Study On Phase Change Material-assisted Silicon Optical Switches

With the development of integrated photonics,the miniaturization of optical devices is increasingly required in order to increase the integration density of optical chips.Miniaturization has the potential to reduce the cost and power consumption and increases the operation speed of active photonic devices.Chalcogenide phase-change materials(PCMs)have two or more stable phase states and have been used in data storage with unique advantages of drastic optical and electrical contrast and fast switching speeds.In recent years,research on hybrid integration of photonic integrated circuit(PIC)and PCM has caused even-increasing attention in academia and industry.Most of the work was based on hybrid integration of silicon nitride(Si N)and phase-change material Ge₂Sb₂Te₅(GST).Although Si N waveguides have lower propagation losses,silicon(Si)waveguides possess compact and more integrated features which enable the implementation of a variety of passive and active devices.Therefore,hybrid integration of silicon waveguide and GST material has important academic research significance and has the potential to be used in optical communications,optical computing,optical storage and etc.Therefore,this dissertation studies the Si-GST hybrid integration technologies.Based on this technology,a variety of optical switching devices have been realized.First of all,this dissertation studies the basic optical properties of GST.The GST material is placed directly on the surface of the silicon waveguide to form a Si-GST hybrid waveguide.Numerical simulation of the optical modes in hybrid waveguides shows that the effective index of the Si-GST hybrid waveguide varies significantly when the GST changes from the amorphous to the crystalline states.To experimentally verify the effectiveness of the Si-GST hybrid waveguide on light wave manipulation,a series of unbalanced Mach-Zehnder interferometers(UMZI)with one arm inserted with a section of Si-GST hybrid waveguide of different lengths were fabricated.The transmission spectra were measured and the complex effective indices were extracted for GST at crystalline,amorphous and intermediate phases.The experimental results overall agree well with the simulation ones.Then,we used directional couplers to further verify that the optical transmission before and after the phase transition of the heterogeneous integrated waveguide can be used to control the optical path.The dissertation also studies and compares three other commonly used PCMs,which provides data support for the selection of PCMs in the future.When phase change is induced by annealing the device on a hot plate,only amorphous-to-crystalline phase transition can be obtained.In order to obtain multiple cycles of back-and-forth switching,we proposed an all-optical switching unit by using evanescent coupling between the optical mode and a GST thin film.We carried out simulations and experiments to investigate the switching processes during amorphization and crystallization.We found that the pump optical pulse can only amorphize a part of the crystalline region,and on the other hand,crystallization only occurs at the boundary of the amorphous region.It helps us better understand the physical principles behind the phase transition.In addition,we designed a silicon AMZI-coupled ring resonator integrated with a small piece of GST material on top of the ring waveguide.By controlling the intermediate states,we obtain all-optical switching with seven clearly distinguishable intermediate levels and more than 20 d B transmission contrast between the amorphous and crystalline GST states.In order to reduce the power consumption,we also implemented a non-volatile silicon nanobeam resonator integrated with a thin film of phase change material.This gives us a reversible,nonvolatile,low-power tuning method to manipulate light propagation in integrated photonic devices.The all-optical switch is not suitable for large-scale integration.Therefore,we proposed three memristive optical switches enabled by different micro-heaters,namely,a silicon-GST-ITO sandwich heater,an ITO heater,and a doped silicon heater.The working principles and the basic characteristics of the three devices were described in detail.It was demonstrated that both binary-level and multi-level switching operations can be performed in these memristive switches.The device with the doped silicon heater has the advantages of low insertion loss at the amorphous GST state and large on-off switching extinction ratio.Therefore,we focused on the optimization of this device.The successful implementation of optical memristive switches actuated by GST marks a significant step forward in realizing an ultra-small and low-power consumption optical circuit with non-volatile reconfigurability.Meanwhile,we have also shown that a silicon microring resonator integrated with GST can be used as an all-optical synapse for neuromorphic computing.Synaptic plasticity was studied under different resonator-waveguide coupling conditions.We demonstrate a Si-GST hybrid photonic integrated device driven by the interaction of electrical and optical pulses to perform multiplication and logic computation.Owning to the self-holding property of GST,the memory function is seamlessly integrated in the device,providing in-memory computing capability.Finally,the dissertation summarizes the research work.The research and technology foresight for the integration of PCM and silicon waveguide is pointed out.