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Characterization of second-harmonic generation in silver nanoparticles for spontaneous parametric down-conversion

Energy-time entangled photon pairs (EPPs), which are at the heart of numerous quantum light applications, are commonly generated in nonlinear crystals. Some highly sensitive quantum applications require the use of ultra-broadband entangled photons that cannot be generated in nonlinear crystals due to phase-matching requirements. Here, we investigate the possibility of using metallic nanoparticles (MNPs) as a means for generating entangled photons through spontaneous parametric down-conversion (SPDC). MNPs are known for their strong light-matter coupling at their localized surface plasmon resonance, and since the propagation length through them is negligible relative to optical wavelengths, we consider them as excellent candidates to serve as non-phase matched sources of ultra-broadband entangled photons. To that end, we report experimental results of classical-light second-harmonic generation in silver nanotriangles and nanocubes embedded in polyvinyl alcohol. Based on the results of our experiments, performed using the reference-free hyper-Rayleigh scattering method [A. Ashkenazy et al., J. Phys. B 145401, 2019], we present an estimation of the characteristics of SPDC in MNPs. We show that, despite of the metals' centrosymmetric structure, MNPs exhibit second order nonlinearity that is mainly of an electric dipole nature, and so they are suitable for SPDC. Moreover, we show that localized surface-plasmon resonance can play a significant role in enhancing the generated EPPs flux. Finally, we compare the SPDC capabilities of MNPs to that of commonly used nonlinear crystals and show that the expected EPPs flux from MNPs is weaker but the EPPs have very large bandwidth, which could be helpful for advanced quantum sensing, spectroscopy and communication applications.