  We design extremely flexible ultrahigh-Q diamond-based double-heterostructure
photonic crystal slab cavities by modifying the refractive index of the
diamond. The refractive index changes needed for ultrahigh-Q cavities with $Q ~
10^7$, are well within what can be achieved ($\Delta n \sim 0.02$). The cavity
modes have relatively small volumes $V<2 (\lambda /n)^3$, making them ideal for
cavity quantum electro-dynamic applications. Importantly for realistic
fabrication, our design is flexible because the range of parameters, cavity
length and the index changes, that enables an ultrahigh-Q is quite broad.
Furthermore as the index modification is post-processed, an efficient technique
to generate cavities around defect centres is achievable, improving prospects
for defect-tolerant quantum architectures.
