Excitation of symmetry protected modes in a lithium niobate membrane photonic crystal for sensing applications

Abstract

Our theoretical study reveals the opportunity to develop an electric field sensor based on the exploitation of the symmetry protected mode (SPM) that we excite within an electro-optical material, namely lithium niobate (LN). The SPM consists of a dark Fano-like resonance that results from the combination of a discrete Bloch mode of a photonic crystal (PhC) with a continuum mode of a membrane, both of them made in LN. The dark character is linked to the structure geometry having a high degree of symmetry. The SPM excitation is then made possible thanks to an illumination under small oblique incidence, which breaks the symmetry of the configuration. This results in several ultra-sensitive and tunable Fano-like resonances with high quality factors up to 10 5 in the telecoms spectral range. Some of these resonances provide modes with a highly confined electric field inside LN. This confinement allows the enhancement of the electro-optic Pockels effect by a factor up to 5 × 10 4 , thus exacerbating the detection sensitivity of the designed sensor.