High-resolution imaging and mapping of the ocean and its floor has been limited to less than 5% of the global waters due to technological barriers. Whereas sonar is the primary contributor to existing underwater imagery, the water-based system is limited in spatial coverage due to its low imaging throughput. On the other hand, aerial synthetic aperture radar systems have provided high-resolution imaging of the entire earth's landscapes but are incapable of deep penetration into water. In this work, we present a proofof-concept system which bridges the gap between electromagnetic imaging in air and sonar imaging in water through the laser-induced photoacoustic effect and high-sensitivity airborne ultrasonic detection. Here, we use air-coupled capacitive micromachined ultrasonic transducers (CMUTs) which is a critical differentiator from previous works and has enabled the acquisition of an underwater image from a fully airborne acoustic imaging system-a task that has yet to be accomplished in the literature. With the entire imaging system located on an airborne platform, there is much promise for the scalability of our system to one which could perform high-throughput imaging of underwater in large-scale deployment. Non-contact acousticbased imaging modalities are also of much interest to the medical imaging and non-destructive testing communities. Incorporating air-coupled transducers, for example CMUTs, or other resonant sensors in these applications could be aided by the analysis presented throughout this work. INDEX TERMS Capacitive micromachined ultrasonic transducer, CMUT, laser doppler vibrometer, laser ultrasound, non-destructive testing, photoacoustic, sonar, ultrasound, underwater imaging This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.

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Information about the root system architecture of plants is of great value in modern crop science. However, there is a dearth of tools that can provide field-scale measurements of below-ground parameters in a non-destructive and non-invasive fashion. In this paper, we propose a multi-modal, non-contact thermoacoustic sensing system to address this measurement gap and discuss various system design aspects in the context of belowground sensing. We also demonstrate the first thermoacoustic images of plant material (potatoes) in a soil medium, with the use of highly sensitive capacitive micromachined ultrasound transducers enabling non-contact detection and cm-scale image resolution. Finally, we show high correlation (adj. R 2 = 0.95) between the measured biomass content and the reconstructed thermoacoustic images of the potato tubers.

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