Quasi-BIC Based Low-Voltage Phase Modulation on Lithium Niobite Metasurface

Efficient phase modulation plays a key role in optical and electro-optic (EO) devices. The rising metasurfaces provide a versatile platform for flexible phase control within subwavelength scale. Lithium niobate (LN) with excellent optical properties is one of the most outstanding EO materials. Here, we numerically design a LN-based metasurface which consists of two identical nanobars in each unit cell. By slightly breaking the structure symmetry, a quasi-bound state in the continuum (quasi-BIC) is excited with a high quality-factor ( $Q$ -factor), which also holds a strong local field enhancement. Significantly, by taking advantage of the EO effect of LN, the reflection phase delay can be modulated in the range of 0- $\pi $ using an external voltage as low as 14 V. Simultaneously, the reflectivity remains higher than 30% as voltage varies. Our design suggests a promising scheme for metasurface-based spatial light modulators.