Electrical field sensing in an etchless lithium niobate heterostructure with Low driving voltage based on quasi-bound states in the continuum

Realizing a lithium niobate-based electric field (E-field) sensor that operates with low driving voltage has attracted widespread attention for a variety of applications such as extremely weak electric field detection, wireless communications, and astronomical observations. However, lithium niobate (LN) is difficult to etch with great accuracy. To address this issue, we proposed a heterostructure E-field sensor consisting of an etchless LN thin-film and one-dimensional SiO2 metagratings that support Fabry-Perot-like BIC mode. This mode has not only an ultra-high quality (Q) factor but also a large extinction ratio. Due to electrode loss, the BIC mode transitions into a quasi-BIC resonance with a dimensionless Q factor of 2050. This resonance confines the electromagnetic field inside the LN layer, leading to an optical field enhancement factor that is 14.2 times greater than that of an unpatterned LN thin film. This resonance also drives the strong interaction between the optical field, the LN material, and the external applied voltage. We carry out numerical simulations to demonstrate that the Fabry-Perot-like quasi-BIC resonance is sensitive to the refractive index change of the LN material. As a result, we achieved a tuning sensitivity of 40.8 nm/V and a low driving voltage of 18.9 mV with a wavelength resolution of 0.38 nm. Meanwhile, we estimate that the 3 dB bandwidth of the E-field sensor should exceed 154 GHz after taking into account the low parasitic capacitance of the LN material and the high conductivity of the electrodes. This LN-based heterostructure E-field sensor has great potential for applications that require ultra-low driving voltage.