Observation of nonequilibrium intrinsic phonon relaxation by ultrafast infrared microspectroscopy

Yang Luo; Heyuan Liu; Kun Yang; Shu Zhang; Zicheng Zhou; Jiajie Qi; Chenchen Wu; Xiangdong Guo; Kaihui Liu; Xiaoxia Yang; Hailong Chen; Qing Dai

doi:10.1103/lgbl-xcng

Observation of nonequilibrium intrinsic phonon relaxation by ultrafast infrared microspectroscopy

Yang Luo^1,2,*, Heyuan Liu ^3,4,*, Kun Yang ⁵, Shu Zhang^1,2, Zicheng Zhou¹, Jiajie Qi ⁶, Chenchen Wu^1,2, Xiangdong Guo⁷, Kaihui Liu ⁶ et al.

Xiaoxia Yang^1,2, Hailong Chen^3,4,†, and Qing Dai^1,2,7,‡

¹CAS Key Laboratory of Nanophotonic Materials and Devices, CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China
²Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
³The Laboratory of Soft Matter Physics, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
⁴School of Physical Science, University of the Chinese Academy of Sciences, Beijing 100049, China
⁵CAS Laboratory of Theoretical and Computational Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China
⁶State Key Lab for Mesoscopic Physics, Frontiers Science Centre for Nano-optoelectronics, School of Physics, Peking University, Beijing 100871, China
⁷School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

^*These authors contributed equally to this work
^†Contact author: hlchen@iphy.ac.cn
^‡Contact author: daiq@nanoctr.cn

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Phys. Rev. B 112, 125424 – Published 26 September, 2025

DOI: https://doi.org/10.1103/lgbl-xcng

Abstract

While free/excited electron-phonon coupling has been extensively investigated, crucial physical processes—such as ultrafast thermal transport and many phase transitions—are mainly governed by nonequilibrium intrinsic phonons, which remain challenging to observe. Here, the relaxation dynamics of nonequilibrium intrinsic phonons in a high-quality hBN flake is studied based on the advanced femtosecond mid-infrared (MIR) transient absorption micro-spectroscopy. We observe a near-unity degree of polarization in the transient absorption after the resonant phonon excitation. Nonequilibrium intrinsic phonons exhibit nonmonotonic relaxation dynamics. Based on time-dependent density functional theory simulations, this behavior is attributed to the cascaded anharmonic interactions that redistribute energy and momentum among phonons and bound electrons. Our work sheds new light on the ultrafast dynamics of collective excitations, which provides critical insights into atomic-scale ultrafast thermal transport and offers a novel strategy for active ultrafast mid-infrared light modulation.

Physical Review B

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Observation of nonequilibrium intrinsic phonon relaxation by ultrafast infrared microspectroscopy

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Observation of nonequilibrium intrinsic phonon relaxation by ultrafast infrared microspectroscopy

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