Phononic crystal sensors - Insides behind the scene

Ultrasonic sensors and acoustic (resonant) microsensors are well accepted devices in many application fields. Both principles are based on acoustic wave propagation where the value of interest perturbs wave propagation in a distinct manner. Acoustic band gap materials, so-called phononic crystals, provide a novel and alternative platform for sensing material properties in small cavities. The sensor employs specific transmission peaks within the band gap to determine properties of a component that builds the phononic crystal. When applied as sensor, the material of interest constitutes one component of the phononic crystal. The typical arrangement used so far consists of periodically arranged holes in a solid matrix filled with a liquid. The value of interest changes its acoustic properties thereby changing the acoustic properties of the phononic crystal. Transmission or reflection coefficients are appropriate parameters for measurement and used to localize a transmission peak within the band gap of the phononic crystal. Such a transmission peak results from a resonant mode created in a confined liquid. A (micro)fluidic channel acting as a defect in an otherwise regular structure can be exploited for this purpose.