Ultrafast Electron Dynamics in CrSBr Studied with Time-Resolved Photoemission Electron Microscopy and Pump-Probe Spectroscopy

The study of ultrafast processes in emerging quantum materials is essential for understanding their potential applications in next-generation electronics, optoelectronics, and spintronic devices. Two of the most advanced techniques used to explore these processes are time-resolved photoemission electron microscopy (TR-PEEM) and pump-probe spectroscopy. These methods provide femtosecond temporal resolution and nanometer-scale spatial precision, enabling researchers to probe electron relaxation dynamics with remarkable detail.

A recent study by Prof. Qihuang Gong’s group from Peking University in China, published in The Journal of Physical Chemistry C,¹ employed these techniques to investigate the ultrafast electron dynamics of chromium sulfide bromide (CrSBr), a layered two-dimensional (2D) magnetic semiconductor known for its strong electron-phonon interactions and anisotropic optical properties. Featuring Radiantis femtosecond optical parametric oscillator Inspire in a pump-probe scheme, researchers use the signal output tuned at 600 nm as the pump pulse to excite charge carriers within the material. The probe pulse at 273 nm, obtained by generating the third harmonic of the fundamental beam, ejects the excited electrons, allowing their relaxation pathways to be studied with high temporal resolution. Figure 1 shows (a) the optical setup to obtain the pump and probe beams with the Inspire, while (b) shows an experimental scheme of the TR-PEEM setup.

Figure 1 – (a) Simplified schemes for the pump-probe optical setup and (b) the Time-resolved Photoemission Microscopy (TR-PEEM) used in [1].

The results obtained by the authors revealed critical insights into the electron-phonon interactions governing the relaxation dynamics in CrSBr. First, they observed that electrons excited to the conduction band rapidly decay in the sub-picosecond timescale, which can be identified as an energy redistribution through the electron−electron interaction. This is followed by a second process that lasts a few picoseconds and can be related to electron−phonon scattering, defect trapping, Auger recombination, or other types of relaxation mechanisms in 2D materials (Figure 2.a). One interesting observation is the strong temperature dependence of the electron relaxation lifetime through electron−phonon scattering. The authors observed that at 100 K, the photoexcited electrons exhibited a relaxation time of approximately 11 ps. However, as the temperature increased, this lifetime decreased significantly, reaching about 2 ps at room temperature. This trend indicates that electron-phonon scattering plays a dominant role in the energy dissipation process, with higher temperatures facilitating more efficient phonon-mediated relaxation.

Another key finding by the authors is the polarization independence of the observed relaxation lifetimes. Despite CrSBr’s known anisotropic optical and electronic properties, the decay characteristics of the photoelectrons remained unchanged regardless of the pump laser polarization. This suggests that, while the material’s electronic structure exhibits strong anisotropic features, the excited-state electrons follow a consistent relaxation pathway dictated by the conduction band structure rather than the external excitation conditions. These findings provide crucial information for understanding carrier transport mechanisms in low-dimensional materials and highlight the robustness of the decay channels in CrSBr.

Figure 2 – (a) Electron dynamics after photon excitation at 600 nm in a few layers CrSBr. Scheme for the photoemission spectra of CrSBr as a function of temperature. The electron dynamics follow a double exponential decay function. Adapted from [1].

The combination of TR-PEEM and pump-probe spectroscopy in this study highlights the power of femtosecond laser systems in ultrafast materials research. By adjusting the time delay between the pump and probe pulses, researchers were able to construct a precise timeline of energy dissipation, resolving both fast electron-electron interactions occurring within femtoseconds and slower electron-phonon coupling effects extending over picoseconds. This capability is essential for investigating a broad range of materials, from transition metal dichalcogenides (TMDs) to emerging 2D magnetic semiconductors, where ultrafast processes are critical in defining their functionality.

The ability to select the pump wavelengths with broadly tunable femtosecond lasers ensures that researchers can excite specific electronic states with high efficiency, while the probe pulse, typically in the ultraviolet range, allows for direct access to photoemission responses with femtosecond precision. This level of control is crucial in pump-probe experiments, where the fine manipulation of the excitation and detection parameters enables the experimental observation of rapid energy transfer and carrier relaxation dynamics.

The findings from this study also reinforce the importance of advanced ultrafast techniques in revealing fundamental interactions within quantum materials. As ultrafast spectroscopy continues to evolve, the integration of high-performance femtosecond lasers will remain at the forefront of research, providing deeper insights into non-equilibrium electronic states, energy dissipation mechanisms, and the development of next-generation electronic and photonic devices.

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1. Jiang, P; Li Y.*; Lyu, X.; Xiao, J.; Li, X.; Wang, T.; Tang, J.; Wang, Y.; Zhang, L.; Liu, Y.; Yang, H.; Hu, X.; Ye, Y.; Chen, Z.; Gao Y.*; Wu, C.*; and Gong, Q. Ultrafast Electron Dynamics Dominated by Electron-Phonon Coupling in CrSBr Revealed by Photoemission Electron Microscopy, JPCC,  2024, 128, 51, 21855-21860.