  Acoustic wave resonators are promising for gravimetric biosensing. However,
they generally suffer from strong acoustic radiation in liquid, which limits
their quality factor and increases their frequency noise. This article presents
an acoustic-radiation-free gravimetric biosensor based on a locally-resonant
surface phononic crystal (SPC) consisting of periodic high aspect ratio
electrodes to ad-dress the above issue. The acoustic wave generated in the SPC
is slower than the sound wave in water, hence preventing acoustic propagation
in the fluid and resulting in energy confinement near the electrode surface.
This energy confinement results in a significant quality factor improvement and
thus reduces the frequency noise. The proposed SPC resonator is numerically
studied by finite element analysis and experimentally implemented by an
electroplating based fabrication process. Experimental results show that the
SPC resonator exhibits an in-liquid quality factor 15 times higher than a
conventional Rayleigh wave resonator with a similar operating frequency. The
proposed radiation suppression method using SPC can also be applied in other
types of acoustic wave resonators. It can thus serve as a general technique for
boosting the in-liquid quality factor and the sensing performance of many
acoustic biosensors.
