Sub-wavelength tunneling barrier in rib waveguide microring modulators with vanishing bending losses

Silicon photonics ring resonators in rib waveguide configuration are among the most important components for wavelength-division-multiplexed communication networks. While the rib waveguide enables simple electrical connectivity in microring modulators and add-drop multiplexers, it also results in unacceptable bending losses once the circumference is shrunk below a few micrometers, limiting achievable free spectral ranges and resonant enhancements. We introduce a sub-wavelength tunneling barrier at the critical radius at which the conformally mapped effective index of the slab exceeds that of the waveguide in order to suppress these bending losses, while increasing the resonator's resistance only slightly. The fundamental working principle is explained and illustrated with a design study based on the finite difference eigenmode method. Three-dimensional finite difference time domain simulations verify the design and a proof-of-concept microring modulator is modeled based on the novel geometry.