High-efficiency Second Harmonic Generation in Periodically Poled Thin Film Lithium Niobate Waveguide

In recent years, thin film lithium niobate (TFLN) has attracted extensive attention in integrated nonlinear photonics due to its large second-order nonlinear coefficient. Moreover, the sub-wavelength optical confinement of nano-waveguide improves the nonlinear efficiency by several orders of magnitude. For wavelength conversion, quasi-phase matching (QPM) is the most efficient method because it utilizes the largest χ
⁽²⁾
(d
33
) of lithium niobate and has the largest mode overlap area between fundamental and second harmonic. However, the fabrication process for this device is challenging to produce high quality periodic poling and high tolerance waveguide. In this work, using electric field poling method, we achieve high yields and high fidelity micro-domain by optimizing the poling waveform and designing the comb electrode. Characterized by piezoresponse force microscopy, nearly 100% ferroelectric domain is inversed, and domain duty cycle of 48% is achieved. We use this periodically poled lithium niobate (PPLN) waveguides to demonstrate a high conversion efficiency of 5425%•W
⁻¹
cm
⁻²
for second harmonic generation in a 4mm long waveguide, which is higher than the majority of counterpart in PPLN devices.