Ultrahigh-Qtoroidal microresonators for cavity quantum electrodynamics

We investigate the suitability of toroidal microcavities for strong-coupling cavity quantum electrodynamics (QED). Numerical modeling of the optical modes demonstrate a significant reduction of the modal volume with respect to the whispering gallery modes of dielectric spheres, while retaining the high-quality factors representative of spherical cavities. The extra degree of freedom of toroid microcavities can be used to achieve improved cavity QED characteristics. Numerical results for atom-cavity coupling strength g, critical atom number N₀, and critical photon number n₀ for cesium are calculated and shown to exceed values currently possible using Fabry-Perot cavities. Modeling predicts coupling rates g∕2π exceeding 7000.3em0exMHz and critical atom numbers approaching 10^-7 in optimized structures. Furthermore, preliminary experimental measurements of toroidal cavities at a wavelength of 8520.3em0exnm indicate that quality factors in excess of 10⁸ can be obtained in a 501000-0ptμm principal diameter cavity, which would result in strong-coupling values of $(g∕(2{π}),{n}₀,{N}₀)=(86{{0.3em}{0ex}}MHz,4.6×{}{10}^{{-}4},1.0×{}{10}^{{-}3})$.