Nanowire captures light
Researchers of the Utrecht University and the Institute for Atomic and Molecular Physics (AMOLF) in Amsterdam have been able to capture light in extremely thin zinc oxide wires. Usually, light-waves do not fit in rooms smaller than their own wavelengths, but by combining the light-waves with electrons in the material the researchers took care of the light not leaking out of the wires. The combined light-electrons are so tight that even at a room temperature they still capture the light in the material. This research opens up perspectives for applications, such as in small optical circuits or on more economical lasers and sensors for labs-on-a-chip. This is the first time that the mechanism behind the light conductors has been elucidated by way of zinc oxide nanowires. The researchers will have their results published on 6 October 2006 in the renowned scientific journal Physical Review Letters.
Components that are as big as or as small as the wavelength of the light that is used, become a large problem when used in smaller optical devices. Visible light has a wavelength between 400 and 700 nanometres (one nanometre is one thousand millionth of a metre); parts of modern computer chips are sized only 90 nanometres. Light only fits into a part that is larger than its wavelength and bursts into narrow structures: this is called the diffraction limit.
Leakage of light causes loss of signal and unwanted interaction between intermediate parts. Therefore, many scientists are investigating how they can capture light in devices that are smaller than the wavelength that is used.
The diffraction limit can be circumvented by coupling light-waves to electrons in the material. Thus, polaritons come into being: compound parts that are stuck to the material like electrons are, but that are moving just as easy as light-waves. Popular polaritons are the plasmon polaritons that attaches the light to electron waves on the surface of metals. When enclosed in polaritons, it is harder for the light to escape the material and it can even be diverted around nooks. The research team at Utrecht University and AMOLF, Amsterdam, now proves that free electrons in nanowires of semiconductor material are combining light tightly. Similar semiconductor wires are also useful as a part of sources of light at a nanoscale, such as nano-LEDs and nanolasers. Combining light with purely metal structures is not successful.
Enclosure elucidated
Scientists have been knowing for a long time that light in nanowires of zinc oxide moves across long distances and through sharp bends, even if the wire is up to four times as thin as the wavelength of the light. 'We have been showing how zinc oxide nanowires are able to do so', reports FOM-PhD student Bert van Vugt, Utrecht. The research team discovered traces of exciton polaritons in the zinc oxide. Excitons are pairs of electrons and holes in a sea of electrons. Such pairs are able to absorb incoming light and to hold it for a while, before they transmit it again.
Van Vugt and colleagues have been using new methods in order to prove that there are exciton polaritons in zinc oxide. Conventional ways of measuring the combined action of light and matter are to transmit light into a material and then to investigate the reflection, absorption or transmission with spectroscopy. Practically, this cannot be carried out with a wire the size of hundreds of nanometres thick and up to one hundred micrometre long. Instead, the researchers moved a laser or electron beam over a nanowire and they investigated the quantity and colour of the radiated light.
The scientists discovered two areas of energy, in which the wire absorbed energy more efficiently at the far ends than in the centre. 'This experiment just shows at what place the reaction of the entire wire is the larger', says Van Vugt. 'At the moment we cannot detect where the light leaves the wire, but with other experiments we are able to investigate this, especially at the far ends.' So, wires of zinc oxide are able to enclose and carry light at their entire length.
World record
Van Vugt: 'The combining with a light wave and zinc oxide is so strong that they remain intact at a room temperture. We have given the wires energy with lasers and even up to a 300 degrees Celsius they kept combining with the light. This is really a world record, for in former research with gallium arsenide as the material used, the polaritons disintegrated over -150 degrees Celsius, which will break off the enclosure!'
The research puts the light-conducting by nanowires in a new light and therefore, it is important for the development of new optical chips. Furthermore, it will be possible to develop subtle gas sensors, labs-on-a-chip and perhaps, even a new kind of laser. 'It is told that we are able to build a more efficient laser of materials that have a strong combining with light wave and exciton', Van Vugt says. The light waves are coherent in a laser beam: they stay in line. 'On the right conditons it is possible to have the exciton polaritons staying in line and the transmitted light will also be coherent. This applies for just one pair of exciton polaritons. The conventional population-inversion laser firstly needs to be supplied with thousands of millions of atom energy before they can produce laser light that is included in the same circuit.'
The film (avi-format) shows a nanowire that is absorbing energy. We see where the various wavelengths of the transmitted light can be started at the wire. The wire can best be started at the far ends in order to be able to transmit ultraviolet light and to hold it, while the wire for the green light can be started anywhere just as efficiently.
The article is entitled 'Exciton Polaritons Confined in a ZnO Nanowire Cacity' by L.K. van Vugt, S. Rühle, P. Ravindran, H.C. Gerritsen, L. Kuipers and D. Vanmaekelbergh. It will be published in Physical Review Letters 97 number 14 on 6 October 2006.
For more information, please contact PhD student Bert van Vugt, Utrecht University, phone: (030) 253 22 14 and Professor dr. Daniël Vanmaekelbergh, Utrecht University, phone (030) 253 22 18.