Research On Silicon Based Plasmonic Polarization Handling Devices

In recent years,the on-chip integration technology represented by Silicon Photonic technology has attracted more and more attention from academia and industry.The high refractive index contrast between silicon and its cladding makes the devices compact,however,the polarization state has a significant impact on the performance of on-chip photonic devices and circuits.Currently,most on-chip polarization control devices are based on dielectric waveguide structures.Due to the weak birefringence effect of dielectric materials,the device footprints are usually relatively large and the structures are complicated.Surface plasmon polaritons(SPPs)is a kind of special surface wave existing at the interface between medium and metal,which can confine the energy of light near the interface and has the ability of limiting the light field in sub-wavelength scale.More importantly,the surface plasmon is a transverse magnetic mode with natural polarization sensitivity,which provides a new idea for the research of ultra small siliconbased polarization control devices.Combined with the silicon photonic waveguide,it can effectively control the effective refractive index and light field distribution of the waveguide mode,and significantly enhance the birefringence effect of the device.It is expected to achieve ultra small and high performance silicon-based surface plasmon polarization control device.This thesis focuses on the on-chip polarization handling technology and utilizes silicon hybrid plasmonic waveguide as the basic structure to study ultra-compact polarization controlling devices.By introducing metal structure to increase the degree of design freedom,the characteristics of the optical field mode distribution of the dielectric waveguide and the hybrid plasmonic waveguide are explored.Based on the SOI material platform,two silicon hybrid plasmonic polarization handling devices are proposed,and the possibility of using polarization conversion to implement on-chip photonic adders is also discussed.The main innovations achieved in this thesis are summarized as follows:1.A transverse magnetic(TM)-pass on-chip polarizer based on hollow hybrid plasmonic waveguide is proposed.With the optimized tapered structure and plasmonics induced optical confinement for TM mode,the propagation of the transverse electric(TE)mode is effectively blocked.Since the width of the metal strip above the silicon waveguide is much larger than the width of the waveguide,this structure avoids the precise alignment between the metal and the waveguide,which increases the process tolerance and effectively reduces the fabrication difficulty.The device length is only4 ?m and the extinction ratio is as high as 57.7 d B at the central wavelength of 1.55?m with low insertion loss of 0.145 d B/?m.In addition,the extinction ratio >34 d B and insertion loss <1 d B are achieved within the whole C-band.2.An ultra-compact on-chip polarization rotating beam splitter is realized by using an asymmetric bent directional coupler.Benefitting from the large birefringence induced by the bent structure and plasmonic effect,the cross-polarization coupling coefficient is significantly increased.The coupling length of the device is only 5.21 ?m,which is the smallest length reported in the known literature.By controlling the optical field distribution and the effective refractive index of the waveguide mode via plasmonics,the TM-to-TE polarization conversion efficiency of the device is as high as 99.9%.At the wavelength of 1.55 ?m,the extinction ratios of TM mode and TE mode are 20.6d B and 32.5 d B,respectively.Furthermore,the polarization conversion efficiency is higher than 90% while maintaining cross talk below-19 d B within the bandwidth of80 nm.3.Photonic neural networks based on silicon surface plasmon adder is proposed.For two-port photonic convolution neural network,the matrix multiplication part is theoretically studied and experimentally verified.The test results show that the structure has the ability of 10 GOPs optical domain matrix multiplication and accumulation.Using the good orthogonality of TM and TE polarization states in the same waveguide and polarization cross coupling between different waveguides,a two-port silicon-based surface plasmon photonic adder is designed.The stable and reliable optical domain adding is realized for the output of the matrix multiplication part.For the multi-port photonic reservoir computing network,which is conducive to the orthogonality of multiple polarization modes,a multi-port silicon-based surface plasmon photon adder is designed to achieve the adding operation of multi-channel output of random projection layer,and the device size is relatively compact.