Research group Achim Wixforth


Surface acoustic waves

The interaction between surface acoustic waves (SAW) and free cariers in semiconductor nanostructurs was one of our maior research topics in 1999.

Enthusiastically driven by the efforts of Markus Rotter, a new family of SAW based high frequency devices could be developed in co-operation with the research labs of SIEMENS. Here, we make use of the fact that the strong piezoelectricity of - say LiNbO3 - can be partially transferred on an active semiconductor layered system employing the epitaxial lift-off technique.

We demonstrated that a field effect tunability of the sound velocity in such hybrid devices can result in a whole whealt of different room-temperature operating devices. Probably the most spectacular of these devices is tthe remotely adressable and intterrogable sensor element, that can be used to monitor many different physical quantities like temperature, voltage, currents, charges, illumination intensity and so forth. A short radio-frequency pulse sent to the sensor will result in an time delayed "echo" from the chip, containing the measured quantity AND the identity of the chip itself.

On the same hybrids we could - for the first time - show that the interacztion between SAW and free charge carriers in a semiconductor heterostructure becomes very non-linear at intense sond amplitude levels. The lateral potential modulation in the plane of the electron system can become so large that a formerly homogeneous electron sheet breakes up into well separated stripes of charge that travele with the SAW at the speed of sound. The interaction between the SAW and the electrons becomes very non-linear in that transition regime. Apart from being a fascinating new physical effect this non-linearity could also have some impact on the high frequency devices mentioned above:

In the linear regime, the interaction between SAW and free charges in a semiconductor heterojunction leads to a strong attenuation and a renormalization of the speed of sound for some critical sheet conductivity. Both quantities are related to each other by a Kramers-Kroning type relation. This would in principle hamper the use of our hybrid structures for real life devices, as the desired large change in SAW velocity is usually accompanied by an equally large attenuation, which of cours would strongly reduce the dynamic of the device. Although we have found some technological ways around this problem, the non-linear physics discovered here is a much more elegant and versatile method to overcome the large attenuation.

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In co-operation with the research group of Prof. J. Peisl (LMU), the SAW fields could be directly visualized using a stroboscopic X-ray technique. Although this is a quite exotic approach for the visualization of a surface wave, a lot of additional information could be taken from these experiments, which would have been unavailable without it. Apart from that, we specially like the idea that a whole synchrotron is phase-locked to one of our samples ..... (or maybe vice versa ?). Thanks to Wolfgang Sauer, who did a great job with his dissertation!

Under the headline "acousto optics" basically two different topics have been adressed: After the recent successful demonstration of a "photon conveyor belt" we concentrated in 1999 mostly on questions being related to the loss mechanisms and re-emission efficiency of the storage of spatially separated bipolar charges in such a conveyor belt. Fortunately, we could welcome a new and very skilled young member (Hans-Jörg Kutschera) to our group, who continues the investigations of this novel optoelectronic system. Secondly, we tried to use the conveying mechanism to acoustically pump semiconductor quantum dots. Christoph Bödefeld, in co-operation with the research group of Prof. H. Lipsanen in Helsinki, and the theoretical help of Christian Wiele and Fritz Haake in Essen, struggles with those little guys and tries to throw electrons and holes at them. A photon train is what we want to see at some point.

Sascha Haubrich also uses some kind of a conveyor belt to propel electrons one by one through a quantum point contact. The idea behind this extremely challenging experiment is to generate an acousto-electric curent that is given only by the electron charge and the frequency of the SAW. The
original work of J.M. Shilton and others at the University of Cambridge triggered our interest in this subject. Also, Sasha looks at the fractional quantum Hall effect at very low temperatures and in high magnetic fields. Without exaggeration: Sashas transducers are the best!

Last but by no means least, we were able to find a way to spatially resolve the interaction between a SAW and "some disturbance of the boundary conditions" at the surface of a piezoelectric. Using specially designed transducer structures, we were not only able to focus the SAW to a desired location of the sample, but also to scan the SAW beam across macroscopic distances perpendicular to its propagation direction. This way, we could for instance probe the spatial distribution of photogenerated charges on a chip, observe a bug walking around on the chip and finally employing tomographic techniques borrowed from the CT medics use, build a simple but new type of "camera" based on surface acoustic waves. Martin Streibl and Florian Beil did a really great job on that!

 


Storage of light in a solid




Based on the knowledge that we had gained during our studies of the SAW based "photon conveyor belt", we could show that basically the same mechanisms (spatial separation of photogenerated charges in a lateral potential landscape) apply for static lateral potentials. As Stefan Zimmermann describes in his PhD thesis, an interdigitated gate electrode on top of a specially designed semiconductor quantum well sample does the same job as the SAW before. However, there is a maior difference between both types of "devices". As the maximum storage time for a SAW conveyor belt is basically given by the length of the sample, in the static case much longer strage times can be achieved here. Like our "photon conveyor belt", this new type of photonic device has attracted a lot of interest even in the public media. The idea to trap a photonic signal in a 'box' and then release it as a photonic signal after some time ist something that obviously captures peoples imagination!


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(there's a story behind each marble)



Meanwhile Jan Krauss, another new member of our group, who joined us coming from the University of Erlangen, is putting all his efforts into this fascinating subject. His new ideas and his enthusiasm will certainly add very valuable for the future!




'Exotic' groundstates in strongly coupled semiconductor quantum wells

In close co-operation with the research group of Prof. Valeri T. Dolgopolov in Chernogolovka, Russia, we continued our research on so-called 'parabolic quantum wells'. The nice features of those quantum structures have been extensively described in our research reports of the last few years. Here, we studied the properties of strongly coupled quantum wells, employing magnetocapacitance and far-infrared spectroscopy techniques. For gate bias controlled asymmetric electron density distributions in such a soft two subband system, we observe both individual gaps as well as double layer gaps at integer filling factors. Some of these gaps turn out to result from a wave function reconstruction in growth direction, induced by intersubband electron transfer in perpendicular magnetic fields.

In tilted magnetic fields, even more exotic states are observed. The competition between two groundstates for certain total filling factors turn out to lead to a new insulator-insulator quantum phase transition, the so-called canted antiferromagnetic phase, which has been recently predicted theoretically.