Abstract
We propose a silicon-based asymmetric directional coupler assisted by subwavelength grating anisotropic metamaterial operating at the emerging 2 μm waveband. The effective medium theory is utilized to explain the principle of the proposed structure. Three devices with mode-order conversions are presented. Firstly, TE0-to-TE1, TE0-to-TE2, and TE0-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 μm and the transmittance difference remains below −10 dB above 40 nm within the waveband from 1.95 μm to 2.05 μm.
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