A sheet of paper weighs almost nothing, but stack a hundred sheets and you get a noticeable pile. That's roughly what scientists did with a material that excels at converting one color of light into another, but only when it's just one atom thick. A single layer is like a single sheet: it works beautifully, but it's too thin to produce much light.
Researchers took molybdenum disulfide — a substance that, as a monolayer, is famous for its strong nonlinear response. This means it can take two photons of invisible infrared light and combine them into one visible photon. But a single atomic layer absorbs and emits too little. So they stacked the layers, inserting organic molecules between them to keep the layers from interfering electrically, while their optical outputs added up. The result was a film just 100 nanometers thick — a thousand times thinner than a human hair.
The stack works like a choir: each layer sings its note, and the voices combine into a loud chord. Brightness grows almost as the square of the number of layers. This allowed them to convert infrared light with a wavelength of about 3000 nanometers into visible green or blue light. Such a converter can be applied to any chip or lens — yielding a compact device for identifying molecules by their infrared "fingerprints."
The film is 100 nanometers thick. If scaled up to the thickness of a sheet of paper, the stack would contain about a million layers of molybdenum disulfide.