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Silicon-based monolithically integrated whispering-gallery mode resonators with buried waveguides
Research publication
Abstract
We report on the realization and characterization of a silicon-based integrated optical platform which implements a vertical coupling scheme between a Whispering-gallery type microresonator and a buried dielectric waveguide. The vertical coupling allows for the separation of the resonator and the waveguide into different planes, which enables one to realize the optical components in different materials/thicknesses. The high optical quality of this cavity micro-optical system follows from the accurate planarization of the waveguide topography, which is achieved by multiple depositions-and-reflows of a borophosphosilicate glass over strip waveguides. Importantly, we demonstrate the feasibility of our approach for wafer-scale mass fabrication of freestanding planar resonators suspended in air and coupled to integrated bus waveguides. This opens the door for the realization of stable all-integrated resonator systems for optomechanical and metrological applications and has the potential to substitute today’s complicated fiber-taper coupling schemes.
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Research fields
Top-down
Wafer-scale processing forms freestanding planar resonators above buried dielectric bus waveguides. Repeated borophosphosilicate-glass deposition and reflow flatten the waveguide step before the cavity layer is defined and selectively suspended.
Simulations & fits
Finite-element mode profiles evaluate the vertically coupled cross-section and guide the separation needed for efficient interaction without compromising suspension. Resonance analysis then quantifies the performance of the first fabricated devices.
Characterization
Surface-profile measurements, optical microscopy and SEM validate planarisation and release before broadband waveguide transmission is recorded. The completed platform reaches quality factors around 15,000 and demonstrates repeatable integrated access to the suspended cavity.
RESEARCH TOPICS
CouplingVertical loading is set mainly by the deposited spacer rather than by a submicrometre lateral lithographic gap. Devices with gaps of 910 nm are undercoupled, around 860 nm approach critical coupling, and 680 or 560 nm are overcoupled. Across an 80 nm spectral interval the transmission contains a homogeneous set of cavity resonances with measured Q values around 10,000–20,000 near the under/critical regimes. This repeatable gap control is the practical advantage over manually positioned tapered fibres.
MaterialsA 290 nm PECVD SiON layer is patterned into 2 μm buried strip guides, covered and planarised by repeated borophosphosilicate-glass deposition and reflow. A 350 nm PECVD SiN layer then forms the resonators. Without that planarisation, the guide step would print through the spacer and create a strongly scattering cavity surface. An optional sacrificial layer permits air-suspended disks and spiderweb rings while the bus remains integrated below, demonstrating that the same silicon-compatible stack supports multiple mechanical and optical geometries.
ResonatorsThe study measures 50 μm microdisks as well as freestanding WGM disks and spiderweb rings. Their resonances remain addressable through the buried guide after suspension, avoiding external fibre alignment. Q varies systematically with the vertical gap because the loaded linewidth combines intrinsic loss with controllable waveguide leakage. The platform is thus not simply an image of fabricated devices: its optical spectra demonstrate a stable, wafer-scale route to integrated WGM cavities whose loading can be designed before fabrication.