Photonic ultrasound sensors promise unparalleled spatial and temporal resolution in ultrasound imaging due to their size-independent noise figure, high sensitivity, and broad bandwidth. Optical materials can further improve performance and stability, but achieving small size, high sensitivity, and wide bandwidth remains challenging. This work introduces amorphous silicon carbide (a-SiC) for ultrasound sensing, offering strong optical confinement, low propagation loss, and high stability for miniaturized microring sensors. We demonstrate a compact detection system with a 20-transducers linear array coupled to a single bus waveguide. The sensors achieve an optical finesse of 1320 and intrinsic sensitivity of 78 fm kPa⁻¹, leading to a noise-equivalent pressure below 55mPa/Hz, calibrated from 3.36 MHz to 30 MHz. High-resolution imaging of fine structures validates real-world applicability. a-SiC is also easily integrated on most substrates due to its low deposition temperature. Our results position a-SiC as a promising solution for optical ultrasound sensing, combining miniaturization, low-loss, and high-sensitivity.

Real-time mapping of small forces with micrometer resolution is essential for studying soft and biological matter. However, existing techniques are slow, limited in spatial sampling or require non-planar substrates that can perturb cell behavior. Here we present silicon sensor arrays for rapid surface force mapping that operate using the elasto-optically induced wavelength shift in thin polymer-cladded optical ring resonators. Using a nano-indenter, we demonstrate that the sensor array reaches a force resolution down to 12 µN and shows a linear response. We present both a five-ring linear array and a 10×5 two-dimensional array at 15 µm pitch, and demonstrate the feasibility of localization and force mapping of a spherical nanoindentation tip. Combined measurement of forces by nano-indenter and the optical ring resonator sensor presents a methodology for calibrating this type of photonic force sensor. Moreover, good correspondence between measurements and finite element simulations provides evidence for the proposed operation mechanism. The shown combination of biocompatible claddings, strong opto-mechanical coupling, and foundry-ready photonics, presents a route towards scalable, real-time force mapping for soft-matter metrology, tactile interfaces, and in vitro mechanobiology.

Ultrasound is widely used in medical imaging, and emerging photo-acoustic imaging is crucial for disease diagnosis. Currently, high-end photo-acoustic imaging systems rely on piezo-electric materials for detecting ultrasound waves, which come with sensitivity, noise, and bandwidth limitations. Advanced applications demand a large matrix of broadband, high-resolution, and scalable ultrasound sensors. Silicon photonic circuits have been introduced to meet these requirements by detecting ultrasound-induced deformation and stress in silicon waveguides. Although higher sensitivities could facilitate the exploration of new applications, the high stiffness of the waveguide materials constrains the intrinsic sensitivity of the silicon photonic circuits to ultrasound signals. Here, we explore the impact of the mechanical properties of a polymer cladding on the sensitivity of silicon photonic ultrasound sensors. Our model and experiments reveal that optimizing the polymer cladding's stiffness enhances the resonance wavelength sensitivity. Experimentally, we show a fourfold increase in the sensitivity compared to the sensors without a cladding polymer and, a twofold sensitivity increase compared to the sensors with a cladding polymer of saturated cross-linking density. Interestingly, comparing experiments with the optomechanical model suggests that the change in Young's Modulus alone cannot explain the sensitivity increase. In conclusion, polymer-coated silicon photonic ultrasound sensors exhibit potential for advanced photo-acoustic imaging applications. It offers the prospect of increasing the ultrasound detection sensitivity of silicon photonic ultrasound sensors while using CMOS-compatible processes. This paves the way to integrate the polymer-coated silicon photonic ultrasound sensors with electronics to utilize the sensors in advanced medical imaging applications.

Miniaturized optomechanical devices are well-suited for applications in the automotive, aerospace, and biomedical sectors due to their compact size and lightweight design, which make them ideal for measuring small forces [1]. The significant refractive index contrast between the silicon waveguide core and the silicon dioxide cladding in silicon-on-insulator (SOI) structures enables submicron core dimensions. This design supports single-mode propagation at a wavelength of 1.55 µm, with strong optical confinement that allows for sharp bends with radii as small as a few micrometers [2]. Micro-optical-electromechanical systems (MOEMS) offer several advantages over traditional micro-electromechanical systems (MEMS), including higher optical sensitivity, simplicity, cost-effectiveness, and suitability for use in electromagnetically active environments and ultra-high vacuum conditions [3].

Ultrasound is widely used in medical imaging, and photo-acoustics is an upcoming imaging modality for the diagnosis of diseases. Future applications require a large matrix of small, sensitive, and broadband ultrasound sensors. However, current high-end systems still use piezo-electric material to detect ultrasound, with limited sensitivity and bandwidth. Silicon photonic circuits can meet the requirements of size, bandwidth, and scalability when designed as ultrasound sensors. Namely, a silicon photonic waveguide deforms when the ultrasound pressure waves impinge on it, leading to a change in effective refractive index, n_e, due to geometrical and photo-elastic effects [1]. However, these effects are weak, which limits the intrinsic sensitivity of silicon photonic ultrasound sensors [2]. To significantly enhance sensitivity, silicon waveguides have been combined with acousto-mechanical structures, which achieved acoustomechanical-noise-limited sensing [3], but this is not compatible with standard photonic platforms. Besides that, recent demonstrations of waveguides coated with polymers also improved sensitivity of the silicon photonic ultrasound sensors significantly, but not sufficient to reach acoustomechnical-noise-limited sensing [4]. Here, we study the effect of mechanical and opto-mechanical properties of polymer claddings on the sensitivity of silicon photonic ultrasound sensors. Our aim is to enhance the sensitivity of these devices by implementing tailored polymer coatings. First, we model the refractive index sensitivity of these type of waveguides, i.e. the change in effective refractive index n_e due to the incident ultrasound plane-wave with a pressure P, and we (Equation presented) where n_c, p₁₂, E, and v are refractive index, elasto-optic coefficient, Young's modulus (stiffness), and Poisson's ratio of the cladding material, respectively. We assume the change in cladding index dominates sensitivity.

Efficient coupling of light from an optical fiber to silicon waveguides is a challenging task in integrated photonics. Couplers based on adiabatic mode evolution have the advantages of high bandwidth and low loss but are often accompanied by longer device lengths. In this paper, we introduce the concept of adiabaticity map and optimize the coupling between an optical fiber and Si waveguides by selecting routes on the map that minimize unwanted mode coupling. The map clearly indicates areas in mode evolution where supermode coupling is large and identifies optimal routes for efficient mode evolution. Optimized interaction length and widths are obtained from the adiabaticity map. We obtain highly efficient coupling (96%) with large bandwidth (1-dB bandwidth 280 nm) and misalignment tolerance (⪆90 nm lateral misalignment range for 1-dB excess losses) for the TE polarization.

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.

Future applications of ultrasonography in (bio-)medical imaging require ultrasound sensor matrices with small sensitive elements. Promising are opto-mechanical ultrasound sensors (OMUS) based on a silicon photonic ring resonator embedded in a silicon-dioxide acoustical membrane. This work presents new OMUS modelling: acousto-mechanical non-linear FEM and photonic circuit equations. We show that initial wafer stress needs to be considered in the design: the acoustical resonance frequency changes considerably and OMUS sensitivity differs for up-or downwards buckled membranes. Simulated acoustical resonance frequency agrees well with measurements, assuming realistic SOI wafer stress. Measured sensitivity showed large device-to-device variation and simulations agree within this order of magnitude. We conclude that careful modeling of stress is necessary (b) for the design of robust and sensitive sensors.

Several types of ultrasound sensors have been developed and are used in the field of medical imaging. Conventional transducers are made of piezo-electric material and show good practical performance. However, when the piezo-electric elements need to be small (below 100 μm × 100 μm), these transducers face challenges in fabrication as well as the electrical impedance matching of the elements. As an alternative, we fabricated an optical micro-machined ultrasound transducer (OMUT). This sensor contains an optical micro-ring resonator, which is coupled to a photonic waveguide, and integrated onto an acoustical membrane. The OMUT is build with standard silicon-on-insulator (SOI) technology, allowing for easy fabrication. In this paper, we present the first measurement results of the sensor. Our prototype has a -6 dB bandwidth of 19% and a noise equivalent pressure (NEP) of 0.5 Pa. These first acoustical measurements show that this prototype may form the basis of future ultrasound transducers.