Marini, LorisHelt, L. G.Lu, YueruiEggleton, Benjamin J.Palomba, Stefano2021-04-220740-3224http://hdl.handle.net/1885/230768Spontaneous parametric downconversion (SPDC) has been predicted in atomically thick crystals, though not yet observed. In this work we uncover physical and experimental constraints of photon-pair generation in such planar nonlinear films with a free-space illumination/collection geometry. We measure the material nonlinear response of monolayer tungsten diselenide via second harmonic generation and subsequently calculate the expected SPDC efficiency from appropriate quantum-classical relations. Energy and momentum conservation shape the wavevectors of photon pairs in free-space and allow us to calculate the loss and coincidence-to-accidental ratio in various configurations. This work improves the understanding of the nonlinear quantum optical potential of these crystals, guides their experimental validation, and provides a performance benchmark for these ultrathin materials.School of Physics, University of Sydney; Australian Research Council (ARC) (CUDOS CE110001010); ARC Discovery Early Career Researcher Award (DECRA) (DE140100805).application/pdfen-AU© 2018 Optical Society of AmericaNonlinear optics at surfacesQuantum opticsNonlinear wave mixingParametric processesPhoton statisticsUltrafast nonlinear optics201810.1364/JOSAB.35.0006722020-11-23