Study Of Silicon-based Guided Mode Manipulation Devices Using Subwavelength Structures

With the progress of optical communication and the big data industry,integrated optical devices are becoming increasingly important as optical information processing devices.Moreover,the increase in information transmission and processing rate further requires a breakthrough in high integration and large bandwidth of the devices.As the size of conventional waveguide devices is difficult to reduce,subwavelength optical waveguide devices become a promising solution for high density integrated devices due to their advantages in flexibility and high integration.Therefore,we focus on guided wave optical devices based on subwavelength structures in this paper.First,we implement a reflective mode converter using the photonic inverse design.Then,we design the bimodal interferometers and the quasi-circular polarization generator using the phase-gradient metasurface antenna array.Lastly,we utilize the phase-change material Sb₂S₃ and the on-chip metalens to design optical switches and optical beam splitters.The details are as follows:（1）Mode converters are fundamental devices for implementing mode-division multiplexing systems.We use the direct binary search（DBS）algorithm to optimize a reflective mode converter that converts the input TE₀ mode to TE₁ mode with a footprint of 2.0μm?2.0μm.In the functional region of the device,the subwavelength distributed Si and Si O₂ pixels can convert the TE₀ mode to the contra-propagating TE₁ mode.This process effectively extends the optical path and makes the device more compactable.In the wavelength range of 1525-1565 nm,the simulated insertion loss and crosstalk are lower than 0.6 d B and-20.3 d B.Meanwhile,the insertion loss and crosstalk of the corresponding fabricated devices are lower than 2.2 d B and-16.2 d B.Using the same methodology,we then designed two reflective mode converters to implement the TE₀-TE₂ and TE₀-TE₃ mode conversion functions.（2）Metasurface has been deemed as a promising way to manipulate the guided wave.Because of the overlap between the scattering fields of the meta-atoms and the mode field at the waveguide cross-section,the phase-gradient metasurface antenna array can flexibly manipulate the guided mode.Using the 1550 nm fundamental mode as input,we design and simulate two bimodal interferometers that generate two guided modes with tunable intensity percentages,and a quasi-circular polarization generator.For bimodal interferometers,the two interferometers have the output of the hybrid TE₀-TE₁ modes and TE₀-TE₂ modes with identical intensity,respectively.The mode purities of all modes for both two bimodal interferometers are higher than 47.35%.The power transmissions for TE₀-TE₁ modes and TE₀-TE₂ modes interferometers are 69.7%and70.3%.For the quasi-circular polarization generator,we generate the hybrid TE₀-TM₀modes with a 87.3°phase difference.The mode purities for the two modes are higher than 44.91%,and its power transmission is 69.1%.（3）On-chip wavefront shaping is indispensable for implementing optical routing and signal processing.Metasurface has aroused great attention to shape the wavefront owing to its ability to freely control the electromagnetic wave characteristics like amplitude,polarization,and phase.By referring to the concept of metasurface used in free space,we can realize the on-chip wavefront shaping to manipulate the guided wave effectively.We combine the phase-change material Sb₂S₃ nanorods with 1D on-chip metalens to manipulate the propagation of light.Because of the phase-change characteristic,the refractive index of the meta-atom varies relatively large in different states.In our design,by varying the length of the nanorods,the phase shift reaches 2πrad or 1.7 rad when switching to the amorphous or crystalline state.Based on such difference,we use the 1550 nm fundamental TE mode as input and then arrange the Sb₂S₃ array to form a specific phase gradient to realize the deflection-tunable optical directional switches.It can realize the functions of 1?2 and 1?3 optical switching.The intensity ON/OFF ratio at the output port reaches 10 and 4.1 for 1?2 switches and47.86,22,and 29.18 for 1?3 switches.In addition,we switched the Sb₂S₃ array to the amorphous state to provide sufficient phase shift to realize 1?2 and 1?3 beam splitting.By using different types of subwavelength structures,we can flexibly manipulate the parameters like polarization,intensity and phase of the guided wave to design devices with different functions,which thus has promising applications in optical routing,optical networks,and optical phased arrays,optical sensing,etc.