image: Figure. 1 | Double-coupled waveguides for polarization-controlled chiral transport. a, b Modal field and index variations for TE and TM polarizations by changing the upper (a) and lower (b) widths of the single L-shaped waveguide. Below the modal field distributions, n denotes the modal index. The lower and upper widths of the baseline waveguide are Wb = 700 nm and Wa= 350 nm, respectively, and their width variations are ΔWa = ΔWb =100 nm. c Dual-coupled waveguides for demonstrating polarization-controlled chiral transport. The cross-sectional parameters are marked in the left panel. The blue line with arrows in the right panel show the evolution trajectory, where its projection onto (β ⁄ κ, γ ⁄ κ) plane is marked by the brown line. d, e β versus WLa and WLb at 1550 nm for TE (d) and TM (e) polarizations, with WR being fixed at 450 nm. The white straight lines represent the structural parameter variations from O to A-. f β as a function of the propagation distance, z. The entire silicon waveguides are covered by a 1-μm-thick SiO2 layer, with the refractive indices of Si and SiO2 being of 3.478 and 1.444 at 1550 nm, respectively. The full height of L-shaped waveguide is ha = 340 nm, and the height of the lower waveguide is hb = 220 nm. The modal field distributions in a, b are obtained by FDTD simulations.
Credit: by Hang Zhu, Jian Wang, Andrea Alù and Lin Chen
Handedness-selective chiral transport is an intriguing phenomenon that not only holds significant importance for fundamental research, but it also carries application prospects in fields such as optical communications and sensing, including quantum computing, asymmetric optical switches, polarization controllers, optical isolators, and more. However, previously reported chiral transport devices are static, with each output port locked to a specific mode regardless of the input, which limits functional reconfiguration and transmission capacity improvement.
In a new paper published in Light: Science & Application, a team of scientists, led by Professor Lin Chen and his team from Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, have proposed using the incident polarization diversity to control the Hamiltonian evolution path, achieving polarization-dependent chiral transport. This work combines the concepts of Multiple-Input Multiple-Output and polarization diversity with chiral transport, and challenges the prevailing notion that the modal outputs are fixed to specific modes in chiral transport, thereby opening pathways for the development of on-chip reconfigurable and high-capacity handedness-selective devices.
This work implements anti-directional evolution paths for TE and TM polarizations by introducing L-shaped waveguide cross-sections in double-coupled waveguides. In the L-shaped waveguide, TE and TM polarizations are distributed differently, with TE polarization mainly distributed in the central region and TM polarization primarily in the right region. Although their effective refractive indices increase when either the top or bottom width increases, their sensitivity to top and bottom widths differs. Therefore, by changing the top and bottom widths, the effective refractive indices of TE and TM polarizations change in opposite directions. If an L-shaped waveguide and a rectangular waveguide are combined to form double-coupled waveguides, optimizing the geometry can cause the detuning of TE and TM polarizations to change in opposite directions (Figure 1). The dynamic Hamiltonian trajectories for TE and TM polarizations are different in the Riemann surfaces (Figure 2). Simulated and experimental results demonstrate that different polarizations yield controllable modal outputs (Figure 3).
Journal
Light Science & Applications
Article Title
Polarization-Controlled Chiral Transport