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Nonlinear frequency mixing is of critical importance in extending thewavelength range of optical sources. It is also indispensable for emergingapplications such as quantum information and photonic signal processing.Conventional lithium niobate with periodic poling is the most widely useddevice for frequency mixing due to the strong second-order nonlinearity. Therecent development of nanophotonic lithium niobate waveguides promisesimprovements of nonlinear efficiencies by orders of magnitude withsub-wavelength optical conferment. However, the intrinsic nanoscaleinhomogeneity in nanophotonic lithium niobate limits the coherent interactionlength, leading to low nonlinear efficiencies. Therefore, the performance ofnanophotonic lithium niobate waveguides is still far behind conventionalcounterparts. Here, we overcome this limitation and demonstrate ultra-efficientsecond order nonlinearity in nanophotonic lithium niobate waveguidessignificantly outperforming conventional crystals. This is realized bydeveloping the adapted poling approach to eliminate the impact of nanoscaleinhomogeneity in nanophotonic lithium niobate waveguides. We realize overallsecondharmonic efficiency near 10^4 %/W without cavity enhancement, whichsaturates the theoretical limit. Phase-matching bandwidths and temperaturetunability are improved through dispersion engineering. The ideal squaredependence of the nonlinear efficiency on the waveguide length is recovered. Wealso break the trade-off between the energy conversion ratio and pump power. Aconversion ratio over 80% is achieved in the single-pass configuration withpump power as low as 20 mW.

Adapted poling to break the nonlinear efficiency limit in nanophotonic lithium niobate waveguides