Have you ever tried to take a picture of a computer screen? You may have noticed strange patterns that appear, disappear, and dance around in the image as you move the camera relative to the screen. The effect is what is known as a moiré pattern, which occurs when two similar, but slightly different patterns are spatially overlapped. In this case, the mismatch in alignment between the grid of pixels in the camera and that of the computer screen can cause weird artefacts in the image. While moiré effects are often a nuisance for photographers, scientists are now are purposefully designing moiré patterns out of two-dimensional (2D) crystals to discover fascinating new phenomena. Just last year, for example, researchers observed the surprising emergence of superconductivity in the moiré pattern of “magic-angle graphene”. The quest is now on to discover other physical phenomena that arise from moiré physics. In our paper, published in Nature, we experimentally show that a moiré pattern formed by stacking two different 2D semiconductors can dramatically impact the light-matter interactions.
What does this mean? Consider the two honeycomb crystal structures as shown above. If you twist one of them a tiny bit, and then overlay them, a hexagonal moiré pattern appears (see figure above). In our experiments, we stacked together two different monolayer semiconductors, MoSe2 and WSe2, in such a fashion. After exciting the system with a laser, negatively charged electrons and positively charge holes are created, which then pair up via electrostatic attraction to form interlayer excitons (a hydrogen-like bound state between electrons in MoSe2 and holes from WSe2). It turns out that the moiré pattern in this system does more than just look pretty, it also drastically modulates the potential energy landscape. In contrast to the excitons in an individual monolayer semiconductor that are free to move about the crystal (see figure below, left), the excitons in the moiré pattern live in very hilly terrain (see figure below, right). Shortly after their creation, the excitons relax to the bottom of this landscape, where they are trapped like eggs in an egg carton until the electron and hole recombine to emit light. We detected photoluminescence from these moiré interlayer excitons and showed that their optical signatures, such as narrow emission energy widths, strong circular polarization properties, and behavior in a magnetic field as a function of moiré wavelength (or twist angle between the layers), are consistent with them being trapped in a moiré potential.
Why are these results significant? Broadly speaking, the moiré modulation creates a new optical material that is much greater than the sum of its parts. One may envision using excitons trapped in a moiré superlattice potential to create large-scale arrays of quantum emitters, which could find use as semiconductor lasers or in quantum information processing. Moreover, a tunable lattice of interacting excitons may be a powerful solid-state analogue to ultracold optical lattices of atoms, in which there has recently been tremendous progress in the quantum simulation of many-body physics. It is too early to tell where exactly these results will ultimately lead, but exciting applications and physics are sure to arise.
For our group and collaborators, this work represents an important culmination of effort over the last several years. While we knew that the moiré potential should influence the excitons, we had to deal with the fact that direct experimental evidence is very hard to obtain. The reason is that each moiré trapping site is much smaller than the size of a focused laser beam, so in our experiments, we necessarily excite and detect many hundreds of trapped excitons at a time. This averaging can obscure the underlying moiré behavior, especially if there is strong inhomogeneity in the heterobilayer stack. But fortunately, we persisted in fabricating high-quality structures and carefully studied the system at different temperatures, excitation powers, polarizations, and magnetic fields on multiple samples with designed twist angles. Finally, combined with invaluable theoretical support and predictions from our colleagues Hongyi Yu and Wang Yao at The University of Hong Kong, we felt confident in assigning our observations to moiré-trapped excitons. We look forward to exploring the vast opportunities along with other researchers in this new field of moiré optics.
Link to paper: Signatures of moiré-trapped valley excitons in MoSe2/WSe2 heterobilayers
Written by Kyle Seyler
Cover art by Nathan Wilson