Physicists observe light-matter interaction at its limit
An international team of physicists from ETH Zurich in Switzerland, including AMOLF group leader dr. Yves Rezus, has succeeded in observing the interaction of a single particle of light (a photon) with a single molecule. To this end they constructed an optical circuit which allowed them to transmit photons between two single dye molecules separated by a distance of several meters. Besides its value in fundamental physics the experiment has possible applications in quantum communication and optical computing. The results of the research are published today in Physical Review Letters.
The interaction of light and matter is one of the most fundamental processes in nature. This interaction is responsible for the blueness of the sky, the green colour of grass and the fact that we see our reflection in the water. Ultimately these optical phenomena can all be traced to elementary collisions between particles of light and matter; that is to say between photons and molecules. However, at the single-particle level the probability of such a collision between a photon and a molecule is extremely small. As a consequence our understanding of optical phenomena comes largely from experiments that use countless numbers of photons and molecules.
Needle in a haystack
Under normal circumstances the probability that a photon is absorbed by a molecule is extremely small. Physicists quantify this interaction probability by picturing the molecule as a small opaque disk that blocks light. The area of the disk is called the absorption cross section, and it provides a measure of how strongly the molecule absorbs light. Most molecules have a very small absorption cross section, i.e. far smaller than the physical size of the molecule. Only in the case of very strongly absorbing molecules does the absorption cross section approach the physical size of the molecule. However, even in this case one still needs to fire approximately ten million photons at a single molecule in order to observe a single absorption event. Studying the interaction of single photons with a single molecule is therefore like looking for a needle in a haystack.
The weak interaction is due to collisions between the single molecule and the molecules in its surroundings. These collisions perturb the molecule in its interaction with the light. In order to eliminate this disruptive effect the researchers cooled the molecule to one degree above absolute zero (-272º C). At this temperature nearly all molecular motion vanishes. As a result the absorption cross section increases dramatically causing the single molecule to behave as an opaque disk that is a million times larger than the molecule itself. The price, however, that has to be paid for this strong interaction is that the frequency of the photons needs to match the resonance frequency of the molecule with extreme precision: to within one part in a hundred million.
The experiment
In their experiment Rezus and colleagues used a molecule of the dye dibenzanthanthrene (DBATT). This molecule was trapped in a transparent organic crystal. In order to obtain photons that had exactly the right frequency to interact with this molecule the researchers used a second DBATT molecule as a light source. This DBATT molecule was located in a different crystal. The source crystal was illuminated with green laser light, which caused the source molecule to emit a stream of orange photons. These photons were collected in an optical fiber and sent to the target molecule, where a photon detector measured the reflection. A key aspect of the setup was that the researchers could tune the frequency, i.e. the colour, of the photons by applying an electric field to the source molecule. When the frequency of the photons exactly matched the resonant frequency of the DBATT molecule, a decrease in the reflection was observed: the unambiguous proof for the absorption of the photons by the target molecule. This constitutes the first experimental demonstration of the interaction of single photons with a single molecule.
Reference
'Single-photon spectroscopy of a single molecule', Yves Rezus, Samuel Walt, Robert Lettow, Alois Renn, Gert Zumofen, Stephan Götzinger and Vahid Sandoghdar, Physical Review Letters.
Information
Yves Rezus (FOM Institute AMOLF), +31 (0)20 754 72 11 or +31 (0)20 754 71 00.
About the author
Yves Rezus carried out this research as a postdoctoral researcher at ETH Zurich in the Nano-Optics Group of Vahid Sandoghdar. The group has recently moved to the newly founded Max Planck Institute for the Science of Light in Erlangen, Germany. Rezus has now started his own research group in Biomolecular Photonics at the FOM Institute for Atomic and Molecular Physics (AMOLF).