Nanowire really is the smallest laser in the world

 
Semiconductor nanowires are extremely small sticks. They can emit light under the influence of electricity or light. In that they resemble larger lasers. However, for a long time it was not clear whether they were real lasers. The diameter of the wires made by the researchers is smaller than the wavelength of light, which they anticipated on the fact that light behaves in a different way as in a larger lasers. FOM Ph D student Bert van Vugt and postdoc Sven Rühle, both active in the Condensed Matter & Interfaces Group of the Debye Institute, Utrecht University, supervised by professor D.A.M. Vanmaekelbergh, have demonstrated that the light from the wires is coherent. They really are lasers. The photons stay accurately in line and are exactly of the same colour, just like larger lasers. This coherence is important for implementations that are using lightpulses for transmitting data. As laserlight is highly orderly, more data over a long distance can be transmitted.

Scattered light
It appeared that nanowires do not have the fixed and stabbed rays of light that are so characteristic for larger lasers. The ray of light of the nanolaser scatters immediately after leaving the wire. This is because the wire is so tiny. The light has to worm itself through a cavity that is smaller than its own wavelength. This causes a refraction through which the light is sent in every direction.

The wide conical beam of light of the nanolasers is important for implementations such as pixels in screens that can be viewed from different angles. Scattering can be a problem for other implementations, for example, when the nanolasers have to be linked to other optical elements. However, the researchers think that they will succeed in linking nanolasers with other nanowires. As long as the light moves from wire to wire, it will not escape. In this way nanolasers can be useful in miniaturised optical circuits.

Faster circuits
The nanolasers offer prospects on a faster data processing, such as is necessary on junctions of systems. The electronics that are used, run into the verges of speed and use of energy. A lot of heat is being released, which needs an extensive cooling down. In addition, the use of electronics on the junctions requires time-consuming transformations. Data enters in the form of light through a glass fibre but at first has to be transformed into electric signals before the electronics are able to process them. Optical elements offer prospects on faster circuits that are unable to heat fast and that do not need a detour of electric signals. In the future the most critical components of computer hardware will probably be replaced by optical circuits that have to be operating alongside electronics that we are familiar with.

Exactly the cooperation between optical and electronic circuits offer the nanowires prospects, as they may conduct light as well as electricity. It turns out to be possible to have them provide for laserlight by electric signals, but this is also possible by light signals. The wires have properties that make them ideal as a transistor, the materials of microelectronics. They may also be used as a kind of LED. So, nanowires have properties that simplify the integration of optical and electronic circuits.

A combination of methods demonstrates laserlight
Bert van Vugt created the nanowires by using steaming (900 degrees Celsius) zinc oxide, carbon and gold. If these materials crystallise, an elongated crystal comes into being. That is the wire. The scientists used a combination of methods to demonstrate that the light from the wires actually is laserlight. By using a spectrometer they established that the light of the nanolasers is exactly one-coloured and that it may diversify between 380 nm and 390 nm, a dark violet colour, dependent on the size of the wire.

They also looked at the light with a microscope (figure 2). As soon as the wire starts to emit real laserlight, circular patterns become perceptible (figure 2, on the right) that are reminding of causing a stir, brought about by the laserlight arising from two places, namely from both ends of the nanowire. The rays of light from both places are influenced by each other. This so-called interference causes orderly patterns, because the light particles of laserbeams stay in line with each other so accurately. Ordinary light is much more disorderly, just a patch of light arises (figure 2, on the left).

It appears from comparison with computer simulations that the patterns (in 2E) are in fact caused by a spherical coherent light arising from both ends of the nanowire.

The article is entitled: 'Phase-Correlated NONdirectional Laser Emission from the end facets of a ZnO nanowire'. The authors are Lambert K. van Vugt, Sven Rühle and Daniël Vanmaekelbergh. It is published in Nanoletters vol. 6 issue 12 on 11 November 2006. 

For more information, please contact Drs. Bert van Vugt, Utrecht University, phone +31 (0)30 253 22 14 or

Professor Daniël Vanmaekelbergh, Utrecht University, phone +31 (0)30 253 22 18