Researchers at TU/e have demonstrated that energy transfer without loss via light or heat can occur over much greater distances than previously thought possible thanks to vibrations in microscopic gold rods. They succeeded in making energy jump from one particle to another over a distance of several millimeters without 'spilling' energy along the way. In the microscopic world in which this research takes place, that is a giant leap, with promising applications in quantum communication, solar energy, and ultrasensitive medical sensors. The researchers have published their findings in the journal Science Advances.
Normally, a molecule that absorbs energy loses it again as heat through vibrations passed on to the surrounding environment or as a particle of light (known as a photon). In Förster resonance energy transfer (or FRET for short, which is named after the German physicist Theodor Förster), something different happens: the energy jumps directly, without radiation, from one molecule to a specific neighboring molecule through an invisible interaction between their electric fields.
In this distinctive form of energy transfer, no energy is lost as heat or light. Instead, all energy arrives exactly where it is needed. The best-known example is photosynthesis in plants. Thanks to FRET, plants can move captured sunlight at lightning speed and without energy loss to the cells where it is converted into food.
Scientists have also learned to use FRET as a precision instrument. As the effect only works over extremely short distances, whether or not it occurs can reveal how close two molecules are to one another. This is useful when studying proteins or detecting disease markers in blood or tissue, for example.
Like jumping from Amsterdam to Brussels
But there is one major limitation to FRET. It only works over extremely short distances of roughly a few nanometers, which is about ten thousand times thinner than a human hair. In their research, Professor Jaime Gómez Rivas , postdoctoral researcher Jie Ji, and graduate researcher Wouter Holman managed to extend this effective range to as much as a few millimeters.
That may not sound like much, but in the world of molecules that's a giant leap, comparable to a person making a 200 kilometer jump from Amsterdam to Brussels.
Moving energy from one side to the other
The team's approach is based on a phenomenon in physics known as bound states in the continuum (BICs). These are electromagnetic waves that, due to a precise cancellation effect, remain completely trapped on a surface and radiate no energy outward. They are present, but invisible to the outside world, and remain intact for an exceptionally long time.
To make use of this, the researchers built a flat surface of microscopically small gold rods on glass, which are arranged in a highly precise pattern. When one of their specially designed measurement probes touched the surface at the correct frequency, it generated a BIC state that transported energy without radiation to a detection probe two millimeters away.
No energy spilled
This transport takes place through resonances (vibrations) in the gold rods. Normally, such resonances emit photons (light particles) into the surrounding environment, making energy transfer inefficient. Using BICs, however, keeps the energy fully confined to the surface, threfore making the energy transfer highly efficient. This just as it would be in FRET, but over a distance that had long been considered impossible.
Another striking aspect is that the transfer is strongly direction-dependent and precisely determined by the orientation of the gold rods: along one direction, the energy travels effortlessly over a full two millimeters, while in the perpendicular direction it fades after only a fraction of that distance. This built-in preference for a single direction could be exploited in future devices to control energy flow, much as how an electrical circuit directs electrical current in one direction.
Excited state
What makes the result particularly interesting is not only the distance, but also that the transfer takes place on a flat surface at room temperature, and all without optical waveguides, optical fibers, or cryogenic cooling.
The information carried by the excited (higher-energy) state remains intact along the way, hopping from one gold rod to the next, showing that coherent transfer can also be achieved in this kind of surface-based platform.
What's next?
The demonstration of this massively enhanced energy transfer using resonant gold rods and the trapping of electromagnetic waves to the surface can find direct applications in ultrasensitive sensors where single (bio-)molecules are detected with unprecedented precision.
In the long term, it should be possible to couple not only two molecules that are separated by a long distance, but also to couple many of them, making 'supermolecules' that behave uniformly (and coherently). These coherent supermolecules could modify how chemical reactions take place, opening a new playground for chemistry. The results of the study also bring promising applications in quantum communication, solar energy, and medical sensors one step closer.
