Scientists at the Vienna University of Technology (TU Wien) have uncovered the source of a special type of quantum light created at certain points in the 2D material tungsten diselenide (WSe2) when it is supplied with energy. They found that this special-nature light, which exhibits an antibunching effect, results from the interaction between single atomic defects in the material and mechanical strain. Computer simulations showed how electrons were driven to specific places in the WSe2, where they were captured by a defect, lost energy, and emitted a photon. WSe2 is an atomically thin material, where tungsten atoms are in the middle, coupled to selenium atoms below and above. If energy is supplied to a WSe2 layer, for example by applying an electrical voltage or by irradiating it with light of a suitable wavelength, it begins to shine. When the researchers analyzed the light emitted by tungsten diselenide in detail, in addition to ordinary light they detected a special type of light with unusual properties. This quantum light consisted of photons of specific wavelengths that were always emitted individually. No two photons of the same wavelength were detected at the same time. “This tells us that these photons cannot be produced randomly in the material, but that there must be certain points in the tungsten diselenide sample that produce many of these photons, one after the other,” professor Florian Libisch said. Local distortions in the surface push electrons close to defects. Only the combination of defects and strain can explain the new kind of quantum light. Courtesy of TU Wien. If an electron in WSe2 changes from a high-energy state to a state of lower energy, a photon is emitted. However, this jump to a lower energy is not always allowed The electron has to adhere to the laws of conservation of momentum and angular momentum that state that an electron in a high-energy quantum state must remain there unless certain imperfections in the material allow the energy states to change. “A tungsten diselenide layer is never perfect. In some places one or more selenium atoms may be missing,” researcher Lukas Linhart said. “This also changes the energy of the electron states in this region.” Moreover, the WSe2 layer is not a perfect plane. Like a blanket that wrinkles when spread over a pillow, WSe2 stretches locally when the material layer is suspended on small support structures. These mechanical stresses also have an effect on the electronic energy states. The researchers performed a multiscale tight-binding simulation for the optical spectra of WSe2 under nonuniform strain and in the presence of point defects. They found that strain locally shifted excitonic energy levels into the bandgap where energy levels overlapped with localized intragap defect states. The resulting hybridization allowed for efficient filling and subsequent radiative decay of the defect states. The team identified intervalley defect excitonic states as the likely candidate for antibunched single-photon emission. At regions of the material where defects and surface strains appeared together, the energy levels of the electrons changed from a high to a low energy state and emitted a photon. The electrons must undergo this process one by one (the laws of quantum physics do not allow two electrons to be in exactly the same state at the same time). This leads to the photons being emitted one by one, as well. Lukas Linhart (l) and Florian Libisch (r). Courtesy of TU Wien. At the same time, the mechanical distortion of the material helps to accumulate a large number of electrons in the vicinity of the defect, so that another electron is readily available to step in after the last one has changed its state and emitted a photon. “The interaction of material defects and local strains is complicated,” Linhart said. “However, we have now succeeded in simulating both effects on a computer, and it turns out that only the combination of these effects can explain the strange light effects.” Single-photon emitters play a key role in present and emerging quantum technologies. The research at TU Wien shows that the ultrathin 2D material WSe2 could be a candidate for a reliable single-photon source. The research was published in Physical Review Letters (https://doi.org/10.1103/PhysRevLett.123.146401).