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Nanophysics

Thin Films, Bright Future

München, 12/01/2017

LMU physicist Alexander Högele studies ultrathin semiconducting films and carbon nanotubes, which possess astonishing physical properties. He has now received the second highly endowed European Research Council grant in his career.

“We study functional materials at the atomic scale”: Alexander Högele in the cleanroom. Photo: Jan Greune / LMU

Exploring new physics is sometimes a simple matter. One can apply a piece of sticky tape to the tip of a pencil, peel it away and with luck an atomically thin sheet made up of a single-layered, two-dimensional lattice of pure carbon will stick to the tape. This is graphene – transparent, ultralight, and extremely tough. A square meter of this marvelous material weighs less than a milligram, and a cat could use the sheet as a hammock without the risk of falling through it. The trick first occurred to Russian physicist Andre Geim. Together with Konstantin Novoselov, he went on to win the Nobel Prize for Physics in 2010 for the isolation and characterization of the properties of graphene.

“It’s amazing how much interesting physics one can do with simple tools and a little curiosity. That was a characteristic of the Soviet school of physics,” says nanophysicist Alexander Högele, who himself grew up in the Soviet Union, and admires this unassuming approach to innovative materials. His own work focuses on the search for nanomaterials with unusual optical properties, principally with a view to applications in photonics and quantum technologies. In a project funded by the European Research Council (ERC), he and his research group successfully developed ways of fabricating carbon nanotubes made of graphene that exhibit specific and potentially useful optical characteristics.

Visitors to his office at the Faculty of Physics enter a world of quantum effects and highly unusual materials, of ‘pseudospins’ and ‘potential wells’. Högele’s goal is to fabricate materials that emit light quanta (‘photons’) with a specific energy (wavelength) and can be used in photonic applications and perhaps as the basis for quantum communication. “We study functional materials at the atomic scale,” he says –nanotubes made of single layers of atoms, and ultrathin 2D semiconductors.

Accounts of the goings-on in the nanoworld often make it sound like a totally unfamiliar cosmos. Researchers who probe the properties of materials like graphene work at the frontiers of physics and encounter effects and phenomena that lie beyond the sphere of classical physics. But it’s not always necessary to use high-tech methods to discover new aspects of this unfamiliar world, Högele says.

Honeycombs, rolled up
For some time, his group has been working with carbon nanotubes, characterizing their properties and behavior at temperatures close to absolute zero (-273°C). Nanotubes are optically active, and the wavelength of the light they emit when excited with a laser depends on their diameter. Moreover, the intensity of the light emitted at these very low temperatures is quantized. This meant that they could, in principle, be exploited for the secure transmission of information via optic-fiber cables, as any attempt to eavesdrop on the message would alter the state of the photons and could be detected. The crucial question was whether the system could be coaxed into emitting photons with a specific energy (i.e. frequency) at room temperature.

One promising approach to the problem was to adjust the emission by chemically modifying the nanotubes. The cylinders are several micrometers long and have a diameter of about 1 nanometer. Each consists of a single-layered sheet of graphene rolled up into tube form. However, it is possible to replace some of the carbon atoms that make up the regular honeycomb lattice of graphene, either with atoms of another element or with chemically reactive functional groups. And indeed, this doping strategy allows one to tune the frequency of the emitted photons. “We are now hopeful that we will be able to produce nanotubes that are tailor-made for applications in quantum technology,” Högele says.

The primary goal is to create materials that can be used for secure quantum-based communication. “We have already succeeded in modifying the color of the emitted single photons so that they conform to the technical specifications of the existing optic-fiber network,”Högele says, although the signal bandwidth is still too broad. However, using technical tricks, such as optical resonators, it should be possible to improve the quality of the signal and the yield of suitable photons.

The major challenge lies in the fact that the modified nanotubes must function as single-photon sources (SPS) –they must consistently emit single photons of precisely the same optical frequency, and nothing else. If this reliability can be achieved, the route to quantum communication technologies will be open. In addition, atoms or electronic excitation states in semiconducting films, and other quantized properties such as spin states, can serve as information carriers. If ongoing research efforts succeed, we will enter an era in which technology is no longer based on the tenets of classical physics, but on the probabilistic laws of quantum mechanics. As yet, none of these visions has resulted in real devices, and even the laboratory models that have so far been tested leave a lot to be desired. But research in the field has already raised great expectations with regard to future applications of nanomaterials and quantum effects.

Page 2: A plethora of promising materials

 

erc_webLMU physicist Alexander Högele studies ultrathin semiconducting films and carbon nanotubes, which possess astonishing physical properties. He has now received the second highly endowed European Research Council grant in his career.