High-sensitivity polymer microring resonators for broadband ultrasound detection and imaging

Diagnostic ultrasound imaging traditionally uses piezoelectric transducers for transmission and reception of ultrasound pulses. As the elements in the imaging array are reduced in size, however, the sensitivity will inherently decrease. We have developed a new, optically-based ultrasound sensor using polymer microring resonators. The device consists of a 100μm-diameter polystyrene ring waveguide coupled to an input/output bus waveguide, and is fabricated by nanoimprint lithography. Acoustic pressure causes change in the waveguide cross-section dimension and strain in the polystyrene material, resulting in a change in the effective refractive index and a shift in resonant wavelength. The ultrasonic waveform can be recovered from this modulation of optical output. The dynamic range and sensitivity of each microring can be tuned appropriately by adjusting the Q during fabrication. Our experiments show a low noise-equivalent pressure on the order of 1 kPa. Sensitivity has been measured by the application of known static pressure and a calibrated 20 MHz ultrasound transducer. A simple ID array is demonstrated using wavelength multiplexing. The angular response is determined by sensing the optoacoustic excitation of a 49μm polyester microsphere and shows wide-angle sensitivity, making the sensors useful for beamforming. The frequency response is relatively flat between DC and 40 MHz, and can be extended further by choice of substrate material, limited only by the electrical bandwidth of the photodetector. The high sensitivity, bandwidth, and angular response make it a potentially useful sensor platform for applications in ultrasound imaging, dosimetry, and non-destructive testing.