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.

Contemporary digitally controlled delay elements trade off power overheads and delay quantization error. This paper proposes a new delay element that provides a balanced design that yields low power with low delay quantization error. The proposed element has a quasi linear delay characteristic, with uniform delay differences between adjacent codewords. The element employs and leverages the advantages offered by a 28nm FD-SOI technology, using its back body biasing feature to add an extra dimension to its programmability. To do so, a novel generic delay shift block is proposed, which enables incorporating both fine and coarse delays in a single delay element that can be easily integrated into digital systems, an advantage over hybrid delay elements that rely on analog design.

Portable and easy-to-use imaging systems are in high demand for medical, security screening, nondestructive testing, and sensing applications. We present a new microwave-induced thermoacoustic imaging system with noncontact, airborne ultrasound (US) detection. In this system, a 2.7 GHz microwave excitation causes differential heating at interfaces with dielectric contrast, and the resulting US signal via the thermoacoustic effect travels out of the sample to the detector in air at a standoff. The 65 dB interface loss due to the impedance mismatch at the air-sample boundary is overcome with high-sensitivity capacitive micromachined ultrasonic transducers with minimum detectable pressures (MDPs) as low as 278 µPa rms and we explore two different designs-one operating at a center frequency of 71 kHz and another at a center frequency of 910 kHz. We further demonstrate that the air-sample interface presents a tradeoff with the advantage of improved resolution, as the change in wave velocity at the interface creates a strong focusing effect alongside the attenuation, resulting in axial resolutions more than 10× smaller than that predicted by the traditional speed/bandwidth limit. A piecewise synthetic aperture radar (SAR) algorithm modified for US imaging and enhanced with signal processing techniques is used for image reconstruction, resulting in mm-scale lateral and axial image resolution. Finally, measurements are conducted to verify simulations and demonstrate successful system performance. Index Terms-Capacitive micromachined ultrasonic transducer (CMUT), non-contact microwave-induced thermoacoustics, piecewise synthetic aperture, ultrasound (US) imaging. I. INTRODUCTION P ORTABLE imaging systems are in demand for a variety of applications, from nondestructive testing and sensing to security screening and point-of-care diagnostic imaging [1]-[3]. Such systems must also be low-cost, safe, Manuscript

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