Highly efficient mode converter for coupling light into wide slot photonic crystal waveguide.

We design, fabricate and experimentally demonstrate a highly efficient adiabatic mode converter for coupling light into a silicon slot waveguide with a slot width as large as 320 nm. This strip-to-slot mode converter is optimized to provide a measured insertion loss as low as 0.08 dB. Our mode converter provides 0.1 dB lower loss compared to a conventional V-shape mode converter. This mode converter is used to couple light into and out of a 320 nm slot photonic crystal waveguide, and it is experimentally shown to improve the coupling efficiency up to 3.5 dB compared to the V-shape mode converter, over the slow-light wavelength region.