Two ultra-thin sheets, each just one atom thick, are placed on top of each other and slightly rotated. Like two sheets of colored tracing paper: while they lie flat, the picture is one thing, but give them a twist and a moiré pattern appears. Here, though, the pattern isn't visible to the eye, but it completely changes how the material handles light.
In the experiment, molybdenum disulfide and tungsten diselenide, two semiconductors familiar from electronics, were stacked and illuminated with an invisible infrared beam at a wavelength of 1260 nanometers. In response, the layers emitted a green glow. Here's how it works: two infrared photons merge into one visible photon — a phenomenon called second-harmonic generation. But the key point is that the polarization of this green light turned out to be left- or right-handed depending on how the layers were rotated. At a twist angle of about 30 degrees, the difference between left and right polarization reached 1.96—nearly the theoretical limit.
Why does this happen? The light wave from each layer arrives with its own delay, and these two waves add up. The delay depends on the direction of polarization rotation: it's like two translators repeating the same phrase, but one is slightly behind. Depending on who lags, the meaning of the final phrase flips to the opposite. In physics, this phase shift is called the Pancharatnam–Berry phase, and it's dictated solely by the geometry of the twist.
The practical value is that by rotating an atomic layer, you can control the material's nonlinear response: that is, how it converts light frequency. This promises tiny converters for optical chips, where color and polarization are governed by geometry rather than bulky crystals.
The effect is so sensitive to rotation that changing the angle by just one degree can reverse the polarization, as if a slight turn of the steering wheel made the car go in reverse.
In Peter Watts's novel "Echopraxia," materials are mentioned whose properties depend on the relative orientation of layers — similar to arbitrarily switching optical properties by rotation.