Integrated silicon photonic crystal sensor for pressure acoustics (Conference Presentation)

Ultrasonic pressure acoustic sensors have been previously demonstrated for aerospace research, sound localization, robotics navigation, photoacoustic tomography, and non-destructive evaluation. Non-contact ultrasound modalities can enable the sterile inspection of materials in harsh environments; however, piezoelectric transducers face issues regarding impedance matching with air. Additionally, the design of sensors suitable for turbulence research is particularly challenging with conventional capacitive, piezoresistive, and hot-wire transduction methods due to the relationship between the small and large Kolmogorov scales of the flow that widens considerably as the Reynold’s number is increased, placing demanding restrictions on sensor element size. Our research focuses on a novel sensing mechanism based on the directional coupling between the dielectric-like modes of two photonic crystal (PC) edges. Changes in the beat-length of the directional coupler can be related to relative position of the PC edges and are observable as changes in transmission at the output ports. Using the dispersive properties of PC waveguides, light may be effectively slowed down leading to enhanced optomechanical interaction and therefore smaller device footprints. Crucially, the planar fabrication does not depend on the membrane-substrate distance and therefore reduces the probability of stiction. The thermal mechanical motion of several vibrational modes of the fabricated membrane could be resolved. For a 15 µm by 15 µm PC membrane, we report a noise floor of 20 fm/√Hz using the fluctuation-dissipation theorem and flat frequency response up to the first resonance at 500 kHz. Ongoing measurements demonstrate the membrane can be driven by acoustic radiation and this motion can be detected at the optical output of the device. The compact size, high bandwidth, immunity to electromagnetic interference, and tunable sensitivity of these integrated photonic sensors make them suitable for array applications, particularly for the study fluid dynamics of high Reynold’s number phenomena.