Portable tiny doughnut-shaped glass structures with exceedingly smooth surfaces and high Q-factors of around 30 million have now been made by scientists in California. The 'microtoroidal silica resonators' allow light to circulate around their outer ring with low loss and are potentially ideal for creating high-performance optical filters and exploring cavity quantum electrodynamics (QED) (Opt. Express 15, 166–175; 2007).

Credit: © 2007 OSA

Fabricating and handling the 50-micrometre diameter microtoroids is a tricky business. Mani Hossein-Zadeh and Kerry Vahala, from the California Institute of Technology, first deposit a thin disk of glass (2 μm thick) onto a silicon wafer. They then etch away the silicon substrate with XeF2 to create a small pillar beneath the silica disk. The glass disk is then irradiated with a CO2 laser beam, which melts its edges, and surface tension causes the glass to flow with exceptional smoothness into a doughnut shape that features a very thin membrane of glass in its centre. Finally, the silicon pillar is etched to a point and the tip of a tapered optical fibre is used to break the microtoroid from the pillar and detach it.

Although detaching the toroid creates a small hole (4–6 μm in diameter) in the central silica membrane, the researchers say that this does not adversely affect the optical properties or Q-factor in any way because the light is confined within the ring part of the structure.

As any contamination or physical damage to the microtoroid would degrade its Q-factor, the team from California have also designed and fabricated so-called silica microforks to help make the detachment and handling much easier and safer. As for its future plans, the team is now gearing up to integrate several such microtoroids together on a silicon–silica bench and couple them to waveguides.