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The notion of a phased array was initially articulated by Nobel Prize recipient K. F. Braun. Phased arrays have subsequently evolved into a formidable mechanism for wave manipulation. This assertion holds particularly true in the realm of ultrasound, wherein arrays composed of ultrasound-generating transducers are employed in various applications, including therapeutic ultrasound, tissue engineering, and particle manipulation.

Importantly, these applications—contrary to those aimed at imaging—demand high-intensity ultrasound, which complicates the electrical driving requirements, as each channel necessitates its own independently operational pulse circuitry and amplifier. Consequently, the majority of phased array transducers (PATs) are constrained to several hundred elements, thereby restricting the capability to shape intricate ultrasound beams.

To date, there exists no scalable methodology for the powering and control of phased array transducers.

Notwithstanding the extensive development over the past five decades that has culminated in enhanced reliability and efficiency of PAT hardware, the fundamental structure of the multi-channel sequential architecture employed for the individual addressing of transducer elements has remained largely unchanged.

Although advancements have been made to facilitate high-power electrical excitation of therapeutic arrays, such advancements are tailored to point-focused geometries and would prove inadequate for the significantly more intricate field configurations required for a variety of applications.

Consequently, there exists a pressing necessity for an innovative architecture capable of concurrently driving thousands of transducer elements with high power, rapid update rates, and stringent timing or phase control to enable precise field synthesis.

We have developed an optoelectronic architecture that necessitates merely a singular amplified electrical input signal, which is subsequently disseminated and modulated independently for each transducer element, thereby obviating the requirement for separate pulse circuitry and amplifiers for every channel. Furthermore, given that the signals driving each channel are intrinsically phase-synchronized, the necessity for an independent clock is eliminated, allowing all phase modulation to be executed via the optical inputs.

Ultimately, our electronic architecture exhibits a broad operational bandwidth, thereby accommodating the employment of transducers across an extensive spectrum of central frequencies.

The pivotal component within the OPAT architecture is the light-activated phase shifter (LAPS), which constitutes an analog circuit designed for high-frequency operation, predicated on photoresistive elements integrated with passive electronic devices within a cascaded architecture. The electrical circuit effectively converts an optical intensity into a precisely phase-shifted electrical signal.

The real-time phase is adjustable by modulating the intensity of the illuminating light, which can be concurrently applied across all channels utilizing standard, commercially available projectors. The conceptual framework of modulating the phase of each transducer within the array independently through light is depicted in fig. 1.

Forthcoming biomedical applications utilizing structured ultrasound fields necessitate the transmission of elevated acoustic power into spatially intricate configurations. To fulfill these requirements, it is imperative to employ transducers characterized by larger apertures and increased element counts compared to those conventionally utilized for imaging, which presents significant fabrication challenges and has, until this point, been impractical for electrical excitation.

In our article in Nature Communications, we have proposed a novel architecture for the electrical excitation of phased arrays through the utilization of programmable light intensity patterns. The principal innovation within our design is a balanced dual-cascaded RC network, wherein the photosensitive resistors facilitate wireless and extensively parallelizable regulation of the electrical output phase. This configuration permits, for the first time, the optical modulation of the driving phase for each individual array element continuously across the range of -π to + π.

Furthermore, our architecture is capable of independently driving all transducer elements within an array utilizing merely one amplified RF signal, thereby significantly streamlining the operation of extensive transducer arrays. The current system operates at a rapid rate of 100 Hz and exhibits scalability, as the light-triggered phase switching can be executed with low-intensity optical projection 66 mW/cm².

Additionally, the circuit is designed to accommodate varying transducer capacitance and a wide frequency spectrum ranging from 100 kHz to 10 MHz. We implement our architecture to actuate a standard 11×11 imaging array, thereby generating complex wavefronts, demonstrating switchable focusing capabilities, and projecting vortex beams.

We employed the Iterative Angular Spectrum Approach (IASA) to ascertain the relative phase shift for distinct transducer elements essential for the generation of the programmed wavefront. The phase distribution across the transducer elements is ascertained by iteratively propagating a wave from the target plane to the transducer plane and subsequently returning, while imposing amplitude constraints in each respective plane.

In our refinement of the IASA algorithm, we implemented an amplitude constraint for each pixel to align with the experimentally observed phase shift-dependent amplitude. The amplitude constraints for each iteration are established independently and explicitly. The algorithm demonstrates convergence within several tens of iterations to produce the requisite phase map.

We executed about 50 iterations for each wavefront to facilitate an accurate comparison of the performance of Optical Phase Array Transducers (OPATs) across various applications. We partitioned the transducer elements into multiple pixels in order to configure the transducer phases for fields exhibiting higher resolutions than the driving array and posited a uniform phase output for all pixels corresponding to a singular element.

The relative phase shift for individual transducer elements is subsequently computed by averaging the resultant phase shift across all pixels that constitute one element. The algorithm yields favorable experimental outcomes that align with the theoretically anticipated wavefronts. The phase distribution computed via IASA is subsequently converted into light intensity through a look-up table derived from the experimentally measured curves depicted in fig. 2.

Due to its parallel addressing mechanism, singular power supply, and performance that is independent of load, the optical phase shifter presented herein is anticipated to scale advantageously, thus facilitating the operation of exceptionally large element transducer arrays and significantly enhancing the capacity to generate dynamic intricate ultrasound fields.

To effectively actuate larger arrays while maintaining a compact and adaptable footprint, the optical modulator may be amalgamated, for example, utilizing µLED arrays in conjunction with a micro-lens array to optimize light collection efficiency. The modulator circuitry could subsequently be seamlessly integrated with an emitter transducer through flip-chip bonding.

This cohesive architecture could thereby endorse additional scaling without necessitating more intricate interconnections and electronics, thereby opening avenues for novel applications in biomedicine, including ultrasound-assisted tissue engineering and ultrasound-based therapeutic interventions.

This story is part of , where researchers can report findings from their published research articles. for information about Science X Dialog and how to participate.

Rahul Goyal received his B.E. degree in 2015 in electrical engineering with a major in semiconductor physics. Thereafter, he worked in New Delhi and later joined the Indian Institute of Science, Bangalore, where he obtained his M.E. degree in 2018 in nanoscience and engineering. He obtained his doctorate (PhD) from Heidelberg University in Engineering Sciences (Dr.-Ing.). Currently, he is a postdoctoral researcher in the MNMS Group at the Max Planck Institute (Heidelberg, Germany). His research focuses on the development of novel acoustic effects and designing acoustic devices for dynamic wavefront shaping. He is also interested in making dynamic magnetic field systems and high frequency integrated circuits. He has four years of experience in carrying out manufacturing processes in an ISO 4 cleanroom. He has a strong knowledge of advanced techniques such as lithography, PVD, RIE, ALD, and PECVD, as well as characterization techniques such as SEM, EDX, XRD, FTIR, and Ellipsometry.