Numerical simulations of tunable ultrashort power splitters based on slotted multimode interference couplers

This paper presents an ultracompact tunable device for power splitting and switching by tuning the Fermi energy level of monolayer patternless graphene underneath a slotted multimode interference (MMI) coupler operating in the mid-infrared, lambda = 9-11 mu m. By introducing a high-index silicon slot in the central region of the MMI structure, which can significantly shorten the beat length, the proposed device has an approximately 4.5-fold reduction in device length and a two-fold improvement in power transmission compared with conventional MMI couplers without slotting. The device has a footprint of only 0.30 x 0.65 mu m(2) (<lambda/10), making it the smallest power splitter and switcher. Over the bandwidth of 2 mu m, the power transmission of the proposed device is nearly uniform. Extending the operating bandwidth is limited only by the practically achievable Fermi energy of graphene. For the fabrication tolerance, the numerical results show that the relative power variations are lower than 5%, even though the dimension variations are greater than 15%. With its advantages of tuna bility, compact footprint, and broadband operation, the proposed device is suitable for highly dense photonic integrated circuits.