Abstract
AbstractThe recent development of ultrasound sensing using the silicon‐photonics platform has enabled super‐resolution optoacoustic imaging not possible by piezoelectric technology or polymeric optical microresonators. The silicon waveguide etalon detector (SWED) design exploits the sub‐micrometer light confinement in the cross‐section of a silicon strip waveguide to achieve a sensor aperture which is 13‐fold to 30‐fold smaller than the cutoff wavelength of the sensor. While its performance in near‐field scanning optoacoustic imaging has been previously studied, the operational characteristics of this sensor as it relates to conventional optoacoustic imaging applications are not known. Here, for the first time, the application of the SWED in optoacoustic mesoscopy is investigated, the interaction of the sensor with ultrasound in the far‐field is characterized, the acoustic point spread function up to a depth of 10 mm is measured, and 3D vasculature‐mimicking phantoms are imaged. The measured point spread function of the sensor shows that surface acoustic waves can degrade the lateral resolution. Nevertheless, superior resolution is demonstrated over any state‐of‐the‐art ultrasound sensor, over the whole range of imaging depths that are of interest to optoacoustic mesoscopy. Silicon photonics is proposed as a powerful and promising new platform for ultrasonics and optoacoustics.
Highlights
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We show that ultrasound can interact with the silicon-photonics platform in two prominent ways: direct detection of longitudinal waves (L-waves) and indirect detection through conversion of L-waves to surface acoustic waves (SAWs)
We demonstrated that the miniaturization and scalability offered by the silicon-photonics platform enable manufacturing of acoustic point sensors that offer superior image resolution, at any imaging depth, and superior integration density over what is possible to achieve using piezoelectric arrays or optical resonators manufactured from polymer
Summary
In order to maximize the transmission efficiency, the facet of the fiber array is polished so the light is incident on the grating couplers with an angle of 7° relative to the normal to the chip surface, and the fibers are oriented to excite the transverse electric (TE) mode in the silicon waveguide.[15] The fiber array is attached to the chip using a ultraviolet-curable epoxy transparent in the C-band. The excited TE mode is strongly confined to the crosssection of the silicon waveguide, ensuring ultrasound detection is limited to within this area, and no cross talk between the SWEDs is possible even if the pitch is greatly reduced. A tunable continuous wave (CW) laser (“Laser2;” Figure 1a) pumps light through the fiber to the SWED. All the images were reconstructed using a back-projection method in Fourier space.[16]
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