Optics in Curved Space
The impact of space-time curvature on the spatial evolution of light as
stated by general relativity becomes significant only on astronomic
scales or in the vicinity of large masses, making a direct experimental
observation in the laboratory impossible. In our paper we demonstrate
the effects of space curvature on light propagation in a table-top
experiment and, hence, develop optics beyond the limits of flat space.
Our approach is to abandon one spatial dimension and to confine light
propagation to two-dimensional curved surfaces. On a positively curved
surface like that of a sphere beams seem to attract each other, giving
rise to a lensing feature of space itself. In contrast, on a negatively
curved surface like a saddle beams look like repelling each other and
light spreads exponentially as it propagates. This is not only a new
kind of tool kit for integrated optics, but may serve as an intuitive
picture of the complex dynamics of light in general relativity.
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LT12266
The benefits of probing continuously: It helps to measure fluxes while
maintaining validity of Fluctuation Theorems
According to quantum mechanics any measurement disturbs the measured
object. In
our research we focused on the flux of electrons through a nanoscopic
junction
connected to the electrodes of a battery, and discovered that important
properties, known as fluctuation theorems, continue to hold unaltered, even
when the flux of electrons is continuously measured. This means that certain
aspects of the dynamics of a quantum system remain unchanged even though the
state of the system is strongly altered by a measurement process. In the
case
of electron transport through a nano-junction the fluctuation theorems say
that some of the electrons go to the negative electrode while most of them
behave in the "normal" way and go to the positive one. These theorems also
quantify the number of exceptional events of electrons flowing in the
"wrong"
direction. Our discovery corroborates recent experimental findings
[Physical Review B, 81, 125331 (2010)], and makes the theory of quantum
fluctuation theorems applicable to a wide class of experiments, enabling
the --otherwise not contemplated-- advantageous continuous measurement
of fluxes.
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LS12715
How nanostructures healthily cope with stress
We have discovered that, by fabricating self-assembled nanostructures on
pre-patterned substrates, we can "feed them" with material and let them
grow in size far beyond what is possible on conventional planar
substrates. For the considered nanostructures (self-assembled
semiconductor quantum dots), elastic stress is the main driving force
governing their "birth and fate". Once formed, dots eagerly compete for
the externally provided material. In this way some of them are able to
rapidly grow in size and need to find ways to release the increasing
amount of stress. In the end, when a dot reaches a certain critical
size, it usually relaxes by crystal defect introduction, which is
analogous to its "death", since defects are deleterious for dot
applications. Now, by guiding the formation of the dots at well defined
positions on a periodically patterned substrate and thus imposing an
equal sharing of the provided material, we find that dots are able to
cope with the increasing stress by cyclically incorporating large
amounts of material from the substrate and corresponding changes of
their shape. This allows them to keep growing in size while delaying
relaxation mediated by defects.
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BS11331
Element-selective insights into Magnetism within reach of any Laboratory
In our work we present an experimental technique for element-selective and ultra-fast investigations of magnetism on the nanometer scale, which is going to be available in any optical laboratory in the near future. It overcomes the restrictions of previous existing methods that either require highly specialized and rare infrastructure, such as large-scale synchrotron radiation facilities, or lack in performance, such as laser-based techniques. Our method is based on reflectivity experiments in the extreme ultraviolet (XUV) range and benefits from recently developed table-top soft x-ray sources that manage to produce ultra-fast and coherent XUV light with moderate effort.
In our publication we investigate magnetic switching processes in multilayer systems consisting of thin magnetic films in the XUV region that have already been studied extensively in the visible range as well as soft x-ray region during the past decades, because of their high relevance for both fundamental research and technology. Just as this region of the electro-magnetic spectrum is located in between the visible and soft x-ray range, its interaction with matter combines the characteristics of both spectral regions potentially contributing an additional perspective. A deeper understanding of such materials paves the road to technologies for faster, smaller and more economical data processing and storage devices, for example.
