Group velocity independent coupling into slow light photonic crystal waveguide on silicon nanophotonic integrated circuits

Slow light in photonic crystal waveguide can significantly enhance the light-matter interaction, which is a promising approach toward building ultra-compact photonic devices. However, optical coupling from strip waveguide to slow light photonic crystal waveguide is challenging due to the group velocity mismatch between these waveguides. This issue can be addressed by designing a photonic crystal taper that allows the defect guided mode in photonic crystal waveguide to slow down gradually when it enters the photonic crystal waveguide from strip waveguide, thereby minimizing the group velocity mismatch. By using the photonic crystal taper design, experimental results show coupling efficiency can be enhanced by more than 20dB in normal group velocity region with 5dB less fluctuation as compared to the control group, which does not have photonic crystal taper. Enhancement right before photonic bandgap cutoff can be up to 28dB. Measurement results show excellent agreement with two-dimensional (2D) finite-difference time domain (FDTD) simulation.