A pulsed semiconductor disk laser shows hysteresis in the output characteristics
Hysteresis in a semiconductor disk laser pulsed with a semiconductor saturable absorber
mirror is reported for the first time. This kind of bistable laser can potentially be utilized
in optical memories and clocks for computers.
Semiconductor disk lasers combine the wavelength versatility of semiconductor lasers with
the high optical power and excellent beam quality of traditional thin disk lasers, which makes
them ideal for projector and display applications. Pulsed semiconductor disk lasers can reach
multigigahertz pulse repetition rates and are well suited for high-speed communications,
switching, sampling, and clocking. The laser studied in this paper could operate at three different
pulsing frequencies in the gigahertz range, exhibiting bistability and hysteresis between
these states in the output power as well as in the laser wavelength and pulse width.
Furthermore, the size of the hysteresis loops could be tuned by varying the design of the light
intensifying semiconductor structure, holding promise for controllable devices in the future.
***
LQ11508B
Improvements in magnetic nano-oscillators as a function of the
orientation of an applied magnetic field
In devices consisting of an alternating stack of magnetic and
nonmagnetic metal layers, a perpendicularly-flowing direct electrical
current can become partially spin polarized upon passing through one
magnetic layer and can then transfer some of that spin into the next
magnetic layer downstream. As a result, under certain experimental
conditions the magnetization of the second layer can be excited into
steady-state oscillation and it emits microwaves. This phenomenon is
under development for making frequency-tunable nanoscale microwave
sources. For these applications, it is desirable that the
oscillations be highly reproducible and periodic, i.e. that they have
narrow linewidths in their frequency spectrum. In this paper, we
study how these linewidths depend on the angle of an applied magnetic
field. We found that the most-commonly studied field orientation, in
the plane of the magnetic layers and parallel to the magnetic easy
axis of the oscillating layer, actually produces the worst microwave
properties. As the field angle is rotated by 90 degrees in the sample
plane, the linewidths can decrease dramatically, by a factor of 20-50
in some devices. This enables a simple way to improve the microwave
source. Computer simulations attribute the improvement to a
transition from spatially incoherent dynamics within the moving
magnetic layer to nearly spatially-uniform oscillations.
***
LQ11476
SLOW ANTIPROTONS UNRAVEL THE SECRETS OF ATOMIC COLLISIONS.
We have used a beam of slow antiprotons to learn more about the dynamics of
atomic collisions in general.
One of the first successes of quantum theory was the accurate description of
the structure of the isolated hydrogen atom. Due to difficulties with the
inclusion of the interaction between its two electrons, it took years to
reach a similar accuracy for the helium atom.
Today, 90 years later, similar progress has not yet been achieved in the
case of atomic collisions, where a description of dynamic systems involving
several electrons are needed. Even for the "theorists dream system", namely
the collision of a slow antiproton on a helium atom leading to the
liberation of one electron, for which a large number of advanced theoretical
calculations exist, these differ by up to 30%.
Using a new scheme for the creation of a DC beam of slow (a few keV)
antiprotons at CERN, this group have obtained benchmark data for the
development of more accurate theoretical calculations.
The use of antiprotons instead of other ions is important, since
antiprotons, due to their negative charge, do not support the process where
electrons are transferred to the slow projectile. This simplifies the
already difficult task of the theorists.
***
BQR1070
Temperature-induced spin coherence dissipation in quantum dots
Electron spins in quantum dots are an attractive candidate for
implementing quantum information technologies in a solid state
environment. This attractivity is mainly based on the long living spin
coherence in these systems, as most of the spin relaxation mechanisms
which work very efficiently in systems of higher dimensionality are
suppressed in quantum dots. The interaction which cannot be avoided at
cryogenic temperatures is the hyperfine interaction with the
background of lattice nuclei limiting the coherence time $T_2$ to the
microseconds range in GaAs based quantum dots. Despite of these
results, the understanding of the spin dynamics is still fragmentary.
For practical applications also the temperature dependence of the
coherence time is of key importance. In this work the authors report
the first measurement of the variation of $T_2$ for quantum dot
electrons with increasing temperature. Such measurements became
possible by addressing quantum dot ensembles and implementing the
recently developed technique of mode-locking by which the spin
ensemble is synchronized with a periodic excitation laser. This
technique allows a refocussing of the spins in the inhomogeneous
ensemble by which the single spin dynamics can be extracted. It was
found that $T_2$ is constant in the microseconds range up to 20 K, but
for higher temperatures shows a sharp drop. Comparison with detailed
model caluculations shows that this drop arises from fluctuations of
the hyperfine interaction due to the temperature induced lattice
vibrations. These results are a key step towards understanding the
limitations of spin relaxation which are under lively debate at the
moment. From this understanding one might develop tools to extend the
coherence towards longer times, as required only not for quantum
information but also for spin-based electronics
***
LS11288
Demystifying the Plutonium Enigma
Plutonium is an element which, while controversial, is central to our national
security, both in terms of energy production and defense. Despite almost half
a century of research, plutonium is remains a fundamental problem
of condensed matter physics. In this paper, we have performed one of the most
accurate simulations to date of the electrons in solid Pu using the Dynamical
Mean-Field Theory (DMFT) and Density Functional Theory (DFT). This achievement
was made possible by both recent theoretical developments and the massive
supercomputing resources at Lawrence Livermore National Laboratory. We
show that the electrons in Pu are delocalized, but are somewhat heavy due to
their interactions. The behavior of the electrons are then studied as the
volume of the Pu lattice is expanded, which is a feat that is not easily
achieved under controlled experimental conditions. The electrons become
progressively heavier as the volume increases, and for a 30% expansion they
behave in a localized fashion at ambient temperatures. Predicted trends of
various physical properties are in favorable agreement with experimental
observation. The success of this study is simply the beginning of a much
longer journey of discovery. Delta Pu displays many anomalies which are not
sufficiently understood, such as a density which is less than liquid Pu (like
water) and the fact that it shrinks when it is heated! Finally, it appears that
we may have a tool, DFT+DMFT, which is capable of capturing the salient physics
of Pu and revealing its secrets.
***
LP11489
The universe is necessarily unpredictable
From the flight of a falling apple to the rising of the sun, the world
around us appears to be highly predictable. However, in contrast to
these examples, we show that there are particular natural processes
which are entirely unpredictable.
A remarkable feature of our world is that numerous physical phenomena
can be calculated with virtual certainty. In fact, such certainty can
only apply to large objects; for smaller ones, our ability to make
precise predictions becomes inherently worse. This is because small
objects are described by an altogether different theory, quantum
mechanics. Consider, for example, the radioactive decay of some atom.
Quantum mechanics tells us the probability of decay within, say, the
next hour, but not the exact time of decay. This is an undesirable
feature and led several scholars, including Albert Einstein, to
speculate that there may exist a (yet unknown) more complete
description of quantum processes. However, as our article shows, this
is impossible. The work of Kochen and Specker and of Bell showed that
outcomes of physical experiments cannot generally be predicted *with
certainty*. We have extended their result to make a stronger claim,
that the outcomes of particular physical experiments are *entirely
unpredictable*.
***
BP10887
A rule of selecting preferred copolymers for organic optoelectronic devices
Organic light-emitting diodes (OLEDs) and photovoltaic cells are optoelectronic devices with contrary operation processes. Surprisingly, it has been revealed that donor-acceptor copolymers are species of promising materials that favor both OLEDs and photovoltaic cells. How to choose efficiently a copolymer for a specific purpose is a realistic issue that must be addressed by experimentalist. In this paper, by employing a dynamic simulation method, we propose a general role associated with the electronic structure of donor-acceptor copolymers, such as the ratio of level offset to polaron or exciton binding energy, and the interfacial electronic coupling, to help experimentalist on this issue. It is found that the most favored level offset for photovoltaic cells is the one with level offset greater than the exciton binding energy while for OLEDs is that between polaron and exciton binding energy. The electronic coupling at the interface is also an important factor that can affect the operation of optoelectronic devices. Our study presents a helpful guidence to experiments in the preliminary selection of preferred copolymers for organic optoelectronic devices.
