Abstract

Lithium niobate (LiNbO₃) is a prime material for photonics thanks to a unique mix of properties, including strong electro-optical and nonlinear optical responses as well as a broad transparency window spanning the visible and near-infrared (NIR) range (0.35 to 4.5 μm). Despite the inherent challenges in the nanofabrication of LiNbO₃, caused by its hardness and resistance to chemical etching, recent technical advances eventually enabled the realization of deeply sub-μm features in LiNbO₃, thereby enabling a series of breakthroughs in the field of nano-and meta-photonics [1]. Nonlinear nano-optics has especially benefitted from such improvements thanks to the large second-order diagonal nonlinearity of LiNbO₃. Several LiNbO₃ metasurfaces have been designed to enhance and steer second-harmonic generation (SHG) [1] emitted up to the visible range [2]. Similar designs were also exploited for generating photon pairs via spontaneous parametric down-conversion (SPDC) [3], a quantum process where a single photon is converted into two photons of the same total energy, see Fig. 1c (right).

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