Contents I. INTRODUCTION
Ultrasound (US) technology, recognized for its distinctive characteristics, has been instrumental in many applications including imaging, non-destructive evaluation, structural health monitoring, flow sensing, range finding and positioning. Each of these applications is associated with different transducer types that also often operate at different frequencies. For instance, medical imaging is typically realized with ultrasound transducers operating at frequencies greater than 2 MHz. On the other hand, low-frequency ultrasound, notable for its lower attenuation and deeper penetration, can address applications such as remote acoustic power and data telemetry including for implantable medical devices. Thus, researchers have shown increasing interest in developing technologies capable of utilizing the full frequency spectrum of US, aiming to take advantages of both high and low-frequency ultrasound within a single device [1]-[3]. Piezoelectric Micromachined Ultrasound Transducers (PMUTs) have emerged as a promising solution to address this challenge, with the potential to integrate multiple transducer designs in a single chip operating in diverse modes and at both high and low frequencies [4], [5]. This multifrequency capability of PMUTs holds promising potential for enhancing the accuracy and sensitivity of ultrasound measurements, and there have been some pioneering studies that have demonstrated this innovation to varying degrees. For instance, Sun et al. and Shi et al. leveraged the multifrequency capabilities of PMUTs in their designs of rectangular micromachined ultrasound devices [6], [7]. They optimized the length-to-width ratios to induce a frequency overlap between different flexural modes, achieving bandwidths of −6 dB (689 kHz) and -10dB (650 kHz) for wireless power transfer. Similarly, Cai et al., Wang et al., and Billen et al. utilized multiple resonant modes of PMUTs, broadening the operating bandwidth and consequently enhancing the resolution in imaging applications [8]-[10]. Despite these promising studies, the existing literature reveals a gap in the research, with few studies investigating the simultaneous utilization of multiple frequencies for diverse applications. A few endeavors have attempted to use traditional ultrasound transducers for multi-frequency operations, achieving concurrent power transfer and backscatter communication [11]. However, these efforts often demand high performance from the traditional US transducers, particularly in terms of inherent bandwidth and sensitivity. Further, these methods typically necessitate integration of electrical circuits for frequency multiplexing, introducing additional complexity.
