Ultrashort slot polarization rotator with double paralleled nonlinear geometry slot crossings

The polarization rotation of an optical slot structure from the vertical to the horizontal direction, or vice versa, is demonstrated theoretically within an ultrashort conversion length. Based on mode-evolution theory and three-dimensional finite-difference time-domain simulation, two parallel nonlinearly tapered crossings are introduced and optimized in the transition region, which efficiently decreases the conversion length from 11 to 5 μm for the wavelength of 1.55 μm in a silicon-on-isolator system. More importantly, the extinction ratio of 22 dB and loss of 0.12 dB are numerically obtained using practical material parameters. The method and result presented here may be extremely valuable for the applications of integrated slot structures.