Spontaneous Parametric Down-Conversion Beaming from a Lithium Niobate Nanostructured Resonator

Lithium niobate (LiNbO
3
) 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 LiNbO
3
, caused by its hardness and resistance to chemical etching, recent technical advances eventually enabled the realization of deeply sub-μm features in LiNbO
3,
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 LiNbO
3
. Several LiNbO
3
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).