Silicon subwavelength grating-assisted asymmetric directional coupler around 2 μm and its applications

We propose a silicon-based asymmetric directional coupler assisted by subwavelength grating anisotmpic metamaterial operating at the emerging 2 mu m waveband. The effective medium theory is utilized to explain the principle of the proposed structure. Three devices with mode-order conversions are presented. Firstly, TEO-to-TE1, TEO-to-TE2, and TEO-to-TE3 conversions with remarkably high transmittance (> -0.17 dB) and extremely high mode purity (>98.81%) are achieved at the central wavelength of 1985 nm. The fabrication tolerance analyses demonstrate reasonable robustness of the proposed structure. Secondly, such structure can realize a reciprocal optical diode with an exceedingly high contrast ratio (>34 dB) based on spatial symmetry breaking. Finally, we present a symmetric 3 dB mode converter and splitter just by adjusting the length of subwavelength grating and the gap of asymmetric directional coupler. The results reveal that the transmittances of both output ports approach -3 dB around 2 mu m and the transmittance difference remains below -10 dB above 40 nm within the waveband from 1.95 mu m to 2.05 mu m.

Automated summary

This study introduces a novel silicon-based asymmetric directional coupler, utilizing subwavelength grating anisotropic metamaterial in the 2 μm waveband. Mode conversions with high transmittance and mode purity are achieved, as well as a reciprocal optical diode with a high contrast ratio. A symmetric 3 dB mode converter and splitter is also presented, demonstrating high transmittance and low transmittance difference within the waveband.