New microscope detects and controls how fast molecules emit light

How quickly is a photon emitted? Researchers at AMOLF, the FOM institute, have built a new microscope that can measure and even control how fast a molecule emits a photon (a particle of light) depending on where the molecule is placed.

Their microscope moves a light source through very small optical structures to within a few nanometres of accuracy. By placing the light source at specific locations in the structures, the researchers influence how quickly the light is emitted. This is an important breakthrough for many applications, from small optical sensors and optical computer chips to quantum information technology. The results appear ... in the prestigious journal, Physical Review Letters.

How is a photon emitted?
Light sources are enormously important in our lives. Their use ranges from lighting up our homes every day to high-tech applications too, such as high-speed data transfer through optical fibres.
Researchers would therefore like to understand exactly how light is produced and how they can influence it.  A molecule that is ‘loaded’ with a package of energy can emit this energy in the form of a light particle after a certain amount of time. How long it takes before the molecule emits the light – its ‘lifetime’ – depends not only on the molecule, but also on its environment. So by changing the environment, researchers can determine when the light is emitted. To do this, they need to design ultra-small structures, smaller than 1000 nanometers, or 1% of the thickness of a hair.
 
Map
Such an ultra-small photonic structure is only of any practical use  if the light source can be placed in exactly the right location. FOM researcher Martin Frimmer has constructed a new experimental set-up in which light sources can be moved very accurately to within a few nanometers. The researchers glue a tiny light source onto the end of a very sharp needle. They move the needle through the photonic environment and measure the lifetime at each position.  In this way, the researchers create a ‘map’ which indicates exactly where the photonic structure accelerates or slows emission of a photon. This map subsequently allows the researchers to place light-emitting molecules exactly where the properties are best.

Metal wire
The researchers have used the technique to demonstrate that light sources emit their photon at least twice as fast when held near the tip of a small metal wire. They also show that if the light source is located directly next to the wire, most of the photons emitted are captured and guided by the wire. This is very similar to the fibre-optic video stream leading into your living room, but 1000 times smaller! In future, photonic structures may be used to make LEDs more efficient and to make light sources for fast and secure data communication.

Reference
Scanning emitter lifetime imaging microscopy for spontaneous emission control
Martin Frimmer, Yuntian Chen, and A. Femius Koenderink
Physical Review Letters 107 (2011) 123602. DOI: 10.1103/PhysRevLett.107.123602

For further information please contact:
Martin Frimmer, email: m.frimmerATamolf.nl
Dr. Femius Koenderink,  phone 020 754 7100, email: f.koenderinkATamolf.nl

Figure 1: The first challenge is to glue a nano-sized light source to the tip of a very sharp needle. Then the researchers move the light source over the photonic structure – in this case, a very thin silver wire – to create a map that shows how fast the light source emits light depending on its position.

Figure 2: (a) Camera image of the wire with the light source, taken at the moment when the source is located directly next to the wire. Not only the source itself lights up (the light point in the middle of the camera image) but also the ends of the wire. This observation shows that the wire captures and guides the emitted photons. (b) Lifetime measurements show that the average time it takes before the light source has emitted its photon is much shorter (steep curve) when the source is next to the wire than when the source is far away from the wire.