Optical cavities provide a route to sensing through the shift of the optical resonant peak. However, effective sensing with optical cavities requires the optimization of the modal quality factor, Q, and the field overlap with the sample, f. For a photonic crystal slab (PCS) this figure of merit, M = fQ, involves two competing effects. The air modes usually have large f but small Q, whereas the dielectric modes have high-Q and small f. We compare the sensitivity of air and dielectric modes for different PCS cavity designs and account for loss associated with absorption by the sensed sample or its host liquid. We find that optimizing Q at the expense of f is the most beneficial strategy, and modes deriving from the dielectric bands are thus preferred.
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