Imaging quantum states directly in two-dimensional materials

(Nanowerk News) When some semiconductors absorb light, excitons (or particle pairs made of electrons bonded to electron holes) can form. Two-dimensional crystals of tungsten disulfide (WS2) has a unique exciton state 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 creating discrete images of these individual quantum states. By tracking the individual quantum states, the researchers demonstrated that the coupling mechanism that leads to the mixing of states may not fully fit current theory.

This research was published in Physical Review Letter (“Momentum Resolved Exciton Coupling and Valley Polarization Dynamics in Monolayer WS2“). Left, an ultrafast pulse of light excites and probes a small sample of WS2 one layer of atoms thick, emitting electrons collected by a new detector called a momentum microscope. That’s right, the full 3-D energy momentum distribution of the emitted electrons. (Image: Stony Brook University)

Scientists are particularly interested in transition metal dichalcogenides, the crystalline family that includes tungsten disulfide, because they absorb light so strongly even though they are only a few atoms thick. Researchers can use these crystals to build new nanoscale solar cells or electronic sensors. Using a new technique called time-resolved momentum microscopy, the researchers can now better track the transitions between different exciton quantum states. The technique is so widely applicable, that scientists can now place materials and other next-generation devices under this momentum microscope to see how they work.

Multiple light-induced excitability states can form in monolayer transition metal (TMD) dichalcogenides such as WS2 under different conditions. Varying the wavelength or the strength of the attractive light or the temperature of the crystal allows different excitation states to form or persist.

Circularly polarized light, in which the direction of the electric field rotates around the direction of travel of the light wave, can selectively create stimuli with a certain configuration of quantum spins within a certain set of energy bands. Researchers at Stony Brook University have developed a unique instrument to directly visualize this effect under different conditions of ultrafast light excitation and unravel the complex mixtures of quantum states that can form.

These new findings show how the forces that bind electrons and electron holes together in the exciton also contribute to the very fast coupling, or mixing, of the different exciton states. The researchers demonstrated that this effect leads to mixing of stimuli with different spin configurations while still conserving energy and momentum in the coupling process.

Surprisingly, the results showed that the mixing rate of the stimuli was not dependent on the energy of the stimuli as previously predicted by the researchers. This study provides important experimental support for some of the current theories about exciton coupling in TMD, but also highlights important differences. Understanding the interactions between these excitation states is a key step towards harnessing the potential of TMDs for nanotechnology and quantum sensing.