Robert Moerland is a researcher in nano technology at the Delft University of Technology in the Netherlands. When his and his coworker’s research, performed at Aalto University in Finland, was published in Nano Letters, the top publication in the field of nano techonology, he created his cover image in Blender. Robert starts out with a physics lesson in order to explain to us what this image is actually demonstrating. Good stuff!
Some basics about coupled oscillators
Nanoscale light fields bound to metal surfaces can interact with molecules and, if the interaction is strong, form a new object that is a mixture of light and matter: the new object inherits properties of both. In order to get a feeling for this interaction, we look at an analog system in the macroscopic world: a mass connected to a spring, or a mass-spring system.
Mass-spring systems are oscillators – they go back and forth once you give them a push and in an ideal world, they would continue forever. The frequency with which they oscillate then only depends on the stiffness of the spring (the higher the stiffness, the faster the system oscillates) and on the amount of mass (the more mass there is, the slower the system oscillates). The basic way a simple mass-spring system with two springs oscillates is shown in the video below:
Things get more interesting when two masses are coupled with springs to the ‘fixed earth’ and to each other. At that moment, the frequency with which the single mass was moving disappears, and the coupled systems oscillate with two new frequencies, one lower and one higher than the original. The two ways this coupled system can move is shown in the videos below.
However, the truly interesting stuff happens when you let the masses oscillate at both frequencies at the same time: then, the energy that is stored in the movement of one mass gradually gets transferred to the other mass, and then back again. This is shown in the video:
How is this principle demonstrated in this picture?
Now back to the cover picture: it turns out that molecules can be thought of as very small (nanoscale) mass-spring systems, where it is the electrons in the molecule that are oscillating when driven by light. But, also very tiny islands of metal (here: silver) can viewed in the same way. Also there the electrons are oscillating when driven by light. By tuning the size of the metal islands, the oscillation frequency of these nanoscale oscillators can be made equal to the oscillation frequency of the molecules.
Just like their macroscopic analog, energy can be exchanged from the molecule to the metal island if they are coupled strongly enough (that is, if there is a ‘stiff enough spring between them’). And just like their macroscopic analog, the islands and molecules that are coupled strongly do not oscillate at their original frequency anymore.
The most interesting part however occurs when the islands are put in a regular two-dimensional array. Because the islands themselves also are coupled with each and every other island in the array, a third oscillator emerges: a so-called surface lattice resonance. The strangest property of this oscillator is that you cannot pinpoint where this oscillator exactly is, since it is the whole array at all times. Because of this delocalized third oscillator, energy can hop from molecule A to a certain metal island, get transferred to a remote metal island that is far away (on the scale of a molecule) and finally end up at molecule B. The coherence of the oscillation, a property important in for example lasers, is preserved during this process.
The cover image gives an artistic impression of the transfer of energy – via a path of metal islands - from a remote molecule at the back of the array to a molecule at the front, close to the viewer. The combination of the properties of both light and matter in one object, together with the extremely intense light fields near the nanoparticles, may lead to, for instance, more efficient lasers requiring much lower powers and also working on the nanoscale.
Image (c) 2014 American Chemical Society