62 research outputs found
Joined optical and thermal characterization of a III-nitride semiconductor membrane by micro-photoluminescence spectroscopy and Raman thermometry
We present the simultaneous optical and thermal analysis of a freestanding
photonic semiconductor membrane made from wurtzite III-nitride material. By
linking micro-photoluminescence (PL) spectroscopy with Raman thermometry,
we demonstrate how a robust value for the thermal conductivity can be
obtained using only optical, non-invasive means. For this, we consider the
balance of different contributions to thermal transport given by, e.g.,
excitons, charge carriers, and heat carrying phonons. Further complication is
given by the fact that this membrane is made from direct bandgap
semiconductors, designed to emit light based on an InGaN
() quantum well embedded in GaN. To meet these challenges, we designed
a novel experimental setup that enables the necessary optical and thermal
characterizations in parallel. We perform micro-Raman thermometry, either based
on a heating laser that acts as a probe laser (1-laser Raman thermometry), or
based on two lasers, providing the heating and the temperature probe separately
(2-laser Raman thermometry). For the latter technique, we obtain temperature
maps over tens of micrometers with a spatial resolution less than
, yielding
for the
-plane of our -thick membrane at around
room temperature, which compares well to our calculations
applied to a simplified structure. Based on these calculations, we explain the
particular relevance of the temperature probe volume, as quasi-ballistic
transport of heat-carrying phonons occurs on length scales beyond the
penetration depths of the heating laser and even its focus spot radius. The
present work represents a significant step towards non-invasive, highly
spatially resolved, and still quantitative thermometry performed on a photonic
membrane.Comment: 28 pages, 14 figures, and Supplemental Materia
Nitric Oxide Induces Cell Death by Regulating Anti-Apoptotic BCL-2 Family Members
Nitric oxide (NO) activates the intrinsic apoptotic pathway to induce cell death. However, the mechanism by which this pathway is activated in cells exposed to NO is not known. Here we report that BAX and BAK are activated by NO and that cytochrome c is released from the mitochondria. Cells deficient in Bax and Bak or Caspase-9 are completely protected from NO-induced cell death. The individual loss of the BH3-only proteins, Bim, Bid, Puma, Bad or Noxa, or Bid knockdown in Bimâ/â/Pumaâ/â MEFs, does not prevent NO-induced cell death. Our data show that the anti-apoptotic protein MCL-1 undergoes ASK1-JNK1 mediated degradation upon exposure to NO, and that cells deficient in either Ask1 or Jnk1 are protected against NO-induced cell death. NO can inhibit the mitochondrial electron transport chain resulting in an increase in superoxide generation and peroxynitrite formation. However, scavengers of ROS or peroxynitrite do not prevent NO-induced cell death. Collectively, these data indicate that NO degrades MCL-1 through the ASK1-JNK1 axis to induce BAX/BAK-dependent cell death
How to verify the precision of density-functional-theory implementations via reproducible and universal workflows
In the past decades many density-functional theory methods and codes adopting
periodic boundary conditions have been developed and are now extensively used
in condensed matter physics and materials science research. Only in 2016,
however, their precision (i.e., to which extent properties computed with
different codes agree among each other) was systematically assessed on
elemental crystals: a first crucial step to evaluate the reliability of such
computations. We discuss here general recommendations for verification studies
aiming at further testing precision and transferability of
density-functional-theory computational approaches and codes. We illustrate
such recommendations using a greatly expanded protocol covering the whole
periodic table from Z=1 to 96 and characterizing 10 prototypical cubic
compounds for each element: 4 unaries and 6 oxides, spanning a wide range of
coordination numbers and oxidation states. The primary outcome is a reference
dataset of 960 equations of state cross-checked between two all-electron codes,
then used to verify and improve nine pseudopotential-based approaches. Such
effort is facilitated by deploying AiiDA common workflows that perform
automatic input parameter selection, provide identical input/output interfaces
across codes, and ensure full reproducibility. Finally, we discuss the extent
to which the current results for total energies can be reused for different
goals (e.g., obtaining formation energies).Comment: Main text: 23 pages, 4 figures. Supplementary: 68 page
Visible spectrum quantum light sources based on InxGa1âxN/GaN Quantum Dots
We present a method for designing quantum
light sources, emitting in the visible band, using wurtzite
InxGa1âxN quantum dots (QDs) in a GaN matrix. This system
is significantly more versatile than previously proposed
arsenide- and phosphide-based QDs, having a tuning range
exceeding 1 eV. The quantum mechanical configuration
interaction method, capturing the fermionic nature of
electrons and associated quantum effects explicitly, is used to
find shapes and compositions of dots to maximize the
excitonic dipole matrix element and optimize the biexciton
binding energy. These results provide QD morphologies
tailored for either bright single-photon emission or entangledphoton-
pair emission at any given wavelength in the visible
spectrum
Systematic design of virtual patient cases for clinical reasoning training: a didactic concept and guide for authors
Thermal Transport in c-plane GaN Membranes Characterized by Raman Thermometry
Excess heat often limits the lifetime or stability of semiconductor devices, like laser strcutures, e.g. by affecting the refractive index or defect formation. It is therefore vital to understand how thermal energy is dissipated from the active region. In this contribution, we analyze the in-plane thermal transport in GaN-based membranes which can be applied in UV-visible light emission. The temperature of the material is probed by the shift and width of Raman modes under heating with a UV laser. This method allows for a contactless characterization without the need for additional processing steps often needed for alternative thermometry. We find, that the thermal conductivity, Îș, is significantly reduced compared to bulk GaN due to the finite thickness of the analyzed membranes. Phonon scattering due to roughness and porosity of the membrane is found to further reduce Îș. Studying in-plane thermal transport lays the foundation for subsequent thermal studies on entire device structures; exploiting a subtle balance of in- and cross-plane thermal transport which could improve device designs
60 GHz Pulsradar zur Detektion von Hochspannungsleitungen Schlussbericht
With 39 figs.SIGLEDEGerman
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