Electron leakage effects on GaN-based light-emitting diodes

Nitride-based light-emitting diodes suffer from a reduction (droop) of the internal quantum efficiency (IQE) with increasing injection current. Using advanced device simulation, we investigate the impact of electron leakage on the IQE droop for different properties of the electron blocker layer (EBL). The simulations show a strong influence of the EBL acceptor density on the droop. We also find that the electron leakage decreases with increasing temperature, which contradicts common assumptions.     

MRICOM–MRI COntrast Modelling using 2D T1–T2 correlation spectra and relaxation signatures

Image contrast is calculated by inputting experimental 2D T₁–T₂ relaxation spectra into the ODIN software interface. The method involves characterising a magnetic resonance imaging pulse sequence with a “relaxation signature” which describes the sensitivity of the sequence to relaxation and is independent of sample parameters. Maximising (or minimising) the overlap between the experimental 2D T₁–T₂ relaxation spectra and the relaxation signature can then be used to maximise image contrast. The concept is illustrated using relaxation signatures for the echo planar imaging and Turbo spin-echo imaging sequences, together with in-vitro 2D T₁–T₂ spectra for liver and cartilage.     

Fast fabrication of super-hydrophobic surfaces on polypropylene by replication of short-pulse laser structured molds

A new two-step method, facilitating the rapid generation of super-hydrophobic surface structures via parallel laser processing followed by a replica generation by injection molding is reported. A self-made fused silica-based diffractive optical element (DOE) is applied to distribute the laser energy into a 25×25 dot matrix. This DOE is used as a transmission mask for surface ablation of metal molds, applying short-pulse UV laser pulses. In a subsequent process step, replicas of the processed stamp are produced by variothermal injection molding, enabling the mass production of the surface pattern on plastics parts. The resulting topography facilitates a super-hydrophobic behavior of the fabricated components.     

Laboratory singing sand avalanches

Some desert sand dunes have the peculiar ability to emit a loud sound up to 110 dB, with a well-defined frequency: this phenomenon, known since early travelers (Darwin, Marco Polo, etc.), has been called the song of dunes. But only in late 19th century scientific observations were made, showing three important characteristics of singing dunes: first, not all dunes sing, but all the singing dunes are composed of dry and well-sorted sand; second, this sound occurs spontaneously during avalanches on a slip face; third this is not the only way to produce sound with this sand.More recent field observations have shown that during avalanches, the sound frequency does not depend on the dune size or shape, but on the grain diameter only, and scales as the square root of g/d - with g the gravity and d the diameter of the grains - explaining why all the singing dunes in the same vicinity sing at the same frequency.We have been able to reproduce these singing avalanches in laboratory on a hard plate, which made possible to study them more accurately than on the field. Signals of accelerometers at the flowing surface of the avalanche are compared to signals of microphones placed above, and it evidences a very strong vibration of the flowing layer at the same frequency as on the field, responsible for the emission of sound.Moreover, other characteristics of the booming dunes are reproduced and analyzed, such as a threshold under which no sound is produced, or beats in the sound that appears when the flow is too large. Finally, the size of the coherence zones emitting sound has been measured and discussed.