Evanescent coupling of asymmetric self-rolled-up microtube and slab waveguide

In this paper, for the first time, optical characteristics of asymmetric self-rolled-up microtube (SRM) are investigated analytically. By considering the asymmetric nature of SRM, modified conformal mapping method is introduced to derive closed-form formulas for resonant modes and field profiles. Intrinsic quality factor (Q-factor), due to whispering gallery and edge scattering losses, is evaluated by combining modified conformal mapping and volume current methods. Also, coupling strength and consequently external Q-factor for evanescent coupling of asymmetric SRM and silicon (Si) slab waveguide is theoretically examined by the coupled mode theory. The simulations show that tailoring the design parameters such as radius of SRM, winding number and asymmetric parameter severely affects the resonant modes and each constituent of Q-factor. On the other hand, Q-factor of edge scattering (~3×106) determines the limit for intrinsic Q-factor. Additionally, the impact of phase mismatch and field overlap on external Q-factor is investigated when the height of Si waveguide and the gap distance between SRM and Si waveguide are changed. Results are in good agreement with the numerical simulation values and the data extracted from experimental reports.