Low driving voltage lithium niobate metasurface electro-optical modulator operating in free space

A simple configuration of only λ/9 thick 2D metallic grating embedded within an electro-optic (EO) material (lithium niobate for instance) is proposed and theoretically studied to act as an EO modulator. On the one hand, this grating is used as an interdigitated comb to apply a very high and spatially periodic modification of the electrostatic field. On the other hand, the grating is designed to exhibit a Fano-like resonance in the NIR spectral range. This resonance is used to confine the electromagnetic field inside the EO material leading to an intrinsic enhancement of the EO effect. Extensive numerical simulations are performed to optimize the geometry in agreement with technological fabrication constraints. We achieved a local field factor of 24.5 leading to a local index modification Δn as large as 1 for 1 V applied voltage. This allows a modulation sensitivity of 14.35 nm/V (2000 times larger than state of the art) together with a resonance depth of 60% and a driving voltage of only 75 mV opening the way to the fabrication of ultra-thin low driving voltage EO devices.