Enhanced Sensitivity of Silicon-Photonics-Based Ultrasound Detection via BCB Coating

In biomedical applications, ultrasound detection is conventionally performed with piezoelectric transducers.
Due to its opacity and reduced sensitivity upon miniaturization, there is a need for the development of new
detection methodology. Recently, ultrasound detection has been demonstrated with waveguides fabricated
in silicon-on-insulator (SOI) substrates. 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 photoelastic response of silicon and silica limited the sensitivity of conventional SOI sensors. In this study, 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. We experimentally show that the sensors’ response to ultrasound in terms of
the induced modulation in the effective refractive index achieved for a BCB-coated silicon waveguide with
TM polarization is comparable to the values 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 (SAWs) and reverberations was reduced for both TE and TM modes
when the BCB over-cladding was used. The use of a silicon waveguide with a polymer over-cladding may
be regarded as a hybrid approach that exploits the advantages of both materials. Using this approach,
optical resonators may be produced that achieve the miniaturization level offered by silicon photonics and
combine it with the sensitivity values comparable to those achieved with polymer waveguides