Counterpropagating Non-Degenerate Frequency Up-Conversion in X-Cut Periodically Poled LiNbO₃ Nanophotonic Wires

Counterpropagating (CP) <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\chi^{(2)}$</tex> interactions have since the inception of nonlinear optics garnered significant interest due to the unique features of their wave-dynamics and spectral response, stemming from their inherent feedback. The extremely short periods required for their quasi-phase matching (QPM) imply significant technology challenges, that are nevertheless being overcome by advances in periodically poled (PP) materials, with most prominent results achieved in bulk PPKTP under pulsed excitations [1]. Very recently, continuous-wave (CW) operation in ultralow-footprint devices has been achieved on the emerging periodically poled thin film LiNbO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> (PPTFLN) platform, with symmetric second harmonic generation (SHG) in Z-cut waveguides [2]. Here we present the first results on CP frequency conversion in X-cut PPTFLN, the most widely used cut for photonic integrated circuits in LN [3], which has however proven harder to pole with short periods and high uniformity. At difference to the work of [2], we extensively explore the spectral response in sum frequency generation (SFG) experiments, considering nondegenerate and asymmetric regimes, which provide efficient unidirectional emission.