Many unique properties in two-dimensional (2D) materials and their heterostructures rely on charge excitation, scattering, transfer and relaxation dynamics across different points in the momentum space. Understanding of these dynamics is crucial in both fundamental study of the 2D physics and their incorporation in optoelectronic and quantum devices. A direct method to probe charge carrier dynamics with momentum resolution is time- and angle- resolved photoemission spectroscopy (TR-ARPES). Such measurements have been challenging, since photoexcited carriers in many 2D monolayers reside at high crystal momenta, requiring probe photon energies in the extreme UV (EUV) regime. These challenges have been recently addressed by developments of table-top pulsed EUV sources based on high harmonic generation, and the successful integration into TR-ARPES and/or time-resolved momentum microscope. Such experiments will allow direct imaging of photoelectrons with superior time, energy, and crystal momentum resolution, with unique advantage over traditional optical measurements. Recently, TR-ARPES experiments of 2D transition metal dichalcogenide (TMDC) monolayers and bilayers has created unprecedented opportunities to reveal many intrinsic dynamics of 2D materials, such as bandgap renormalization, charge carrier scattering, relaxation, and wavefunction localization in moiré patterns. This perspective aims to give a short review on recent discoveries, and discuss the challenges and opportunities of such techniques in the future.

This article is Open Access

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