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

TL;DR

This paper introduces an adapted poling technique in nanophotonic lithium niobate waveguides that overcomes nanoscale inhomogeneity limitations, achieving near-theoretical second harmonic efficiency and high conversion ratios at low pump powers.

Contribution

The authors develop an adapted poling method that eliminates nanoscale inhomogeneity effects, significantly enhancing nonlinear efficiency in nanophotonic lithium niobate waveguides.

Findings

Achieved near 10^4 %/W second harmonic efficiency without cavity enhancement.

Recovered the quadratic dependence of efficiency on waveguide length.

Realized over 80% energy conversion ratio with only 20 mW pump power.

Abstract

Nonlinear frequency mixing is of critical importance in extending the wavelength range of optical sources. It is also indispensable for emerging applications such as quantum information and photonic signal processing. Conventional lithium niobate with periodic poling is the most widely used device for frequency mixing due to the strong second-order nonlinearity. The recent development of nanophotonic lithium niobate waveguides promises improvements of nonlinear efficiencies by orders of magnitude with sub-wavelength optical conferment. However, the intrinsic nanoscale inhomogeneity in nanophotonic lithium niobate limits the coherent interaction length, leading to low nonlinear efficiencies. Therefore, the performance of nanophotonic lithium niobate waveguides is still far behind conventional counterparts. Here, we overcome this limitation and demonstrate ultra-efficient second order nonlinearity in nanophotonic lithium niobate waveguides significantly outperforming conventional crystals. This is realized by developing the adapted poling approach to eliminate the impact of nanoscale inhomogeneity in nanophotonic lithium niobate waveguides. We realize overall secondharmonic efficiency near 10^4 %/W without cavity enhancement, which saturates the theoretical limit. Phase-matching bandwidths and temperature tunability are improved through dispersion engineering. The ideal square dependence of the nonlinear efficiency on the waveguide length is recovered. We also break the trade-off between the energy conversion ratio and pump power. A conversion ratio over 80% is achieved in the single-pass configuration with pump power as low as 20 mW.