Enhanced sensitivity of an all-dielectric refractive index sensor with an optical bound state in the continuum

The sensitivity of a refractive index sensor based on an optical bound state in the continuum is considered. Applying Zel'dovich perturbation theory we derived an analytic expression for bulk sensitivity of an all-dielectric sensor utilizing symmetry protected in-Gamma optical bound states in a dielectric grating. The upper sensitivity limit is obtained. A recipe is proposed for obtaining the upper sensitivity limit by optimizing the design of the grating. It is shown that the maximal sensitivity can be achieved regardless to the permittivity of the constituent dielectric of the system. The results are confirmed through direct numerical simulations.