Ultrahigh-efficiency second-harmonic generation in nanophotonic PPLN waveguides (1810.09235v1)

Abstract: Periodically poled lithium niobate (PPLN) waveguide is a powerful platform for efficient wavelength conversion. Conventional PPLN converters however typically require long device lengths and high pump powers due to the limited nonlinear interaction strength. Here we use a nanostructured PPLN waveguides to demonstrate an ultrahigh normalized efficiency of 2600%/W-cm$^2$ for second-harmonic generation of 1.5-$\mu$m radiation, more than 20 times higher than that in state-of-the-art diffused waveguides. This is achieved by a combination of sub-wavelength optical confinement and high-fidelity periodic poling at a first-order poling period of 4 $\mu$m. Our highly integrated PPLN waveguides are promising for future chip-scale integration of classical and quantum photonic systems.

• The paper demonstrates a breakthrough in SHG efficiency, achieving 2600%/W-cm² through sub-wavelength optical confinement in thin-film PPLN waveguides.
• The study employs an innovative fabrication process with precise 4 μm periodic poling to significantly enhance nonlinear interactions.
• Experimental validation reveals a 53% conversion efficiency on a 4-mm device with 220 mW pump power, pointing to advances in integrated photonic systems.

Ultrahigh-Efficiency Second-Harmonic Generation in Nanophotonic PPLN Waveguides

The manuscript presents a comprehensive investigation into the utilization of nanophotonic periodically poled lithium niobate (PPLN) waveguides to achieve exceptional second-harmonic generation (SHG) efficiencies. Authors Cheng Wang, Carsten Langrock, et al., detail a significant breakthrough in nonlinear optical conversion efficiencies through the innovative implementation of highly confined optical modes with pronounced nonlinear interactions. By leveraging thin-film PPLN waveguides, the paper demonstrates a normalized conversion efficiency of 2600%/W-cm² for SHG, exceeding previously established benchmarks by over an order of magnitude.

The core of this research revolves around the challenge of enhancing conversion efficiencies in PPLN waveguides, which traditionally require extensive device lengths and substantial pump powers due to limited nonlinear interaction. The paper successfully marries sub-wavelength optical confinement with precise periodic poling at a period of 4 μm, achieving optical confinement that greatly enhances interaction strengths. This approach addresses the limitations of conventional waveguides which suffer from low core-to-cladding refractive index contrast, resulting in less efficient mode interactions and larger device dimensions.

Crucial to their findings is the use of thin-film lithium niobate, which inherently offers a higher nonlinear coefficient (e.g., $d_{33} = 25 \, \text{pm/V}$), bolstered by advanced waveguide fabrication processes that ensure minimal propagation losses and tight modal confinement. These precisely engineered waveguides exhibit confinement to areas less than 1 μm², facilitating high overlap and thus enhanced nonlinear efficiency. The research presents numerical simulations reflecting theoretical efficiencies upwards of 4000%/W-cm², contingent upon the meticulous alignment between fundamental and second-harmonic modes.

Experimental validation reinforces the theoretical projections. A detailed fabrication process involving high-fidelity periodic poling using a thin LN film and subsequent waveguide formation enabled the authors to achieve high SHG conversion efficiencies. The practical demonstrations include a measured total conversion efficiency of 53% with an on-chip pump power of 220 mW from a 4-mm device. Such outcomes are indicative of robust device performance with a broad quasi-phase matching (QPM) bandwidth.

This work has significant implications for the field of photonics, especially in contexts demanding efficient wavelength conversion for classical and quantum communication systems. The advancement suggests the potential imminent development of further integrated photonic systems with lower power requirements without compromising bandwidth. Future explorations could involve extending device lengths for near-unity conversion efficiencies under sub-milliwatt pump powers or undertaking dispersion engineering to further optimize efficiency-bandwidth trade-offs.

The implications of this paper are profound, particularly in facilitating chip-scale integration, vital for the advancement of compact and efficient photonic devices. It paves the way for more sophisticated configurations that could potentially address broader spectrum applications while being more adaptive to varying operational environments due to their design and fabrication flexibilities. This research thus constitutes a pivotal contribution to nanophotonics, with tangible impacts on the continued evolution of photonic technologies.