© (2006) AIP

Optical microcavities can alter the way in which atoms and photons interact. Microphotonic systems provide efficient guiding and trapping of light, recent developments in atom-on-a-chip technology also allow the trapping and transport of cold atoms. The intriguing possibility of enhanced light–matter interaction has led researchers to investigate methods of integrating these two technologies. A promising route to this is provided by silicon-based materials. Gold wires on silicon oxide have already produced high-quality atom chips, and the optical properties of silicon nitride make it an excellent material for the on-chip guiding and localization of light. The next step, which has now been taken by P. E. Barclay and colleagues at Caltech1, is to combine the two.

The team fabricated silicon nitride microdisks having a thickness of 250 nm and a diameter of 9 μm on a silicon substrate. Their low-volume and high-quality factor meant that the microdisks could operate in the strong atom–photon coupling regime. This approach also has the advantage that wet etching allows the resonant wavelength of the cavities to be tuned to a chosen atomic transition. The resulting structures were then mounted on an atom chip where microwires could guide caesium atoms to the vicinity of the microdisks. With the aid of an optical-fibre waveguide permanently placed within a few hundred nanometres, the modes of the microdisks were optically probed to observe the strong coupling effects. With all the essential elements in place, the authors foresee that this their work will lead to a fully monolithic atom-cavity silicon chip.