Beams of light inherently produce a mechanical pressure. The interplay between incoming light and the vibrations it provokes (for example, on a mirror) is exploited by cutting-edge technological systems, from massive gravitational-wave detectors to tiny microdevices that are sensitive to the smallest quantum-mechanical motion. This work shows that the same behavior underpins how bonds in molecules interact with photons, and it reveals how molecules under short pulses of light can shake themselves apart.

We create the smallest optical cavity in the world, a millionfold smaller than previously used in any optomechanics experiment, by supporting a gold nanoparticle just a few atomic diameters above a metal surface. In this configuration, we can trap light in the subnanometric volume between the particle and the substrate, where the particle acts as a sort of nanometer-scale lens, which allows us to enhance and directly observe interactions between the light and molecules. The appropriate color of light causes molecules placed within these gaps to vibrate. As the intensity of incoming light increases, we show how molecular vibrations increase rapidly above a critical threshold, almost reaching the vibratory equivalent of a laser, or “phaser.” We develop a quantum theory to describe this effect, which suggests how a hundred molecules vibrate together. However, we then also observe breaking of molecular bonds in real time.

Although this irreversible breaking might ultimately limit how molecules can be used as quantum-mechanical oscillators, our results suggest that novel chemical reactions might be accessed via such tightly confined light.