  Exceptional point (EP)-based optical sensors exhibit exceptional sensitivity
but poor detectivity due to their acute sensitivity to perturbations such as
noise. When the optical budget is limited as in applications on mobile
platforms, high detectivity might be equally important as high sensitivity. In
such scenarios, off-EP sensing is advantageous where a slight loss in
sensitivity can lead to a significant gain in detectivity. Here, we show that a
passive parity-time symmetric plasmonic-photonic hybrid shows a peak in its
detectivity off EP where the sensitivity is still very high-surpassing the
sensitivities of equivalent fully-plasmonic and fully-photonic systems. We
demonstrate the high detectivity of off-EP sensing of anti-mouse IgG protein
using this plasmonic-photonic hybrid system. We report a sensitivity of 1.2
nm/nM while requiring a minimum optical budget of 1.3 nJ. Thus, non-Hermitian
plasmonic-photonic hybrids could be the best class of nanophotonic sensors for
real-world applications.
