Scientists Reveal Hidden Quantum States That Form in Millionths of a Second
Researchers have directly observed how light-excited particles in ultrathin semiconductors transform into dark, invisible quantum states—a process lasting just 85 to 150 femtoseconds. The finding could improve design of next-generation optoelectronic devices, from solar cells to quantum computers, by clarifying how to harness these fleeting states before they disappear.
Originaltitel: Role of Nonequilibrium Populations in Dark-Exciton Formation
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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mn>2</a:mn> <a:mi mathvariant="normal">H</a:mi> <a:mtext>−</a:mtext> <a:msub> <a:mrow> <a:mi>MoS</a:mi> </a:mrow> <a:mrow> <a:mn>2</a:mn> </a:mrow> </a:msub> </a:mrow> </a:math> , 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.