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Sensitive, small, broadband and scalable optomechanical ultrasound sensor in silicon photonics

Abstract

Ultrasonography1 and photoacoustic2,3 (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 size4. The demonstrated sensor matrix with on-chip photonic multiplexing5,6,7 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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Fig. 1: Concept of new OMUS and innovative split-rib waveguide.
Fig. 2: CMOS fabrication process developed to produce the 15 nm gap.
Fig. 3: Ultrasound sensor characterization.
Fig. 4: Ten OMUSs with photonic WDM .
Fig. 5: Photoacoustic tomography with the OMUS.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Code availability

Photonic and mechanical numerical modelling was performed using commercially available Lumerical and COMSOL softwares. The code that analyses the experimentally measured data is available from the corresponding author upon reasonable request.

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Acknowledgements

We thank B. Du Bois, M. Dahlem, R. Jansen, C. Pieters and H. Tilmans for fruitful discussions; N. Hosseini for simulations of the directional coupler; P. Coene for the Process Design Kit; R. Demeyer and T. Raes for preparatinon of the CMOS masks; K. Baumans, J. de Coster, J. He and B. Snyder for fibre coupling; Imec’s 200 mm Pilot Line Engineering and FAB teams for fabrication process development; M. Billen, P. Czarnecki, T. D. Kongnyuy and M. Zunic for lab assistance; T. A. La and O. Ülgen for preparing the photoacoustic phantom; P. Absil, J. De Boeck, L. Lagae and H. Osman for support. V.N. acknowledges financial support from the Deutsche Forschungsgemeinschaft (DFG; Gottfried Wilhelm Leibniz Prize 2013; NT 3/10-1).

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Authors

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W.J.W., M.M.-U.-H., S.S. and V.R. invented the sensor and fabrication concept. W.J.W. designed the sensor, carried out the ultrasound experiments, analysed the data and wrote the manuscript. W.J.W. and R.S. carried out the photoacoustic experiments and analysed these data. M.M.-U.-H. and S.S. designed and supervised the CMOS-compatible process integration. V.N. supervised the photoacoustic experiments. X.R., S.S. and V.R. and supervised the work.

Corresponding author

Correspondence to Xavier Rottenberg.

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Competing interests

The authors declare the following competing interests: Imec (Leuven, Belgium) has patents pending related to the reported sensor concept.

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Peer review information Nature Photonics thanks the anonymous reviewers for their contribution to the peer review of this work.

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Supplementary Information

Supplementary Sections 1–11.

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Westerveld, W.J., Mahmud-Ul-Hasan, M., Shnaiderman, R. et al. Sensitive, small, broadband and scalable optomechanical ultrasound sensor in silicon photonics. Nat. Photonics 15, 341–345 (2021). https://doi.org/10.1038/s41566-021-00776-0

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