ArXiv · 2026
Tong Shen (State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, People's Republic of China), Yiyang Xie (State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, People's Republic of China), Yucheng Dai (State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, People's Republic of China), Yifan Zhang (School of Information Science and Technology, ShanghaiTech University, Shanghai 201210, People's Republic of China), Yuanze Li (State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, People's Republic of China), Xufeng Kou (School of Information Science and Technology, ShanghaiTech University, Shanghai 201210, People's Republic of China, ShanghaiTech Laboratory for Topological Physics, School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, People's Republic of China), Tian Liang (State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, People's Republic of China, Frontier Science Center for Quantum Information, Beijing 100084, People's Republic of China)
The topological magnetoelectric effect (TME) in three-dimensional topological insulators manifests as a quantized surface charge accumulation proportional to an applied magnetic field. Here we demonstrate an optical method using terahertz time-domain spectroscopy (THz-TDS) to detect surface charge accumulation in a chromium-doped (Bi,Sb)₂Te₃ thin film under oblique incidence, achieving sub-milliradian Faraday rotation precision. Unlike transport probes that require ultralow longitudinal conductivity, this optical technique is robust against finite σ_L, degrading by less than 0.3% even when σ_L ∼ σ_T. We extract the charge accumulation η/B_z from the measured Faraday rotation and show results at 45^∘ and 60^∘ coincide within experimental uncertainty. Extending this to axion insulators, we predict that the TME produces an imaginary Faraday rotation linear in frequency, whose slope directly reflects the single-surface charge density. With improved sample thickness and precision, this optical scheme provides a viable pathway toward direct verification of the TME and four-dimensional quantum Hall effect.
