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Left, ultrafast pulses of light excite and probe a tiny sample of WS2 that’s one layer thick, emitting electrons that are collected by a new detector called a momentum microscope. Right, full 3-D energy-kinetic distribution of the emitted electrons. Credit: Stony Brook University

When some semiconductors absorb light, arrows (or pairs of particles from an electron bound to an electron hole) can form. Two-dimensional crystals of tungsten disulfide (WS2) have unique excitation states not found in other materials. However, these states are short-lived and can change from one to another very quickly.

Scientists have developed a new approach to create separate images of these unique quantum states. By observing the individual quantum states, the researchers showed that the coupling mechanisms that lead to the mixing of the states may not fully match existing theories.

Scientists are excited about transition metal dichalcogenides, the family of crystals that include tungsten disulfide, because they absorb light so strongly despite being only a few atoms thick. Scientists could use these crystals to build new nanoscale solar cells or electronic sensors. Using a new technique called time-resolved momentum microscopy, scientists are now able to better observe transitions between different excited quantum states. This technique is broadly applicable, so scientists can now put other next-generation materials and devices under this high-speed microscope to see how they work.

A variety of light-induced excited states can be generated in monolayer transition metal dichalcogenides (TMDs) such as WS2 under different conditions. Changing the wavelength or power of the exciting light or the temperature of the crystal allows different excited states to form or persist. Circularly polarized light, where the direction of the electric field is about the direction of the light wave travel, can selectively create excitation with a specific quantum spin configuration in a specific set of energy fields.

Researchers at Stony Brook University have developed a unique instrument to directly observe this effect under different ultrafast light excitation conditions, disentangling the complex mixture of quantum states that can form.

Published in Physical audit letter, these new results show how the force that binds an electron and an electron hole together in the excitation also contributes to very fast coupling, or mixing, of different excited states. The researchers demonstrated that this effect leads to the mixing of atoms with different spin configurations while still conserving both energy and momentum in the coupling process.

Surprisingly, the results showed that the rate of excitation mixing did not depend on the excitation energy, as researchers had previously predicted. This study provides important experimental support for some current theories of arousal coupling in TMD, but also highlights important discrepancies. Understanding the interplay between these excited states is a key step toward exploiting the potential of TMDs for nanotechnology and quantum sensing.

More information:
Alice Kunin et al., Momentum-Resolved Exciton Coupling and Valley Polarization Dynamics in Monolayer WS2, Physical audit letter (2023). DOI: 10.1103/PhysRevLett.130.046202

Diary information:
Physical audit letter

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