571 research outputs found
Ortsaufgelöste Spektroskopie an Halbleiterstrukturen
Im Rahmen dieser Arbeit wurden die optischen Eigenschaften von verschiedenen Halbleiterheterostrukturen auf kleinen LĂ€ngenskalen untersucht. Dazu wurden drei unterschiedliche Methoden der ortsaufgelösten Spektroskopie angewandt, fĂŒr die jeweils eine eigenstĂ€ndige Messapparatur entwickelt und aufgebaut wurde.
Zur ortsaufgelösten Messung der Photolumineszenz wurden ein Mikro-Photolumineszenz-Messplatz aufgebaut. Dabei konnte die Ortsauflösung durch den Einsatz einer Festkörperimmersionslinse auf ein viertel der WellenlÀnge des Lichtes gesteigert werden. Mit diesem Aufbau wurde der Einfluss der Unordnung auf die optischen Eigenschaften von GaAs-Quantenfilme untersucht.
Eine weitere Möglichkeit zur Untersuchung der rÀumlich eng begrenzter PhÀnomene bietet die ortsaufgelöste Anrege-Abfrage-Spektroskopie. Sie erlaubt die Untersuchung der raumzeitlichen Dynamik optisch generierter LadungstrÀger mit einer Zeitauflösung von 100 fs und einer Ortsauflösung von 1 ”m. Eine weitere Steigerung der Ortsauflösung ist durch den Einsatz einer Festkörper-Immersionslinse möglich.
Mit der optische Nahfeld-Mikroskopie wurden erstmals ferromagnetische MnAs-Cluster untersucht. Dabei konnte das Drehvermögen einzelner Cluster qualitativ nachgewiesen werden
Study of leakage currents in pCVD diamonds as function of the magnetic field
pCVD diamond sensors are regularly used as beam loss monitors in accelerators
by measuring the ionization of the lost particles. In the past these beam loss
monitors showed sudden increases in the dark leakage current without beam
losses and these erratic leakage currents were found to decrease, if magnetic
fields were present. Here we report on a systematic study of leakage currents
inside a magnetic field. The decrease of erratic currents in a magnetic field
was confirmed. On the contrary, diamonds without erratic currents showed an
increase of the leakage current in a magnetic field perpendicular to the
electric field for fields up to 0.6T, for higher fields it decreases. A
preliminary model is introduced to explain the observations.Comment: 6 pages, 16 figures, poster at Hasselt Diamond Workshop, Mar 2009,
accepted version for publicatio
Spin flip from dark to bright states in InP quantum dots
We report measurements of the time for spin flip from dark (non-light
emitting) exciton states in quantum dots to bright (light emitting) exciton
states in InP quantum dots. Dark excitons are created by two-photon excitation
by an ultrafast laser. The time for spin flip between dark and bright states is
found to be approximately 200 ps, independent of density and temperature below
70 K. This is much shorter than observed in other quantum dot systems. The rate
of decay of the luminescence intensity, approximately 300 ps, is not simply
equal to the radiative decay rate from the bright states, because the rate of
decay is limited by the rate of conversion from dark excitons into bright
excitons. The dependence of the luminescence decay time on the spin flip time
is a general effect that applies to many experiments.Comment: 3 figure
Perceptual learning in patients with macular degeneration
Patients with age-related macular degeneration (AMD) or hereditary macular dystrophies (JMD) rely on an efficient use of their peripheral visual field. We trained eight AMD and five JMD patients to perform a texture-discrimination task (TDT) at their preferred retinal locus (PRL) used for fixation. Six training sessions of approximately one hour duration were conducted over a period of approximately 3 weeks. Before, during and after training twelve patients and twelve age-matched controls (the data from two controls had to be discarded later) took part in three functional magnetic resonance imaging (fMRI) sessions to assess training-related changes in the BOLD response in early visual cortex. Patients benefited from the training measurements as indexed by significant decrease (p = 0.001) in the stimulus onset asynchrony (SOA) between the presentation of the texture target on background and the visual mask, and in a significant location specific effect of the PRL with respect to hit rate (p = 0.014). The following trends were observed: (i) improvement in Vernier acuity for an eccentric line-bisection task; (ii) positive correlation between the development of BOLD signals in early visual cortex and initial fixation stability (r = 0.531); (iii) positive correlation between the increase in task performance and initial fixation stability (r = 0.730). The first two trends were non-significant, whereas the third trend was significant at p = 0.014, Bonferroni corrected. Consequently, our exploratory study suggests that training on the TDT can enhance eccentric vision in patients with central vision loss. This enhancement is accompanied by a modest alteration in the BOLD response in early visual cortex
The wavefront of the radio signal emitted by cosmic ray air showers
Analyzing measurements of the LOPES antenna array together with corresponding
CoREAS simulations for more than 300 measured events with energy above
eV and zenith angles smaller than , we find that the radio
wavefront of cosmic-ray air showers is of approximately hyperbolic shape. The
simulations predict a slightly steeper wavefront towards East than towards
West, but this asymmetry is negligible against the measurement uncertainties of
LOPES. At axis distances m, the wavefront can be approximated by
a simple cone. According to the simulations, the cone angle is clearly
correlated with the shower maximum. Thus, we confirm earlier predictions that
arrival time measurements can be used to study the longitudinal shower
development, but now using a realistic wavefront. Moreover, we show that the
hyperbolic wavefront is compatible with our measurement, and we present several
experimental indications that the cone angle is indeed sensitive to the shower
development. Consequently, the wavefront can be used to statistically study the
primary composition of ultra-high energy cosmic rays. At LOPES, the
experimentally achieved precision for the shower maximum is limited by
measurement uncertainties to approximately g/cm. But the simulations
indicate that under better conditions this method might yield an accuracy for
the atmospheric depth of the shower maximum, , better than
g/cm. This would be competitive with the established air-fluorescence
and air-Cherenkov techniques, where the radio technique offers the advantage of
a significantly higher duty-cycle. Finally, the hyperbolic wavefront can be
used to reconstruct the shower geometry more accurately, which potentially
allows a better reconstruction of all other shower parameters, too.Comment: accepted by JCA
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