Microsphere resonator for optoacoustic detection of high frequency ultrasound

Optical techniques offer a promising approach to realizing high frequency ultrasound imaging arrays. Exceptional pressure sensitivity has been demonstrated with high Q-factor optical resonators, such as polymer microring devices. However, excessive bend loss restricts the microring diameter to roughly 90 um or larger. We are exploring an alternative optoacoustic sensor based on a microsphere resonator. Optical whispering gallery modes (WGMs) circulate within the microsphere surface via total internal reflection. For proof of concept, we use a prism coupling technique to excite WGM resonances in a 25 um diameter polystyrene microsphere. The microsphere is placed on the surface of an equilateral prism made from a UV-curable polyurethane. The microsphere was encapsulated within an approximately 50 um cured layer of polydimethylsiloxane (PDMS). Light is focused into the prism with a 10X microscope objective. The incident angle is approximately 70 degrees in order to excite WGM resonances. Ultrasound excitation is provided by a 25 MHz unfocused transducer with ultrasound coupling gel. As expected for a 25 um diameter polystyrene microsphere, the light reflected from the prism-microsphere interface has optical resonance dips separated by approximately 5 nm. The Q-factor is about 800. Optoacoustic performance was evaluated by illuminating the microsphere with a temperature controlled Fabry-Perot AlGaAs diode laser tuned to a wavelength of 780 nm. The measured signal has a peak frequency of 18 MHz and -6 dB bandwidth of 22 MHz. Based on single-shot measurements, the noise equivalent pressure (NEP) of the microsphere detector is 75 kPa over a 50 MHz bandwidth. Although the NEP is fairly high, the Q-factor of our microsphere resonator can be improved by at least an order of magnitude by optimizing the separation between the microsphere and coupling prism. We believe these encouraging results suggest the potential of microsphere resonators for optoacoustic detection.