Physicists Directly Observe Light Trapped in a Room-Temperature Moire Grid

Researchers at the University of Twente in the Netherlands, working with collaborators in Utrecht, Brazil and Japan, have directly observed how a nanoscale "moire" pattern traps packets of light energy at room temperature, a step toward ultrasmall light sources and optical sensors. The findings were published in Nature Physics.

A moire pattern is the wavy interference effect that appears when two fine, repeating grids overlap and are slightly misaligned, familiar from photographs of computer screens. The team created one deliberately by stacking two sheets of molybdenum disulfide, a semiconductor crystal just three atoms thick, and rotating the top sheet by 2 degrees. The resulting pattern repeats every 9 nanometers, a spacing fine enough that 10,000 repeats fit across the width of a single human hair.

When light strikes a semiconductor, its energy can be absorbed to create particle pairs called excitons, which largely determine how the material absorbs and emits light. Scientists had long suspected that a moire pattern could trap excitons at fixed positions, offering a way to control light in a programmable grid, but nobody had directly observed this before: optical measurements average over thousands of repeats, and sharper microscopy methods worked only at extremely low temperatures.

First author Laurens Westenberg scanned the stacked layers with an atomically sharp needle while tuned light illuminated them from below, detecting the tiny electric current generated at each point to map where light energy was absorbed with nanometer precision. The maps showed that different kinds of excitons settle in different spots: one group gathers at the crossing points of the moire lattice, while another settles in the regions between, each confined to an area roughly 2 nanometers across.