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Light | Polarization control chiral transmission

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Introduction

Chiral transport is not only significant in fundamental research but also holds vast potential in fields such as optical communication and sensing. However, current on-chip chiral transport devices are static and non-tunable, limiting their practical applications. To overcome this limitation,Professor Jian Wang/Professor Lin Chen's team at Huazhong University of Science and Technologyproposed a polarization-controlled chiral transport method. Experiments have demonstrated that different polarizations can produce controllable mode outputs. This achievement is published inLight: Science & Applications，titled “Polarization-controlled chiral transport”.Master's student Hang Zhuis the first author of the paper,Professor Lin Chenis the corresponding author of the paper.

In parity-time symmetric non-Hermitian systems, eigenvalues and eigenstates are simultaneously degenerate at exceptional points (EPs). When the eigenvalues forming the Riemann surface dynamically encircle the EP in the Hamiltonian parameter space, due to the special topological properties of the Riemann surface, the output state of the system only depends on the encirclement direction and is independent of the input state. This phenomenon is chiral transport. Mapping the Hamiltonian parameters to optical structure parameters (such as optical waveguides) allows the observation of chiral transport phenomenon on optical platforms. This phenomenon has great application prospects in optical communication and sensing, such as quantum computing, asymmetric optical switches, polarization controllers, optical isolators, etc. However, previously reported chiral transport devices are static and non-tunable, each output port is locked to a specific mode, regardless of the input light mode and polarization, which limits the device's reconfigurability and transmission capacity.

This research analyzes the refractive index changes of TE and TM polarizations in the L-shaped waveguide and finds that the sensitivity of the refractive index of different polarizations to changes in the top and bottom widths of the L-shaped waveguide is different: when the top width (Wa) changes, the TM polarization refractive index change is more sensitive; when the bottom width (Wb) changes, the TE polarization refractive index change is more sensitive. Therefore, by changing the top and bottom widths, the effective refractive indices of TE and TM polarizations can be made to change in opposite directions. In this case, combining the L-shaped waveguide with a rectangular waveguide with unchanged width to form a dual-coupled waveguide can make the detuning (the difference between the propagation constants of the two waveguides) of TE and TM polarizations change in opposite directions, thus producing polarization-controlled chiral transport (Figure 1).

Figure 1: Dual-coupled waveguide for realizing polarization-controlled chiral transport. (a, b) Mode fields and refractive index changes of TE and TM polarizations in the L-shaped waveguide; (c) Dual-coupled waveguide used to demonstrate polarization-controlled chiral transport; (d-e) Relationship between detuning and L-shaped waveguide width changes for TE (d) and TM (e); (f) Relationship between detuning and propagation distance z

By corresponding the structural parameters of the dual-coupled waveguide system to the Hamiltonian parameters, the corresponding Riemann surface of the system and the parameter evolution trajectories of different polarizations on the Riemann surface can be constructed (Figure 2). When light is transmitted from left to right (from right to left), the parameters of TE polarization will evolve clockwise (counterclockwise) on the Riemann surface, while the parameters of TM polarization will evolve counterclockwise (clockwise) on the Riemann surface. This result shows that the output not only depends on the direction of light transmission but also on the polarization of the input light.

Figure 2: State evolution on the Riemann surface. (a-d) Symmetric mode input from the left end; (e-h) Symmetric mode input from the right end

In addition to using theory to explain the polarization-controlled chiral transport phenomenon, it is also proved through simulation and experiment. The constructed dual-coupled waveguide structure is fabricated on a silicon-based chip, characterized, and experimentally tested to obtain the transmittance curves of TE and TM polarizations (Figure 3). Both simulation and experimental results show that the device has good performance within a certain bandwidth and demonstrate the phenomenon of polarization-controlled chiral transport.

Figure 3: Experimental verification. (a) SEM image of the chiral transport device; (b, c) Simulation (b) and experimental (c) transmittance results for TE₀input; (d, e) Simulation (d) and experimental (e) transmittance results for TM₀input

This research realizes polarization-controlled chiral transport by controlling the Hamiltonian parameters of TE and TM polarizations to evolve in opposite directions around the EP. Compared with the previously reported static and non-tunable chiral transport devices, the device reported in this research has polarization reconfiguration function. From an application perspective, this new type of device can be used as a routing unit for high-performance optical communication and optical computing networks, effectively improving the capacity of optical information transmission and providing a reconfigurable on-chip transmission scheme.

Paper Information

Zhu， H.， Wang， J.， Alù， A. et al. Polarization-controlled chiral transport. Light Science & Applications 14, 77 (2025).

https://doi.org/10.1038/s41377-025-01762-9

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