  Our theoretical study reveals the opportunity to develop an electric field
sensor basedon the exploitation of Symmetry Protected Mode (SPM) that we excite
within an electro-opticalmaterial, namely lithium niobate (LN). The SPM
consists of a dark Fano-like resonance thatresults from the combination of a
discrete Bloch mode of a Photonic Crystal (PhC) with acontinuum mode of a
membrane, both of them made in LN. The dark character is linked tothe structure
geometry having a high degree of symmetry. The SPM excitation is then
madepossible thanks to an illumination under small oblique incidence which
breaks the symmetry ofthe configuration. This results in several
ultra-sensitive and tunable Fano-like resonances withhigh quality factors up to
10^5 in the telecoms spectral range. Some of these resonances providemodes with
a highly confined electric field inside LN. This confinement allows the
enhancementof the electro-optic Pockels effect by a factor up to 4x10^5, thus
exacerbating the detectionsensitivity of the designed sensor.
