Interdisciplinary QC-CM Seminar | Strong Terahertz electrodynamics in emergent 2D materials

Terahertz (THz) sensing and imaging are critical in both quantum information technology and biomedical sensing because THz frequencies (0.1-10 THz) resonate with key low-energy information carriers (e.g., coherent phonons and magnons) in quantum materials and molecular vibrations in biological matter (e.g., skin tumor tissues and blood cells). In addition, materials with THz response are essential building blocks for the next generation telecommunication technology. However, the widespread use of THz technology has long been hindered by a lack of materials with strong THz light-matter interactions for high-performance devices.

In this talk, I will present our recent advances in two-dimensional (2D) quantum materials to overcome these limitations by leveraging their unique topological properties and exploiting the resulting strong light-matter interactions. One remarkable example is the recently discovered nonlinear Hall effect (NHE) in 2D topological semimetals, mediated by their diverging quantum geometrical properties [1-3]. In the first part of the talk, I will report how we use this new notion to demonstrate the long-sought THz sensing metrics [4]. Specifically, we have experimentally studied the unique interplay among the quantum geometrical properties, gate-tunable electron correlation and THz electrodynamics in atomically thin topological semimetals TaIrTe 4 . Building upon the nonlinear Hall effect as a new mechanism for THz rectification, we have observed a large zero-bias responsivity (~ 0.3 A/W), ultralow NEP (~pW/Hz 1/2 ), broadband THz response (0.1 to 10 THz) and ultrafast intrinsic speed (~ ps) at room temperature. The device performance can be further enhanced by introducing gate-tunable electron correlations. Thanks to the new topological physics and strong electron correlation, the demonstrated device metrics show tremendous advantages over the attainable THz detectors based on other 2D materials and conventional technology. Beyond light probing, the rich interplay physics in this platform also allows using light to induce more exotic order. If time permits, I may present our ongoing efforts along this way.

Detecting terahertz waves is only one half of the equation, in the second half of the talk, I will introduce our report of colossal THz emission from a van der Waals (vdW) ferroelectric semiconductor NbOI 2 [5]. Using THz emission spectroscopy, we observe a THz generation efficiency that is an order of magnitude higher than that of ZnTe. We uncover the underlying generation mechanisms tied to its substantial ferroelectric polarization by investigating the dependence of THz emission on excitation wavelength, incident polarization and fluence. Leveraging the long-lived coherent ferron-mediated THz emission, we further demonstrate the ultrafast coherent amplification and annihilation of the THz emission and associated coherent ferron oscillations by using an ultrafast double-pump scheme.

References:

[1] Q. Ma et al., Nature 565, 337 (2019).
[2] K. Kang et al., Nature Materials 18, 324 (2019).
[3] J. Xiao et al., Nature Physics 16, 1028 (2020).
[4] T. Xi et al., Nature Electronics 8, 578 (2025).
[5] S. Subedi et al., Advanced Optical Materials 13, 2403471 (2025).

Short Bio: 
Dr. Xiao is an assistant professor in the Department of Materials Science and Engineering at the University of Wisconsin-Madison from August 2021. Prior to joining Madison, Dr. Jun Xiao worked as a postdoctoral scholar with Prof. Aaron Lindenberg and Prof. Tony Heinz at Stanford University and SLAC National Accelerator Laboratory. He earned his Ph.D. in applied science and technology from UC Berkeley (2018) under Prof. Xiang Zhang’s supervision. He received his bachelor’s degree in physics from Nanjing University (2012). His research experience and interests focus on structure-property relationships and light-matter interactions in 2D quantum materials for robust quantum computing, efficient energy conservation and high-performance THz optoelectronics. His findings are published in many high-impact journals including Nature, Science, Nature Physics, Nature Nanotechnology, Nature Electronics and Physical Review Letters. He is the recipient of the 2023 NSF CAREER Award.