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discovered 02_2012

FOCUS// The HZDR Research Magazine WWW.Hzdr.DE 34 35 position of this energy level depends on the semiconductor material and on the size of the nano-pyramid. So, when dealing with quantum dots, one doesn’t have the usual wide valence and conduction bands, rather sharply delimited sections or energy levels on these bands, thus resembling an artificial atom. “If electrons then jump from a higher energy to a lower energy state, they also give out radiation, just like LEDs emit light, but the colour of the light is much more precisely controllable. The Semiconductor Spectroscopy Division at HZDR, to which I belong, has concentrated more on quantum dots recently. We see great potential in them and we could use their special properties to develop very energy- efficient, quantum-dot-based lasers, and be able to adjust the colour of such lasers precisely,” Stephan Winnerl explains his interest in these nano-pyramids. As all too often, however, there is a snag: While the pyramids form spontaneously during a specific process of crystal growth, their size varies within a certain range. Studying them with infrared light, for example, one obtains blurred signals because electrons in different sized pyramids respond to different infrared energies. This is why the Semiconductor Spectroscopy Division scientists are so keen to obtain a detailed view of the electrons trapped inside a single quantum dot. Using a special method to focus laser light onto a single pyramid, the light donates energy to its electrons, boosting them to an excited state. This excitation is detected by measuring the amount of light that is scattered from the tip. The method of choice for these experiments is called near- field microscopy, and has been perfected for the free-electron laser in Rossendorf in cooperation with Lukas Eng of TU Dresden. The method is now very successful. The researchers in Dresden were recently the first to succeed in determining states only within the conduction band in single quantum dots using infrared light. To do so, they exploited the special advantage of near-field microscopy: Laser light is shone onto a metallic tip less than 100 nanometers thick, by which the light can be strongly collimated to a hundred times smaller than the limiting wavelength of light that “conventional” optics using lenses and mirrors could ever achieve. While this involves major signal losses, the light beam is still strong enough to excite the electrons inside the observed pyramids. This allowed Stephan Winnerl and his colleagues from HZDR, Technische Universität Dresden and the Leibniz Institute for Solid State and Materials Research Dresden (IFW) to study the behavior of the electrons in great detail. The free-electron laser is an ideal infrared radiation source for such experiments because the energy of its light can be adjusted to precisely match the energy level inside a quantum dot. The laser at HZDR also delivers such intense radiation that it more than makes up for the unavoidable losses inherent to the method. “Next, we intend to reveal the behavior of electrons inside quantum dots at lower temperatures,” Stephan Winnerl says. “From these experiments, we hope to gain even more precise insights into the private life of these electrons. In particular, we want to gain a much better understanding of how the electrons interact with one another as well as with the vibrations of the crystal lattice.” Thanks to its intense laser flashes in a broad, freely selectable spectral range, the free-electron laser offers ideal conditions for researching many other interesting things in the remarkable lives of electrons. Stephan Winnerl’s investigations are funded, among other sources, from the priority program “Graphene” of the Deutsche Forschungsgemeinschaft (DFG) and other resources of the European Union. LITERATURE S. Winnerl et al.: “Carrier dynamics in epitaxial graphene close to the Dirac point”, in Physical Review Letters, vol. 107 (2011), p. 237401 (DOI: 10.1103/PhysRevLett.107.237401) R. Jacob, S. Winnerl et al.: “Intersublevel spectroscopy on single InAs-quantum dots by terahertz near-field microscopy”, in Nano Letters, vol. 12 (2012), p. 4336 (DOI: 10.1021/ nl302078w) HONEYCOMB STRUCTURE: Hexagonal crystalline graphene structure in the form of a honeycomb. Image credit: © AlexanderAlUS – Wikipedia Contact _Institute of Ion Beam Physics and Materials Research at HZDR Dr. Stephan Winnerl Prof. Manfred Helm