Multiple Quasi-phase-matching Parametric Down-conversion Processes In 2D Nonlinear Photonic Crystals

In recent years, utilizing the quasi-phase-matching technology, optical superlattice (OSL) material not only shows extraordinary talents in nonlinear optics and quantum optics for its unique advantage in allowing multiple and efficient nonlinear processes, but also goes out of the laboratory. It has been widely used in the manufacture of wideband, easy-tuning laser device for its high conversion efficiency, and has taken a stable place in the fiercely competitive market of laser device. With the developments of optical superlattice, lots of optical new effects has been found in two-dimensional optical superlattice (2D OSL),it has attracted the great attention of researchers for its abundant phase-matching schemes. In this paper, based on 2D OSL, we comprehensively study the multiple quasi-phase-matcing (QPM) parametric down-conversion (PDC) processes from both experimental and theoretical aspects, especially the parametric gain, spatial distribution and spectrum of both coherent and incoherent multiple parametric processes. We demonstrate that large conversion efficiency of multiple parametric processes can be achieved by employing common parametric beams. Besides, theoretically, we propose a new scheme of parametric process which satisfies the group velocity matching (GVM) condition, which would improve the spectral properties of entangled photons, especially the generation rate of entangled multi-photons. The paper is constructed as follows:1.We experimentally investigated multiple parametric down conversion processes in a 2D OSL, both degenerate and nondegenerate concurrent parametric downconversion processes were studied. Through making analysis of its conversion efficiency and spatial distribution, we demonstrated that conversion efficiency of the coherent dual-PDC process is much larger than that of the incoherent dual-PDC process and found a way to achieve high parametric gain in 2D OSL.2.We theoretically studied multiple parametric down-conversion processes in 2D OSL, investigated the effective nonlinear coefficients and output spectrum of several typical multiple PDC processes, and made out the difference between the coherent multiple parametric process and incoherent case.Besides, we also deduced the coupled equations of multiple parametric processes, which can give a better insight into the evolution of multiple beams in the crystal.3.We theoretically studied the spontaneous parametric down-conversion (SPDC) process which satisfied group velocity matching when pumped by an ultrashort pulse. The photon pair generation rate, correlation and bandwidth are calculated. We demonstrated theoretically that the bandwidth can be shorten and the generation rate can be improved by satisfying the GVM condition.