We demonstrate that the all-optical inscription of second-order nonlinearity through the coherent photogalvanic effect allows not only degenerate but also non-degenerate sum-frequency generation in silicon nitride waveguides. Several multiphoton absorption processes can occur simultaneously, such that their quantum interference allows for the inscription of distinct charge gratings for quasi-phase matching of several second-order nonlinear processes within the same waveguide. In this work, we investigate the effect theoretically and experimentally validate the generalized sum-frequency generation.
All-optical poling leads to an effective second-order nonlinearity (χ(2)) in centrosymmetric materials without the need for sophisticated fabrication techniques or material processing, through the periodic self-organization of the charges. The absence of the inherent χ(2) in prevailing silicon-based platforms can be surmounted through all-optical poling. Using the induced effective χ(2) in silicon nitride (Si3N4) waveguides, nonlinear frequency up-conversion processes, such as second-harmonic generation, were previously demonstrated on Si3N4. Here, we report near- and non-degenerate difference-frequency generation in all-optically poled Si3N4 waveguides. We show the agreement between the theory and the measurements and optimize achievable QPM bandwidth range, reaching conversion efficiency of 1 %/W.
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