Silicon Integrated Photonic Devices With Subwavelength Structures For Multi-Dimensional Multiplexing

With the rapid development of micro-electronics and explosive growth of data-processing needs,the optical-bandwidth bottle-neck has become a severe problem for inter/intra-chip interconnects and data-center communications.The multi-dimensional multiplexing,which is the simultaneous multiplexing that involves wavelength,polarization and mode dimensions,has been regarded as a promising solution to address the conundrum.The silicon integrated photonics is a competitive platform to realize the multi-dimensional multiplexing for its potential ro build blocks with mature fabrications and low power consumption.Recently,the silicon subwavelength structures,e.g.,subwavelength grating(SWG),metamaterial and metagrating,have emerged as a powerful tool to mold optical responses with a high degree of freedom.In this paper,a series of low-loss,low-crosstalk and broadband photonic devices are developed by combining silicon integrated photonics and silicon subwavelength structures for future applications on inter/intra-chip and free-space data interconnects.The simulation method,fabrication technology and measurement platform are firstly introduced.After that,some key elements are thoroughly studied for polarization-,mode-,wavelength-handling as well as free-space multi-dimentional communications.Firstly,for polarization handling,three different types of polarization-handling devices,i.e.polarization beam splitter(PBS),polarizer and polarization rotator(PR),are realized by utilizing anisotropy-and dispersion-engineered SWGs.A novel SWG hetero-junction is proposed to obtain a high-performance PBS.An anisotropy-enhanced coupler is built by assembling SWGs with orthogonal axis orientations,so the launched dual-polarizations can be efficiently separated.Such a device can work over an ultra-broad bandwidth>215 nm.A novel SWG/bent-waveguide hybrid structure is proposed to attain a high-performance polarizer.By properly choosing the structural parameters,the effective-medium index difference can be significantly enhanced between TE and TM polarizations.Thus,the polarization selectivity of bending losses can be greatly boosted,so the input TM mode can be totally filtered out.This device can work over an extremely wide wavelength range(>415 nm).A SWG/cut-cornered-waveguide hybrid structure is proposed to achieve a high-performance PR.The structure is basically a cut-cornered waveguide with corner region replaced by SWGs.The dispersions of polarization-conversion length can be eliminated taking advantages of the abnormal-dispersion properties of SWGs,leading to an ultra-large working bandwidth of>415 nm.Secondly,for mode handling,three different types of mode-handling devices,i.e.multimode-waveguide bend,multimode-waveguide crossing and multimode-waveguide splitter,are realized by exploiting index-engineered SWGs and metamaterials.For multi-mode bending,a novel index-engineered mode converter is introduced.The input straight-waveguide modes can completely convert to be the corresponding bent-waveguide modes,so the bending-induced inter-modal crosstalk can be depressed.We experimentally demonstrate a four-channel MDM system where two mode converters and a sharp multi-mode waveguide bend(<30 ?m radius)are involeved.The experimental results show low insertion losses of<1.5 dB and low inter-modal crosstalk of<-20 dB over a braod bandwidth.For multi-mode crossing,a novel Maxwell's fisheye lens is introduced.The perfect conjugating imaging property of MFL is exploited to realize the lossless power transferring over a crossing section.We experimentally demonstrate a two-channel MDM system where eight mode(de)multiplexers are converged at a single crossing junction.The measurement results show ultra-low insertion losses of<0.28 dB and crosstalk of<-20 dB.For multi-mode splitting,a novel effective-medium transflector is introduced.The SWGs can be treated as a thin-film transflector that is made of effecitve medium,so the incident light can be partially reflected into the output port.Moreover,the reflectivity can be modified by tuning the duty cycle of SWGs,which leads to arbitrary splitting ratios.The low losses and low crosstalk can be achieved over an ultra-wide wavelength span(>415 nm).Thirdly,for wavelength handling,we propose and demonstrate a silicon metagrating optical filter by tailoring bound state in the continuum(BIC).The lateral leakage loss for Bragg resonance can be eliminated by adopting a destructive interference within the shallow-ridge,which transforms the low-quality Bragg resonance into a high-quality quasi-BIC mode.Such transformation process has been experimentally observed.The measurement results show high quality factor of>5000 with an ultra-wide free spectral range of>100 nm.Fourthly,for free-space optical communications,we propose and demonstrate a silicon metagrating optical antenna by harnessing BIC.The diffraction strength can be significantly inhibited by building the destructive interference between different diffraction channels.The measured diffraction strength is as low as 3.3×10-3 dB/?m.The BIC effect can also flatten the diffraction strength dispersion curve,leading to a stable beam steering process.An extremely small far-field divergence<0.027o is attained.Finally,the main works are summarized and an outlook is given.