Research Of On-Chip Brillouin Scattering In Silicon-Based Hybrid Photonic-Phononic Waveguides

With the rapid development of advanced information technologies such as big data,cloud computing and artificial intelligence,there are unprecedented requirements for the transmission,processing and storage of high-speed communication data.Traditional microelectronic chips have increasingly prominent problems in terms of heat generation,power consumption,crosstalk and delay,which impedes the further application of electrical interconnection technology in high-speed communication networks.The silicon-based optical interconnection uses light waves as the information carrier,which has many advantages such as large bandwidth,low power consumption,small delay and strong anti-interference ability.More importantly,the silicon-based photonic integrated circuits are compatible with mature complementary metal-oxide-semiconductor（CMOS）technologies,making it the most potential on-chip optical interconnection solution.The waveguide dimension of silicon-based photonic integrated devices is generally on the order of submicron,which results in sub-wavelength modal confinement and enhances various nonlinear effects.These nonlinear effects can be exploited to a variety of on-chip functional devices.Stimulated Brillouin scattering（SBS）is a common nonlinear effect that originates from the coherent coupling between optical and acoustic waves.The unique acoustic nature of this acousto-optic interaction brings novel functionality that is lacking in all-optical systems,playing an important role in the fields of on-chip microwave signal processing,photonic-phononic information storage and non-reciprocal optical transmission.Therefore,the realization of a controllable and efficient silicon-based SBS integrated device has broad application prospects.In this dissertation,a comprehensive theoretical and simulation study on the SBS effects of silicon-based waveguides has been implemented,and the hybrid photonic-phononic waveguide devices have been fabricated on the silicon-on-insulator（SOI）platform to achieve an efficient forward SBS effect.The specific research results are summarized as follows:（1）The interaction process of SBS in the integrated waveguides is analyzed in depth,and the main physical mechanism of acousto-optic coupling is clarified.Starting from the basic optical and acoustic wave equations,the universally applicable acousto-optic coupled mode equations are derived,and the calculation formulas for SBS gain in translationally invariant and periodic waveguide systems are given.Finally,the three types of SBS that may exist in the integrated waveguides are classified,and how the phase matching conditions lead to their different spatial dynamics are explained in detail.（2）To achieve the appreciable on-chip Brillouin gain in silicon waveguides,a hybrid photonic-phononic waveguide structure combining the ridge waveguide and phononic crystal slab is designed.This design not only provides the simultaneous confinement of the optical and acoustic modes,but also is able to independently control their dispersion characteristics.Then,a series of straight waveguide devices with different parameters were fabricated using CMOS-compatible technology.The experimental results demonstrate the radically enhanced Brillouin nonlinearity,low optical transmission loss and tailorable Brillouin frequency in the hybrid waveguides.The direct gain experiment indicates that a 1.085-cm long straight waveguide device can achieve a small-signal Stokes gain of 0.9 d B at a pump power of 22.4 m W.The net Brillouin amplification is achievable in this hybrid waveguide by optimizing the manufacturing process and coupling scheme.This design can also be applied to the intermodal SBS and other silicon-based material platforms,providing a pathway toward on-chip microwave photonic filters,Brillouin amplifiers and non-reciprocal devices.（3）The silicon suspended racetrack and spiral microrings based on the hybrid photonic-phononic waveguides are designed and fabricated to obtain the resonantly enhanced SBS interactions.According to the acousto-optic properties of the waveguide,the parameters of the microrings are determined to achieve the matching between the free spectrum range and the Brillouin frequency.The experiment characterizes the transmission characteristics of the two types of microrings,showing high Q value（>6×10⁵）,high extinction ratio（～22 d B）and single-mode transmission spectrum.The experiment demonstrates the resonantly enhanced forward SBS in the two kinds of microrings with the measured Brillouin frequency shift of 4.27 GHz and the maximal mechanical quality factor up to 500.In addition,the stronger inhomogeneous broadening effect in the spiral microrings caused by the dimensional variations is also observed,indicating that the racetrack microring is a better choice for on-chip SBS applications.These microring structures can be further cascaded or integrated with other photonic devices,which is expected to realize on-chip photonic-phononic signal processing and Brillouin lasers.（4）To eliminate the detrimental impacts of nonlinear absorption in the waveguides on the Brillouin amplification,a partially suspended silicon nitride（Si₃N₄）vertical slot waveguide structure is designed,which takes advantage of the low propagation loss,wide transparent window and high optical power handling capability of Si₃N₄ material.This design significantly enhances the radiation pressure close to the slot,and suppresses the radiation loss of the acoustic field to the substrate through the acoustic coupling,leading to an enhanced forward SBS gain.The dependence of different structural parameters on the acousto-optic properties of the system is theoretically analyzed,and the optimal waveguide dimensions are determined.Finally,the Stokes amplification performances between the Si₃N₄ slot waveguides and silicon nanowire waveguides are compared,and it is concluded that the Si₃N₄ slot waveguides can provide the larger Stokes amplification under the conditions of large pump power and low optical loss,indicating that Si₃N₄ is a promising on-chip SBS material platform.