Fig. 7: Characterization of the micro-ring ultrasound sensor.

a Schematic illustration of the characterization system that focuses pulsed light onto a golden thin film to generate a point-like ultrasound source. L lens, TL tunable laser, SR signal receiver, MR micro-ring. LTS linear translational stage. b The measured PA signal as a function of time. A time window (red dashed box) was implemented to keep only the first arriving signals while excluding the following ones due to multiple reflections. c The frequency response of the micro-ring sensor. The center frequency is around 60 MHz, and the −6 dB bandwidth is about 175 MHz. d Amplitude map of the measured PA signal as a function of time and translational distance. e Frequency response as a function of the acceptance angle. Two experimentally achieved −3 dB lines are also provided in white, matching the theoretical estimations in red. f Noise amplitude spectral density of the micro-ring sensor. g Noise-equivalent-pressure (NEP) spectral density of the micro-ring sensor. h Reconstructed images of the cross-section of the carbon fiber buried inside the agar. i One-dimensional profile along the lateral direction, suggesting a lateral resolution of 50.4 μm. j One-dimensional profile along the axial direction, suggesting an axial resolution of 43.6 μm.