Nanotaper coupler for the horizontal slot-waveguide

Slot-waveguides have attracted considerable attention recently due to the high-intensity electric fields and power densities that can be achieved in very small volumes of low-index materials. Latest applications of this concept have led to new designs of photodetectors, modulators and CMOS-compatible light-emitting devices. However, the coupling of light to and from fiber optics and slot-waveguides remains a challenge. In this paper we present the numerical analysis of a slotted nanotaper for coupling between a fiber and the horizontal slot-waveguide. We used numerical simulations to study the coupling process and found a minimum mismatch loss of 0.4 dB for a tip width of 105 nm. The mode conversion from the tip of the coupler to the full width of the slot-waveguide was performed with a loss less than 0.2 dB when the length was at least 80 microns. This inverse taper increases significantly the coupling efficiency, compared to other approaches such as direct butt coupling and an improved rectangular silicon nanotaper.

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