Continuously tunable acoustic Fano resonance in side-coupled Helmholtz resonator array assisted by a surface phononic crystal

Fano-like asymmetric line shape in a side-coupled series Helmholtz resonator array-waveguide system is continuously tuned by means of a one-dimensional surface phononic crystal, whose dispersion is exploited to adjust the phase factor appearing in indirectly coupled resonators. Finite-element method simulations reveal that the quality factor of the transmission spectrum can reach values on the order of 107, which can be finetuned by varying either waveguide width or phononic crystal groove depth. The Fano line shape dip, which appears around 25 kHz, red-shifts linearly with respect to the waveguide width at a rate of 308 Hz/mm. The quality factor exhibits a two order of magnitude drop for 0.01 mm waveguide width detuning from the optimal value. In comparison, three orders of magnitude decrease are observed for the same variation from the optimal value of the surface phononic crystal groove depth. Fano-like line shape is preserved when thermo-viscous losses are taken into account, where tunability with geometrical factors is still possible. Fine-tuning of either the dip frequency or quality factor can be utilized in demanding applications such as measurement of distance and concentrations of fluid mixtures with ultra high sensitivity.