Ultrasound detection via silicon waveguides relies on the ability of acoustic waves to modulate the effective refractive index of the guided modes. However, the low photo-elastic response of silicon and silica limits the sensitivity of conventional silicon-oninsulator sensors, in which the silicon core is surrounded by a silica cladding. In this paper, we demonstrate that the sensitivity of silicon waveguides to ultrasound may be significantly enhanced by replacing the silica over-cladding with bisbenzocyclobutene (BCB)-a transparent polymer with a high photo-elastic coefficient. In our experimental study, the response to ultrasound, in terms of the induced modulation in the effective refractive index, achieved for a BCB-coated silicon waveguide with TM polarization was comparable to values previously reported for polymer waveguides and an order of magnitude higher than the response achieved by an optical fiber. In addition, in our study, the susceptibility of the sensors to surface acoustic waves and reverberations was reduced for both TE and TM modes when the BCB over-cladding was used.

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One of the main challenges in miniaturizing optoacoustic technology is the low sensitivity of sub-millimeter piezoelectric ultrasound transducers, which is often insufficient for detecting weak optoacoustic signals. Optical detectors of ultrasound can achieve significantly higher sensitivities than their piezoelectric counterparts for a given sensing area but generally lack acoustic focusing, which is essential in many minimally invasive imaging configurations. In this work, we develop a focused sub-millimeter ultrasound detector composed of a silicon-photonics optical resonator and a micro-machined acoustic lens. The acoustic lens provides acoustic focusing, which, in addition to increasing the lateral resolution, also enhances the signal. The developed detector has a wide bandwidth of 84 MHz, a focal width smaller than 50 µm, and noise-equivalent pressure of 37 mPa/Hz1/2 – an order of magnitude improvement over conventional intravascular ultrasound. We show the feasibility of the approach and the detector’s imaging capabilities by performing high-resolution optoacoustic microscopy of optical phantoms with complex geometries.

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