Fig. 1

Working principle of MRR-based ultrasound detection. a A schematic illustration of the MRR consisting of the ring waveguide and the bus waveguide separated by a low dielectric gap. b A representative resonance spectrum of the MRR and intensity-based PA signal detection using a narrow band laser. The shift in the resonance wavelength can be detected as the intensity modulation of the transmissive optical signal at the output port by using the narrow band laser and then converted to the electric PA signal through the photodetector. c A schematic illustration of MRR based PA signal generation. An acoustic wave from a biological sample photoacoustically generated by a pulsed laser deforms the polymer MRR resulting in the change of its effective optical path length. This induces the shift of the resonance spectrum of the MRR described in the panel (b). d A schematic illustration of deformation of the polymer MRR for an incident acoustic pressure with numerically calculated mode profile of ring waveguide. Scale bar 500 nm