State-of-the-art intracortical neural recording and stimulation systems rely on subdural implants tethered to a cranial implant which itself has a wireless power and data link to the outside world [1] (Fig. 6.2.1). However, this tethered configuration poses challenges such as scarring and potential damage to the surrounding tissue due to strain and micromotions, making this approach unsuitable for chronical implants [2]. Consequently, there is growing interest in wireless connections between cranial and subdural implants. This paper focuses on wireless powering between implants, traversing the dura and cerebrospinal fluid (CSF) tissue layers over distances of 0.5 to 1cm (transdural powering). With modern burr-hole craniotomy, the hole drilled in the skull is 6mm in diameter, limiting the available size for the TX. Moreover, the power dissipation of the TX must be low to keep tissue heating below 1°C [3]. RF and optical modalities suffer from higher attenuation in tissue compared to ultrasound (0.6dB/cm/MHz) [4]. Furthermore, for transdural powering, power losses from reflections at medium interfaces (e.g., skull) are avoided, making ultrasound (US) a prime candidate for efficient in-body wireless power transfer. US is also preferable to inductive powering since US beam steering up to large angles (>45°) is needed to maximize power delivery and compensate for brain micromotions of up to ±4mm [5] and misalignment during surgery. However, prior art US driving systems either use single-phase transducer driving [6, 7], incapable of beam steering, or use class D drivers with low power transfer efficiency (PTE) [8, 9]. A phased array with increased driving efficiency was presented in [10], but it cannot perform beam steering without grating lobes that can be eliminated with miniature transducers with a pitch close to λ/2. To facilitate direct integration between CMOS and the transducer array, the CMOS driving units should also be pitch matched [8, 9].

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Photoacoustic tomography defines new challenges for ultrasound detection compared to ultrasonography. To address these challenges, a sensitive, small, scalable, and broadband optomechanical ultrasound sensor (OMUS) has been developed. The OMUS is an on-chip optical ultrasound sensor, using optical interferometric ultrasound detection. It consists of an acoustic membrane on top of an optical ring resonator that modulates the optical ring resonance with high efficiency enabled by an innovative optomechanical waveguide. Raster scanning photoacoustic tomography has been demonstrated with a single-element OMUS. Based on performance and form factor, the OMUS combined with passive optical multiplexing may enable new applications in photoacoustic imaging.

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We present sensitive ultrasound sensors with an innovative silicon photonic optomechanical waveguide that features a 15-nm gap between movable parts [Nature Photonics 15, 341 (2021)]. Sensors are fabricated using CMOS-compatible processing and tested for biomedical imaging. @en

Future applications of photo-acoustic imaging require a matrix of small (wavelength/2) and sensitive ultrasound sensors with read-out through a flexible cable. Integrated optical sensors have good prospects: small and sensitive sensors, wafer-scale fabrication, and matrix read-out via single optical fiber using on-chip optical multiplexing. We propose a new type of opto-mechanical ultrasound sensor (OMUS) in silicon photonic chip technology. By using an acoustically resonant membrane in combination with an innovative split rib-type photonic waveguide, we achieve extremely high sensitivity. We present our experimental results in the frequency range up to 30 MHz.@en

Future applications of ultrasound and photoacoustic imaging require a matrix of small and sensitive ultrasound sensors with read-out through a flexible cable. Silicon photonic ultrasound sensors have good prospects: small and sensitive sensors, wafer-scale fabrication, and matrix read-out via single optical fiber using photonic multiplexing. Here, we discuss different types of silicon photonic ultrasound sensors and their applications. This includes our optomechanical ultrasound sensor with extreme sensitivity that is achieved with an innovative optomechanical silicon photonic waveguide in an acoustical membrane. We discuss limitations of state-of-the-art piezoelectric sensors, how silicon photonic sensors overcome these, and applications in medical imaging.@en

Ultrasonography ¹ and photoacoustic ^2,3 (optoacoustic) tomography have recently seen great advances in hardware and algorithms. However, current high-end systems still use a matrix of piezoelectric sensor elements, and new applications require sensors with high sensitivity, broadband detection, small size and scalability to a fine-pitch matrix. This work demonstrates an ultrasound sensor in silicon photonic technology with extreme sensitivity owing to an innovative optomechanical waveguide. This waveguide has a tiny 15 nm air gap between two movable parts, which we fabricated using new CMOS-compatible processing. The 20 μm small sensor has a noise equivalent pressure below 1.3 mPa Hz ^−1/2 in the measured range of 3–30 MHz, dominated by acoustomechanical noise. This is two orders of magnitude better than for piezoelectric elements of an identical size ⁴. The demonstrated sensor matrix with on-chip photonic multiplexing ^5–7 offers the prospect of miniaturized catheters that have sensor matrices interrogated using just a few optical fibres, unlike piezoelectric sensors that typically use an electrical connection for each element.

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We propose a new opto-mechanical ultrasound sensor (OMUS) enabled by an innovative silicon photonics waveguide. We present experimental results up to 30 MHz, a 10-sensor array proof-of-concept and our latest findings.@en

Optical ultrasound sensing is a promising technique for the emerging field of biomedical photoacoustic imaging. Previously at imec, micro-opto-mechanical sensors with integrated Mach-Zehnder interferometers were designed and demonstrated as highly sensitive for static pressure sensing. At quasi-static pressures, they are demonstrated to operate as a sensitive microphone. In this study, the application range is extended to include dynamic pressures targeting a proof of concept for a photoacoustic imaging application. To achieve this, the dynamic behavior of the pressure sensor is firstly characterized, showing its resonance frequency at 73.8 kHz. Since this is below the range desired in photoacoustic imaging, several approaches for resonance frequency increase are investigated, resulting positively for incomplete membrane release and new designs. These approaches are analyzed based on the evaluated sensor sensitivity, allowing the selection of optimal design. The mentioned evaluation of sensor sensitivity is possible due to a developed methodology based on measurement data, analytical and accurate acousto-mechanical finite element models. The developed methodology is demonstrated throughout a range of membrane sizes, frequencies and modes of vibration allowing the selection of maximum sensitivity design. In this study, a micro-opto-mechanical ultrasound sensor based on integrated Mach-Zehnder interferometer is designed for 1 MHz operation in a photoacoustic imaging environment and optimized for sensitivity.

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Ultrasonography is widely used in (bio-)medical imaging and especially photo-acoustic imaging is rapidly advancing towards new applications. Future applications require a matrix of small (λ/2) and sensitive ultrasound sensors with read-out through a thin and flexible cable [1]. We target ∼15 MHz ultrasound for e.g. catheter-based intravascular imaging or for freely-moving mouse brain imaging. Integrated optical sensors have good prospects: small and sensitive sensors [2,3], wafer-scale fabrication, and matrix read-out via single optical fiber using on-chip wavelength-division multiplexing (WDM) [4]. Polymer waveguide ring-resonator showed good sensitivity and bandwidth [2] but large device footprint hampers WDM. Silicon photonic waveguides are less susceptible to direct ultrasound but opto-mechanical ultrasound sensors (OMUS), with the photonic resonator integrated in an acoustically resonant membrane, showed high sensitivity [3].

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