Phononic crystals (PnCs) are structures designed to better control the flow of phonons, or vibrational quasiparticles, in frequency and wavelength domains. application. Characterization of bespoke crystal designs is typically done with the scanning tools utilizing ultrasonic transducers for measuring the distribution of sound intensity in space. However, this approach is both painstaking and time-consuming. A visual method for characterization of acoustic field around a PnC is preferential since it can significantly accelerate analysis of PnC performance.The Background Oriented Schlieren (BOS) technique was utilized to visualize the acoustic fields around two different 2D PnCs immersed in water on varying frequencies of ultrasound. Optical distortions were initially observed on a speckled background with a charge-coupled device (CCD) camera with a frame rate of 44 frames per second after an ultrasound transducer began to output a signal of a given frequency. The CCD camera was then placed at a distance away such that the resolution of the randomly generated background could still be observed in a frame. The transducer was set to a given frequency, and the signal was output. In this study, two different PnCs with specific frequency bands were considered to demonstrate the proposed technique. One, a PnC lens, is engineered for biomedical imaging applications, such as spatial characterization and subwavelength resolution in a biosystem beyond the Fresnel zone limit, utilizing collimation at around 576 kHz. The other, PnCs with the cavity tailored to trap the sound energy, designed to produce a sound amplification by stimulated emission of radiation (SASER) at 620-628kHz. More data analyzing the 2D lens oriented along the correct axis would be beneficial, as only the preliminary set of two sets of data was collected with the correct orientation with respect to the application of the crystal. Rudimentary analysis of the PnCs using the given algorithms indicates the peak optical distortions occur somewhere in between 626-6228kHz for the first PnC used to produce a SASER. Further data collection and analysis is ongoing for the remaining two PnCs for the SASER application.During the initial experiment, the optical distortions were not visible without the aid of algorithms like particle image velocimetry (PIV) and optical flow algorithms based on Horn Schunck method. However, after appropriate optimization of experimental design, ultrasound signals were observable on screen in real-time even without the aid of such algorithms. Validations and optimizations of the codes for the two algorithms are ongoing, but our current results are quite promising. Similarly, the optical approach to the analysis of acoustic phenomenon looks auspicious for determining the transmission of sound through PnC and observe possible deviations in the acoustic pattern from the expected one.

Optical Imaging of Ultrasonic Fields Surrounding Phononic Crystals

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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