Research On On-chip Mode Asymmetric Transmission Based On Encircling Exceptional Point

Information and data are crucial to the development of contemporary technology and economy.With the advent of the big data era,traditional electronic chips are gradually unable to meet the increasing demands for data computation.Photonics chips,with their low delay,small loss,and parallel computing,have become an effective way to break through the bottleneck of data computing.The development of photonics chips urgently requires on-chip nanophotonics devices with rich functions and high efficiency.Exceptional point(EP)originates from the unique degeneracy of non-Hermitian physical systems,and the related theories have been widely used in the fields of optical sensing,lasing,and optical field manipulation,providing new potential for the construction of on-chip photonics devices.Among them,encircling EP in the parameter space can achieve asymmetric optical transmission.Compared with traditional asymmetric transmission devices,it has the characteristics of large bandwidth,strong optical field controllability,and good expansion for high-order modes.It not only has application value as an optical diode in optical logic gates and matrix operations,but also has potential applications in bidirectional information processing and laser output mode shaping.This thesis focuses on the problems still existing in EP-encircling asymmetric transmission devices,such as energy efficiency,polarization control,and multimode multiplexing,and conducts theoretical and experimental research.The major research contents are as follows:(1)High-transmittance asymmetric transmission.High transmittance of photonic devices is crucial to ensure the security and reliability of information data.To overcome the challenge of low transmittance in EP-encircling asymmetric transmission devices,the Hamiltonian hopping assisted encircling EP loop is proposed.Τhe required Hamiltonian parameters for encircling EP are mapped onto suitably designed coupled waveguides on standard silicon-on-insulator platform,resulting in the experimental fabrication of an asymmetric transmission device between fundamental mode and first-order mode.The first-order mode outputs in the forward direction,and the fundamental mode outputs in the backward direction.The experimentally demonstrate transmittance of fabricated sample is-0.56 d B at 1550 nm,which is nearly one order of magnitude higher than that of previous EP-encircling devices.(2)Asymmetric transmission of different polarization modes.Polarization is one of the important dimension of light multiplexing,it can be used to increase the capacity of parallel information processing.To overcome the challenge that most asymmetric modes transmission in optical waveguides only operating a single polarization direction,an EP-encircling path crossing the north pole on the Riemann sphere is proposed.By using waveguides with L-shaped cross-section to control the polarization direction and introduce polarization dependent coupling loss,the path around EP that crossing the north pole is mapped on silicon waveguides structure.An asymmetric polarization transmission device is fabricated,in which the transverse electric mode outputs in the forward direction and the transverse magnetic mode outputs in the backward direction.The measured transmittance is greater than-0.9 d B over the wavelength from 1535 to 1575 nm,and the mode crosstalk is bellow-10 d B.(3)Asymmetric transmission with multiple modes.The multiplexing of optical modes is an important way to increase information capacity.To overcome the challenge that only single mode can be output in encircling EP,a grouping mechanism with non-interfering mode evolution for asymmetric transmission is proposed.By analyzing and designing the symmetry of the Hamiltonian during the process of encircling EP,an axisymmetric four-coupled waveguides structure is fabricated,which enables independent asymmetric transmission of two groups of modes with even and odd orders.And the feasibility of independently asymmetric transmission for four groups of modes in eight-coupled waveguides is simulated verified.This thesis,through theoretical research and experimental verification,improves the transmittance of asymmetric transmission that utilize encircling EP,and expands the functions of polarization control and multimode multiplexing,providing new solutions for the application of on-chip optical diodes and lasers in high-efficiency,high-capacity,and multifunctional photonics chips.