Nonlinear metasurfaces for the generation of quantum photon-pair states

We present the fundamental concepts for enhancing photon-pair generation through spontaneous parametric downconversion in nonlinear dielectric metasurfaces. We show that in metasurfaces with specially designed two-dimensional (2D) or one-dimensional (1D) nanopatterns, which support optical bound states in the continuum, the photon rate and spectral brightness can be enhanced by orders of magnitude when compared to unpatterned thin films. In the case of our 2D structure, the photon pairs are emitted close to the normal, while our 1D lattice can mediate emission in a broad angular pattern. We also identify the distinct physical mechanism of the enhancement associated with 2D hyperbolic transverse phase matching of the frequencies with nondegenerate photons. These results can provide a foundation for future advances in the development of ultraminiaturized quantum sources of entangled photons tailored for various applications.