Tunable Mode-Splitting In Indirectly Coupled Photonic Crystal Nanobeam Cavity Systems

This paper first introduces the characteristics of photonic crystal and photonic crystal nanobeam(PCN)cavity,as well as the physical mechanism and generation mechanism of mode splitting.The concept of slow light and the principle and typical structure of slow light in PCN cavity are introduced in detail.Then,the theory and numerical calculation methods in photonic crystal waveguide are introduced,such as Finite Difference Time Domain(FDTD)and Coupled Mode Theory(CMT).Finally,the simulation values are verified by theoretical and numerical calculation methods,and the physical mechanism and application prospect of mode splitting are explored.This paper takes the mode splitting phenomenon in PCN cavity as the research topic,and focuses on the active regulation of mode splitting in the system.The specific research contents are as follows :(1)A cavity-waveguide coupling system is designed based on the PCN cavity,and the mode splitting phenomenon in the system is simulated by FDTD.The mode splitting phenomenon and the corresponding transmission characteristics and resonance characteristics are studied.Firstly,the coupling distance between the resonant cavity and the waveguide in the system structure is changed,and the mode splitting phenomenon occurs in the system.With the decrease of the coupling distance between the resonant cavity and the waveguide,the system is split from a single resonant valley to a double resonant valley.Since the change of system structure is extremely inconvenient for the modulation of mode splitting,we explored the active regulation of mode splitting without changing the system structure parameters.By introducing graphene material to cover it on the PCN cavity and electrically tuning the Fermi level of graphene,we completed the active regulation of mode splitting.Finally,the numerical simulation is analyzed by CMT.The results show that the active control of mode splitting is due to the change of the Fermi level of graphene,which directly affects the loss of the resonator.In addition,the slow light effect realized in the system is discussed and the group delay is calculated.(2)A double PCN cavity indirect coupling system is designed and proposed.Two identical PCN cavities are arranged on both sides of the waveguide in a certain horizontal spacing.By adjusting the horizontal spacing between the two PCN cavities,mode splitting occurs in the system.The research shows that the change of the horizontal spacing between the two side cavities changes the round-trip phase difference between the two cavities,which determines the indirect coupling strength between the two resonant cavities.When the round-trip phase difference is close to π an integer multiple,a strong indirect coupling effect occurs between the two cavities.The slow light effect realized in this system is studied and the group delay under different double-cavity spacings is calculated.Finally,without changing the system structure parameters,the influence of cavity refractive index on mode splitting is discussed.The results show that mode splitting can be controlled by changing the cavity refractive index.