High efficiency asymmetric directional coupler for slow light slot photonic crystal waveguides.

An asymmetric directional coupler scheme for the efficient injection of light into slow light slot photonic crystal waveguide modes is proposed and investigated using finite-difference time-domain simulation. Coupling wavelengths can be flexibly controlled by the geometrical parameters of a side-coupled subwavelength corrugated strip waveguide. This approach leads to a ~1dB insertion loss level up to moderately high light group indices (nG≈30) in wavelength ranges of 5-10nm. This work brings new opportunities to inject light into the slow modes of slot photonic crystal waveguides for on-chip communications using hybrid silicon photonics or sensing based on hollow core waveguides.