Understanding the Correlation Between Structure and Entangled Photon Pair Properties with Metal-Organic Frameworks

Spontaneous parametric down conversion (SPDC) is a quantumsecond-ordernonlinear optical process where the photons generated are frequentlyused in quantum information processing. Materials with large second-ordernonlinearities (chi((2))) can be used as entangled photonsources with a high brightness. The source brightness scales as thesquare of the effective nonlinearity (d (eff)), which is an intrinsic property of the material. Understandingmaterial factors that can significantly alter this intrinsic propertyis useful in developing new materials that are SPDC efficient. Inour work we focus on understanding factors affecting the entangledphoton pair properties, such as the arrangements of ligands withinthe Zn(3-ptz)(2) metal-organic framework (MOF) crystaland temperature. We find that the arrangement and alignment of thepyridine rings in the crystal structure significantly affect the d (eff) and birefringence (Delta n). Smaller pyridine ring alignments relative to the optic c-axis increase the Delta n, which inturn leads to larger photon pair correlation times (tau(c)) in coincidence measurements. Our work has significant implicationsin understanding the effect of ligand arrangement on d (eff) and tau(c) for any MOF crystal structure,providing a tool to rationalize the optimization of MOF crystals forthe development of efficient nonlinear optical devices.