Role of Nonequilibrium Populations in Dark-Exciton Formation

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Abstract
The optical excitation of a bright exciton may be followed by the formation of lower-energy dark states. In these formation and relaxation processes, nonequilibrium exciton and phonon populations play a dominant role but remain so far largely unexplored, as most states are inaccessible by regular spectroscopies. Here, on the example of homobilayer 2 H − MoS 2 , we realize direct access to the full exciton relaxation cascade from experiment and theory. We find distinct changes in the time-, energy-, and in-plane momentum-resolved photoemission spectral function that can be explained only when considering the formation and subsequent thermalization of excitonic nonequilibrium occupation distributions. In agreement with microscopic many-particle calculations, we quantify the timescales for the formation of a nonequilibrium dark-excitonic occupation and its subsequent thermalization to 85 and 150 fs, respectively. Our results provide a previously inaccessible view of the complete exciton relaxation cascade, which is of importance for the future characterization of nonequilibrium excitonic phases and the efficient design of optoelectronic devices.
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Funding Information
  • Deutsche Forschungsgemeinschaft (432680300/SFB 1456)
  • Deutsche Forschungsgemeinschaft (B01)
  • Deutsche Forschungsgemeinschaft (217133147/SFB 1073)
  • Deutsche Forschungsgemeinschaft (B07)
  • Deutsche Forschungsgemeinschaft (B10)
  • Deutsche Forschungsgemeinschaft (223848855/SFB 1083)
  • Deutsche Forschungsgemeinschaft (B9)
  • Deutsche Forschungsgemeinschaft (SPP 2244)
  • Deutsche Forschungsgemeinschaft (535247173)
  • Deutsche Forschungsgemeinschaft (542873258)
  • Deutsche Forschungsgemeinschaft (571553489)
  • Engineering and Physical Sciences Research Council (EP/T001038/1, EP/P005152/1)
  • Japan Society for the Promotion of Science (21H05233)
  • Japan Society for the Promotion of Science (23H02052)
  • Core Research for Evolutional Science and Technology (JPMJCR24A5)
  • Japan Science and Technology Agency
  • Ministry of Education, Culture, Sports, Science and Technology
  • World Premier International Research Center Initiative
  • LOEWE-Exploration

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