Computational Designs Of Metasurface-based Photonic Integrated Devices

The growing maturity of nanophotonics and integrated photonic technologies has triggered tremendous interest in developing photonic integrated circuits(PICs),which surpass their electronic counterparts in high-speed and energy-efficient information processing for applications including artificial intelligence,neuromorphic computing,quantum computing,and automotive Light Detection and Ranging(Li DAR).The success in these applications requires highly scalable photonic building blocks.Conventional devices in PICs are limited by the diffraction limit of light resulting in bulky schemes.Moreover,existing switching configurations in PICs mainly rely on materials with weak,volatile thermo-optic or electro-optic modulation effects,resulting in large footprints and high energy consumption.Therefore,advances in PICs require radical solutions to address key challenges of the field associated with the system's scalability,including increasing the operation bandwidth and reducing the footprint,insertion losses,and energy consumption.On the other hand,optical metasurfaces have demonstrated remarkable flexibility,efficiency,broadband operation,and compactness to control light incident from free space.Very recently,metasurfaces have been used to control the propagation of guided light.Controlling guided light via metasurfaces is expected to be a disruptive paradigm that can lead to high-efficiency,compact and large-scale PICs.To date,reported metasurface-based integrated devices are passive and application-specific because the optical properties of the constitutional meta-atoms are permanent once fabricated.However,the real-time active control of device functionality is crucial for many applications,including programmable PICs and neuromorphic computing.To actively tune the optical properties of metasurfaces,several approaches have been introduced with considerable interest in using Phase change materials(PCMs),as they exhibit a substantial optical contrast in a static,self-holding fashion upon phase transitions.So far,investigations on PCMs-based metasurfaces have been focused on controlling light propagating in free space.Harnessing PCMs-based metasurfaces for controlling guided light has remained relatively unexplored.Therefore,this dissertation is dedicated to developing novel photonic integrated devices using passive and active metasurfaces.The first part presents a metasurface-based Lithium Niobate waveguide power splitter with an ultra-broadband and polarization-independent performance.The design consists of an array of amorphous silicon nanoantennas that partially converts the input mode to multiple output modes creating multimode interference such that the input power is equally split and directed to two branching waveguides.FDTD simulation results show that the power splitter operates with low insertion loss(<1 d B)over a bandwidth of approximately 800 nm in the near-infrared range,far exceeding the O,E,S,C,L,and U optical communication bands.The metasurface is ultracompact with a total length of 2.7?m.The power splitter demonstrates a power imbalance of less than 0.16 d B for fundamental TE and TM modes.Our simulations show that the device efficiency exhibits high tolerance to possible fabrication imperfections.The second part reports a novel concept for devices with functionality to control guided light in the near-visible spectral range dynamically,which is illustrated by a reconfigurable and non-volatile(1×2)switch using an ultra-compact active metasurface.The switch is made of two sets of nanorod arrays of Ti O₂ and antimony trisulfide(Sb₂S₃),a low loss phase change material(PCM),patterned on a Silicon Nitride waveguide.The metasurface creates an effective multimode interferometer that forms an image of the input mode at the end of the stem waveguide and routes this image towards one of the output ports depending on the phase of PCM-nanorods.Remarkably,our metasurface-based 1×2 switch enjoys an ultra-compact coupling length of 5.5?m and a record high bandwidth(22.6 THz)compared to other PCM-based switches.Furthermore,our device exhibits low losses in the near-visible region(?1 d B)and low cross-talk(-11.24 d B)over a wide bandwidth(22.6 THz).This work paves the way towards realizing compact and efficient waveguide routers and switches for applications in quantum computing,neuromorphic photonic networking,and biomedical sensing and optogenetics.The final part introduces a design approach for realizing on-chip wavelength division demultiplexing(WDD)schemes by integrating all-dielectric metasurfaces of Ti O₂ nanorod arrays into a Si N waveguide.The designed metasurface locally modifies the effective refractive index of the Si N waveguide,creating an effective WDD that selectively passes a certain band of wavelengths into a specific output port.A set of representative 2-channel and 3-channel WDDs schemes were demonstrated for input TE₀₀/TM₀₀ modes and operating in different bands,showing the flexibility of our design approach.The proposed WDDS schemes are compatible with visible to infrared wavelengths,photolithography-based fabrication,efficient with maximum transmission of 91%,and a few microns footprint.This work paves the way towards realizing other compact metasurface-based integrated photonic devices for applications in optical data processing and biological sensing.