High-frequency ultrasound transduction using polymer microring resonators

High-frequency ultrasound detection using polymer microring optical resonators is demonstrated. An optical microring resonator consists of a ring waveguide closely coupled to a straight bus waveguide, serving as light input and output. Acoustic waves irradiating the polymer ring waveguide can induce strain to deform the waveguide dimensions and change the refractive index of the waveguide via the elasto-optic effect. These effects lead to the modification of the effective refractive index of the guided mode inside the waveguide. The sharp wavelength dependence of the microring resonator can enhance the optical response to acoustic strain. A polymer microring resonator with a width of 2.4 μm and a height of 1.85 μm, and a diameter of 95 μm was fabricated using nano-imprint technique. Its frequency response measured using a 50-MHz transducer shows that the microring resonator can detect ultrasound at least up to 55 MHz. A calibrated 10 MHz transducer generating a peak pressure of about 5 MPa was used to estimate the sensitivity and the noise equivalent pressure of the microring resonator to be 21 pm/MPa and 150 kPa, respectively. Furthermore, a two- dimensional scan was performed and the effective detecting area of the microring was estimated to be 1.4 times the ring size, implying that array operation would be possible with minimal cross talk between elements. These results suggest polymer microring resonators may be well suited to high-frequency ultrasound detection arrays.