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Silicon-based monolithically integrated whispering-gallery mode resonators
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 the realization of the optical components in different materials/thicknesses. The high optical quality of this 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 and it has the potential to substitute todays complicated fiber-taper coupling schemes.
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Research fields
Top-down
Standard deposition, lithography, cladding reflow and selective release create freestanding planar resonators above buried SiON waveguides. Keeping the cavity and bus in different planes allows their materials, thicknesses and horizontal alignment to be selected independently.
Characterization
Optical and SEM inspection evaluates planarisation and suspension, followed by visible and infrared waveguide-transmission measurements. The spectra demonstrate stable vertical coupling and quality factors sufficient for an integrated alternative to fibre-taper excitation.
RESEARCH TOPICS
CouplingThe platform places the whispering-gallery resonator and dielectric bus waveguide in different vertical planes. Multiple BPSG deposition-and-reflow cycles flatten the step above the buried strip guide, after which the coupling gap is set by a conventional etch. With the vertical distance fixed near 850 nm, Figure 3 compares lateral alignments of 0, −1 and −2 μm and shows how moving the guide beneath different radial field lobes selects which families are excited. Coupling strength and modal selectivity can therefore be adjusted independently of resonator patterning.
MaterialsSeparate fabrication levels allow the guide and cavity to use different silicon-compatible materials and thicknesses: SiON or SiNx guides and resonators, silicon-rich oxide where required, BPSG cladding and an optional amorphous-silicon sacrificial layer. This material freedom is a practical consequence of vertical integration, not simply a list of process ingredients. It enables conventional disks, rings, wedges and freestanding membranes to share the same buried access waveguide while maintaining a planar optical surface.
ResonatorsDisks support several radial families, whereas rings suppress many of the higher-order modes; Figure 6 makes this difference visible in their transmission spectra. The first radial family can exhibit a linewidth less than one tenth that of the second near critical coupling, reflecting its lower loss and different overlap. Selective removal of the sacrificial layer also produces air-suspended resonators without losing the integrated guide. The work demonstrates a wafer-compatible alternative to fragile tapered-fibre positioning for stable, mass-fabricated WGM systems.