Aug 3 2015
Order versus disorder: Electrons are diffracted differently in the crystalline structure of a compound of germanium, antimony and tellurium (GST) than in the amorphous one. It is possible to distinguish clearly between the diffraction image of the crystal (left) and that of the amorphous material (right). © Fritz Haber Institute of the MPG.
DVD material holds tremendous potential. This has been proven by an international research team, which has discovered that the material is suitable as a highly rapid light switch for data processing or optical communication.
DVD material includes germanium, antimony and tellurium wherein data media store information. The study was performed by an international team that collaborated with researchers at the Fritz Haber Institute of the Max Planck Society in Berlin and the ICFO-Institut de Ciències Fotòniques in Barcelona.
The DVD storage mechanism depends on the fact that laser pulses reorder the material’s structure, switching it from a transparent to a non-transparent state. As presently discovered by scientists, the optical properties are modified more rapidly when compared to the structure, which could be leveraged for the design of advanced photonic components.
DVDs may become outdated, but this is not applicable to their storage material. This is due to the fact that data media, which are mainly recognized as media for films, are being substituted with other storage methods. However, the Ge2Sb2Te5 compound, also known as GST, may be used for other purposes.
In rewritable DVDs, laser pulses can quickly transform GST from a strongly reflective crystalline state to a much lesser reflective disordered version. Encoding the zeroes and ones of digital information is then performed by the two states.
“The work we have done shows that the material can also be used for applications other than data storage,” as Ralph Ernstorfer, Research Group Leader at the Fritz Haber Institute of the Max Planck Society in Berlin states. “GST could be suitable for modulators in optical communication or for components in optical computer technology, for example.”
The reason for the new capabilities of this material is a new property discovered by the team headed by Ralph Ernstorfer and Simon Wall, scientist at the ICFO-Institut de Ciències Fotòniques. The material tended to modify its optical properties, such as absorptivity, reflectivity and transparency quickly, while the structure showed only a delayed reaction to the excitation. This was surprising for the scientists.
“We used to think that optical behavior changed so quickly because the structure changed,” says Ralph Ernstorfer.
As the structural change from the normal crystalline to the irregular amorphous structure would take place in such a rapid manner, some researchers compared it to an inside-out flipped umbrella. Even though the GST structure is actually modified between these two forms, in terms of using the same analogy, it is less sudden when compared to a storm blowing an umbrella inside out.
As discovered by Max Planck researchers, this is due to a change in the electronic structure of the material initially and is the decisive factor for optical properties like reflectivity, transmission, and absorptivity. For understanding what actually happens, it is useful to see how the electrons in crystalline GST are arranged, where individual atoms are bonded together by individual electrons as well as electron pairs. These electrons are not restricted to an atomic bond. Instead, the electronic loners take part in several bonds concurrently, and they have a resonant bonding as stated by physicists.
The optical characteristics of crystalline GST are dictated by the resonantly bonded electrons but it is possible to move them to traditionally bonded states. This is exactly what the researchers accomplish with a short, intense laser pulse. With a direct consequence, the material does not absorb light, but transmits it unhindered to a certain extent, thus becoming transparent.
The changes that took place in the electronic and optical properties by firing a second short pulse onto a thin GST sample, after the initial laser pulse, were observed by physicists. As they altered the interval between the two light pulses, they could record, as in a film, the fact that the electronic structure instantly rearranges itself.
“We were able to distinguish between the changes in the optical properties and those in the structure only because we combined this method with a second one,” explains Lutz Waldecker, who played a critical role in the experiments performed as part of his doctoral research. He and his team studied the change in structure using short bursts of electrons, which traverse through a crystal in a different manner when compared to materials with an irregular structure.
As the researchers also observed a time delay for the electrons subsequent to the exciting laser pulse, they determined that the regular atomic arrangement is maintained longer when compared to the electronic structure.
Based on observations, over 5ps elapse after the exciting light pulse arrives prior to the onset of the melting of the crystal. In this process, the crystal fails to maintain its regular structure. Even though 5ps is a very brief time, it is enough to show that the material can be used in applications other than data storage.
As atomic realignment causes stress and material fractures, it is not possible to rearrange infinitely often a substance’s atomic lattice. It is, however, this aspect that forms part of the specification profile for a rapid switch in the optical data stream. “If we succeed in quickly removing the energy which is required for the structural change, the crystalline structure could be maintained,” says Lutz Waldecker.
In case a GST layer is sandwiched between two graphene layers or two thin graphite layers, the energy can be removed quickly. The cross-linked carbon atom sheets constituting graphene satisfy the needed requirements.
The physicists in Ralph Ernstorfer’s group is willing to conduct further experiments on sandwiches of different materials. “We want to investigate which states the electrons arrive at as they are excited and how the energy can flow away in sandwich structures,” says Ralph Ernstorfer.
In this manner, the research team is working towards bringing GST to a position where it can also behave as a light switch to process optical data.