An Optomechanical Transducer Platform for Evanescent Field Displacement Sensing

We demonstrate an integrated waveguide platform and optomechanical transduction circuit for chip-scale displacement sensing. The waveguide consists of a thin silicon nitride core layer, a thick silicon oxide bottom cladding, and a top air cladding with a large evanescent field at the waveguide surface. Although the structures feature subwavelength (<;λ/4n
_core
) vertical confinement, they are fabrication tolerant with micrometer-scale lateral features. We demonstrate via simulations and measurements that the waveguides exhibit a low confinement with a maximized evanescent field as well as an effective index only slightly larger than the SiO
₂
bottom cladding index. Despite the low confinement, the waveguide platform enables complex photonic circuits. As a demonstration of this technology, we fabricate and characterize an unbalanced Mach-Zehnder interferometer for chip-scale displacement sensing. A micrometer-scale fiber taper interacts with the waveguide's evanescent field and induces a phase shift proportional to displacement, thereby acting as an optomechanical transducer. We analyze the responsivity, displacement limit of detection, and strength of optomechanical coupling for high-resolution sensing. An outlook toward other applications is also given.