  To take existing quantum optical experiments and devices into more practical
regimes requires the construction of robust, solid-state implementations. In
particular, to observe the strong-coupling regime of atom-photon interactions
requires very small cavities and large quality factors. Here we show that the
slot-waveguide geometry recently introduced for photonic applications is also
promising for quantum optical applications in the visible regime. We study
diamond- and GaP-based slot-waveguide cavities (SWCs) compatible with diamond
colour centres e.g. nitrogen-vacancy (NV) defect, and show that one can achieve
increased single-photon Rabi frequencies of order O(10^11) Hz in ultra-small
cavity modal volumes, nearly 2 orders of magnitude smaller than previously
studied diamond-based photonic crystal cavities.
