334 research outputs found
A thin polymer insulator for Josephson tunneling applications
The use of an organic monolayer formed from a vapor as an insulating barrier for thin film Josephson junctions is considered, and the effect of an organic monolayer on the transition temperature of a thin film superconductor is investigated. Also analyzed are the geometric factors which influence Josephson junctions and Josephson junction interferometers
Tunneling between a metal and silicon separated by a polymer insulator
Tunneling between metal and silicon separated by thin polymer film insulato
Determination of mechanical stress distribution in Drosophila wing discs using photoelasticity
Morphogenesis, the process by which all complex biological structures are formed, is driven by an intricate interplay between genes, growth, as well as intra- and intercellular forces. While the expression of different genes changes the mechanical properties and shapes of cells, growth exerts forces in response to which tissues, organs and more complex structures are shaped. This is exemplified by a number of recent findings for instance in meristem formation in Arabidopsis and tracheal tube formation in Drosophila. However, growth not only generates forces, mechanical forces can also have an effect on growth rates, as is seen in mammalian tissues or bone growth. In fact, mechanical forces can influence the expression levels of patterning genes, allowing control of morphogenesis via mechanical feedback. In order to study the connections between mechanical stress, growth control and morphogenesis, information about the distribution of stress in a tissue is invaluable. Here, we applied stress-birefringence to the wing imaginal disc of Drosophila melanogaster, a commonly used model system for organ growth and patterning, in order to assess the stress distribution present in this tissue. For this purpose, stress-related differences in retardance are measured using a custom-built optical set-up. Applying this method, we found that the stresses are inhomogeneously distributed in the wing disc, with maximum compression in the centre of the wing pouch. This compression increases with wing disc size, showing that mechanical forces vary with the age of the tissue. These results are discussed in light of recent models proposing mechanical regulation of wing disc growth
Oxide-apertured microcavity single-photon emitting diode
We have developed a microcavity single-photon source based on a single
quantum dot within a planar cavity in which wet-oxidation of a high-aluminium
content layer provides lateral confinement of both the photonic mode and the
injection current. Lateral confinement of the optical mode in optically pumped
structures produces a strong enhancement of the radiative decay rate. Using
microcavity structures with doped contact layers, we demonstrate a
single-photon emitting diode where current may be injected into a single dot
Polarization properties in the transition from below to above lasing threshold in broad-area vertical-cavity surface-emitting lasers
For highly divergent emission of broad-area vertical-cavity surface-emitting
lasers (VCSELs) a rotation of the polarization direction by up to 90 degrees
occurs when the pump rate approaches the lasing threshold. Well below threshold
the polarization is parallel to the direction of the transverse wave vector and
is determined by the transmissive properties of the Bragg reflectors that form
the cavity mirrors. In contrast, near-threshold and above-threshold emission is
more affected by the reflective properties of the reflectors and is
predominantly perpendicular to the direction of transverse wave vectors. Two
qualitatively different types of polarization transition are demonstrated: an
abrupt transition, where the light polarization vanishes at the point of the
transition, and a smooth one, where it is significantly nonzero during the
transition
Ultrahigh frequency oscillations and multimode dynamics in vertical cavity surface emitting lasers
Includes bibliographical references (page 187).We report the observation of ultrahigh frequency oscillations of up to 240 GHz in optically gain switched vertical cavity surface emitting lasers. These oscillations are shown to be produced by multimode emission through mode competition (10-30 GHz) or mode beating (above 100 GHz). Although these oscillations are not related to the intrinsic modulation bandwidth, some of them could be mistaken for relaxation oscillations, calling for careful interpretation of the results of this type of experiments. The highest frequencies observed for single mode relaxation oscillations were about 9 GHz in agreement with values of modulation bandwidth reported in the literature
Externally mode-matched cavity quantum electrodynamics with charge-tunable quantum dots
We present coherent reflection spectroscopy on a charge and DC Stark tunable
quantum dot embedded in a high-quality and externally mode-matched microcavity.
The addition of an exciton to a single-electron charged quantum dot forms a
trion that interacts with the microcavity just below strong coupling regime of
cavity quantum electrodynamics. Such an integrated, monolithic system is a
crucial step towards the implementation of scalable hybrid quantum information
schemes that are based on an efficient interaction between a single photon and
a confined electron spin.Comment: 10 pages, 4 figure
Transverse mode dynamics in vertical cavity surface emitting lasers excited by fast electrical pulses
Includes bibliographical references (page 592).We report measurements of the transient multimode dynamics of vertical cavity surface emittin glasers excited by short (100-200 ps rise times, 1 ns duration) electrical pulses. Fast changes on the spatial distribution of the output power and strong mode competition are observed. Numerical simulations show that the observed dynamics are due to the partial overlap of the different transverse modes through spatial hole burning
Photo-elastic properties of the wing imaginal disc of Drosophila
In the study of developmental biology, the physical properties and constraints of the developing tissues are of great importance. In spite of this, not much is known about the elastic properties of biologically relevant tissues that are studied in biology labs. Here, we characterize properties of the wing imaginal disc of Drosophila, which is a precursor organ intensely studied in the framework of growth control and cell polarity. In order to determine the possibility of measuring mechanical stresses inside the tissue during development, we quantify the photo-elastic properties of the tissue by direct mechanical manipulation. We obtain a photo-elastic constant of [Formula: see text]
Cell-Sorting at the A/P Boundary in the Drosophila Wing Primordium: A Computational Model to Consolidate Observed Non-Local Effects of Hh Signaling
Non-intermingling, adjacent populations of cells define compartment boundaries;
such boundaries are often essential for the positioning and the maintenance of
tissue-organizers during growth. In the developing wing primordium of
Drosophila melanogaster, signaling by the secreted protein
Hedgehog (Hh) is required for compartment boundary maintenance. However, the
precise mechanism of Hh input remains poorly understood. Here, we combine
experimental observations of perturbed Hh signaling with computer simulations of
cellular behavior, and connect physical properties of cells to their Hh
signaling status. We find that experimental disruption of Hh signaling has
observable effects on cell sorting surprisingly far from the compartment
boundary, which is in contrast to a previous model that confines Hh influence to
the compartment boundary itself. We have recapitulated our experimental
observations by simulations of Hh diffusion and transduction coupled to
mechanical tension along cell-to-cell contact surfaces. Intriguingly, the best
results were obtained under the assumption that Hh signaling cannot alter the
overall tension force of the cell, but will merely re-distribute it locally
inside the cell, relative to the signaling status of neighboring cells. Our
results suggest a scenario in which homotypic interactions of a putative Hh
target molecule at the cell surface are converted into a mechanical force. Such
a scenario could explain why the mechanical output of Hh signaling appears to be
confined to the compartment boundary, despite the longer range of the Hh
molecule itself. Our study is the first to couple a cellular vertex model
describing mechanical properties of cells in a growing tissue, to an explicit
model of an entire signaling pathway, including a freely diffusible component.
We discuss potential applications and challenges of such an approach
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