferometry Publisher School of Science Unit Department of Applied Physics Series Aalto University publication series DOCTORAL DISSERTATIONS 112/2014 Field of research Applied Physics Manuscript submitted 15 October 2013 Date of the defence 22 August 2014 Permission to publish granted (date) 7 January 2014 Language English Monograph Article dissertation (summary + original articles) Abstract The research summarized in this dissertation focuses on the development of a heterodyne scanning laser interferometer and of data-analysis techniques for the characterization and analysis of the surface vibration fields in microacoustic devices and test structures.The research summarized in this dissertation focuses on the development of a heterodyne scanning laser interferometer and of data-analysis techniques for the characterization and analysis of the surface vibration fields in microacoustic devices and test structures. The heterodyne laser interferometer enables a phase-sensitive, absolute-amplitude detection of the out-of-plane component of a surface vibration field, with a minimum detectable amplitude of less than a picometer, while the lateral resolution is better than 1 micrometer. The instrument features a flat frequency response up to 6 GHz. The phase-sensitive absolute-amplitude data enables the visualization of the actual wave behavior in electromechanical components and test structures, but more importantly, it is the basis for further analysis. The research instrument is applied to the study of electroacoustic devices based on surface acoustic wave (SAW) and bulk acoustic wave (BAW) technologies. Two novel SAW devices are studied in detail: a phononic crystal (PnC) structure and a scattering structure resulting in a random wave field. PnCs are acoustic metamaterials that can provide engineered material properties. The laser interferometric measurements were amongst the first to directly characterize the wave interaction with the PnC. SAW slowness curves of an anisotropic substrate material are extracted by measuring and analyzing the scattered random wave field. Data analysis methods are developed further in the context of BAW research by experimentally addressing two important aspects of device design: the correct operation of the acoustic reflector used to confine the energy in the resonator, and investigation of the role of the dispersion and standing wave resonances to the spurious responses often observed in high-Q resonators. Fourier transform techniques are used for selective visualization of wave fields and for the extraction of the dispersion characteristics of the plate-waves. The dispersion data are then further used to analyze the transfer characteristics of an acoustic reflector and to study the properties of lateral eigenresonances and the lateral energy confinement in detail. The research described in this thesis provides a detailed characterization of the operation of SAW and BAW devices and effects within, highlighting and further developing the experimental characterization capabilities and data-analysis methods.