Lithium Niobate Meta-Optics

The rapid development of optical metasurfaces, 2D ensembles of engineered nanostructures, is presently underpinning a steady drive toward the miniaturization of many optical functionalities and devices. The list of material platforms for optical metasurfaces is rapidly expanding as, over the past few years, we have witnessed a surge in establishing meta-optical elements from high-index, highly transparent materials with strong nonlinear and electro-optic properties. In particular, crystalline lithium niobate (LiNbO₃), already a prime material for integrated photonics, has shown great promise for novel meta-optical components, thanks to its large electro-optical coefficient and second-order nonlinear response and its broad transparency window ranging from the visible to the mid-infrared. Recent advances in nanofabrication technology have indeed marked a new milestone in the miniaturization of LiNbO₃platforms, hence enabling the first demonstrations of LiNbO₃-based metasurfaces. These seminal works set a steppingstone toward the realization of ultrathin monolithic nonlinear light sources, efficient quantum sources of correlated photon pairs, as well as electro-optical modulators. Here, we review these recent advances by providing a perspective on their potential applications and examining the possible setbacks and limitations of these emerging technologies.