  We propose a new label-free ultrasonic sensor, which comprises a slot
wave-guide and double silicon-on-insulator (SOI) slot micro-ring resonators.
The all-optical sensors do not suffer from electromagnetic interference. We
choose to integrate a silicon slot double micro-ring (SDMR) resonators in an
acoustically resonant membrane. Optimization of the several key structural
parameters is investigated to achieve the mode-field distributions of
transmission spectrum based on Comsol Multiphysics software. Our numerical
studies show that the proposed ultrasonic sensor offers higher sensitivity and
a larger detection frequency range than conventional piezoelectric based
ultrasound transducer. For a SDMR system with an area of 15um*30um, sensitivity
as high as 2453.7mv/kpa, and over a bandwidth range of 1-150MHz. The
sensitivity value is 36 times higher than that of single slot micro-ring
ultrasonic sensor. The theoretical-factor of the SDMR can be approximately
1.24*10^6 with bending radius of 5um. The investigation on the SDMR system is a
valuable exploration of the photo-acoustic microscopy for the ultra-high Q
factor and large frequency range.
