The efficiency and engineering capabilities afforded by parametric processes in second-order [Chi(2)] nonlinear media have tremendously benefitted from periodic poling technologies in ferroelectric materials such as LiNbO3, nowadays routinely exploited in optical parametric generators, amplifiers and oscillators, as well as high brightness sources of entangled photons . Nonlinear photonic crystals (NPCs) implemented by periodic poling, have also attracted a great deal of attention, since they provide additional degrees of freedom to coherently shape the angular and spectral response of parametric devices . Following the first demonstration of coherently coupled twin-beam optical parametric generation (OPG) in a Chi(2)NPC, a number of studies investigated their potential for classical and quantum applications .
Here we demonstrate twin beam OPG in a hexagonally poled LiTaO3 NPC pumped at 527 nm and generating signal and idlers in the near-infrared wavelengths (~800 nm) and telecom (~1550nm) ranges. We perform a complete spectral and angular mapping of its OPG response for different pump pulse energies (10 – 100mJ) and incidence angles (-2.5 to 2.5°), finding very good agreement between predictions and experiments.
We observed a marked gain enhancement afforded by the coherent cross-seeding of OPG processes that share a common idler or a common signal, which results in the appearance of high-intensity peaks (hot-spots) in the OPG maps recorded at the NPC output. The result is a three-mode entanglement at the quantum level. We also discovered that the NPC can be tuned into a novel ‘superresonant’ regime, whereby multiphoton OPG resonances further coalesce, coherently locking the shared-signal and shared-idler processes to each other. The overall result is a 4-mode OPG output where the parametric gain is further enhanced and unconventional signal-idler quantum correlations emerge, which hold promise for novel devices and functionalities in both classical and quantum regimes, currently under investigation.
 L. Myers et al., J. Opt. Soc. Am. B 12 2102 (1995); S. Tanzilli et al., Nature 437, 116 (2005).
 N. Broderick et al., Phys. Rev. Lett. 84, 4345 (2000); K. Gallo et al., Appl. Phys. Lett. 98, 161113 (2011).
 M. Levenius et al., Appl. Phys. Lett. 101, 121114 (2012); H. Jin et al., Phys. Rev. Lett. 111, 023603 (2013).