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PHYSICS
Taking a good look at transparent objects
Researchers from Japan’s National Institute for Materials Science (NIMS) have succeeded in confining a multi-electron quantum system into a three-dimensional nano-space. The result, achieved by precisely engineering the materials used, has not been accomplished before and paves the way for harnessing the properties of the confined quantum system in technological applications. In semiconducting materials when an electron is excited from its place in an atom the hole that is left can be thought of as carrying a positive charge. A pair of an electron and hole can form a quasi-particle called an “exciton”, analogous to a hydrogen atom. Just as an extra proton or electron can be added to a hydrogen atom to produce a hydrogen ion, an extra electron or hole can be added to an exciton to produce a complex state of three particles, called a charged exciton. Unlike hydrogen, however, it is possible to confine electrons and holes in quantum dots – three dimensional spaces in the order of only a few nanometres. If charged excitons could be successfully confined in quantum dots, the energy released when an electron returns to a hole, the “stabilisation energy”, would be expected to increase. In order to achieve the confinement of charged excitons, the researchers fabricated quantum dots of gallium arsenide embedded in a host material of aluminium gallium arsenide. By using an original method developed at NIMS, it was possible to produce an unprecedented clean quantum structure. The scientists then observed that charged excitons had been successfully confined by measuring emissions of energy from single quantum dots, which were much greater than emissions from unconfined particles. This result elucidates for the first time the effect of confining a multi-electron quantum system in a nanospace. In terms of technological applications the control gained from using such engineered nano-structures could lead to the development of low-power semiconductor devices.
A new technique for distinguishing transparent objects without the standard practice of using a reference beam has been developed by researchers at the University of the Philippines Diliman. The method is both simpler and more accurate than previous procedures for differentiating between similar objects. “Phase-only” objects do not affect the intensity of light passing through them and only shift the positions of light waves. Objects such as biological specimens and optical lenses are phase-only, and by measuring the phase-shifts of light transmitted through them a map of the refractive index and thickness profile of an object can be obtained. Conventional methods of mapping phase-only objects require the use of a reference beam to make up for errors arising from the interaction of light with atoms, but the reference beam could also introduce additional errors into the phase maps. Unlike conventional methods where the light is all the same wavelength, the new method uses diffused light – light with mixed wavelengths – to produce phase maps of different objects. The correlation of the phase maps produced is then used to give a quantitative measure of the similarity of the spatial features between the objects. By varying the polarisation of the illumination beam, which is the degree to which all the light waves are travelling in the same plane, the polarisation properties of the test objects can also be found. The use of polarisation information and the reconstruction without a reference beam mean the new method enables high discrimination capability, yet with considerably reduced optical requirements.
For further information contact: Dr Takashi Kuroda Quantum Dot Research Center National Institute for Materials Science, Japan Email: kuroda.takashi@nims.go.jp
SPENCER DIAMOND
NATIONAL INSTITUTE FOR MATERIALS SCIENCE
Packing electrons in a nano box
A phase contrast image of a cheek cell
For further information contact: Dr Percival F. Almoro National Institute of Physics University of the Philippines Diliman Email: pfalmoro@gmail.com