695 research outputs found
Electrically pumped semiconductor laser with low spatial coherence and directional emission
We design and fabricate an on-chip laser source that produces a directional
beam with low spatial coherence. The lasing modes are based on the axial orbit
in a stable cavity and have good directionality. To reduce the spatial
coherence of emission, the number of transverse lasing modes is maximized by
fine-tuning the cavity geometry. Decoherence is reached in a few nanoseconds.
Such rapid decoherence will facilitate applications in ultrafast speckle-free
full-field imaging
The Effects of Air and Underwater Blast on Composite Sandwich Panels and Tubular Laminate Structures
The resistance of glass-fibre reinforced polymer (GFRP) sandwich panels and laminate tubes to blast in air and underwater environments has been studied. Procedures for monitoring the structural response of such materials during blast events have been devised. High-speed photography was employed during the air-blast loading of GFRP sandwich panels, in conjunction with digital image correlation (DIC), to monitor the deformation of these structures under shock loading. Failure mechanisms have been revealed by using DIC and confirmed in post-test sectioning. Strain gauges were used to monitor the structural response of similar sandwich materials and GFRP tubular laminates during underwater shocks. The effect of the backing medium (air or water) of the target facing the shock has been identified during these studies. Mechanisms of failure have been established such as core crushing, skin/core cracking, delamination and fibre breakage. Strain gauge data supported the mechanisms for such damage. These studies were part of a research programme sponsored by the Office of Naval Research (ONR) investigating blast loading of composite naval structures. The full-scale experimental results presented here will aid and assist in the development of analytical and computational models. Furthermore, it highlights the importance of support and boundary conditions with regards to blast resistant design
Photonic Band Gaps in 3D Network Structures with Short-range Order
We present a systematic study of photonic band gaps (PBGs) in
three-dimensional (3D) photonic amorphous structures (PAS) with short-range
order. From calculations of the density of optical states (DOS) for PAS with
different topologies, we find that tetrahedrally connected dielectric networks
produce the largest isotropic PBGs. Local uniformity and tetrahedral order are
essential to the formation of PBGs in PAS, in addition to short-range geometric
order. This work demonstrates that it is possible to create broad, isotropic
PBGs for vector light fields in 3D PAS without long-range order.Comment: 6 pages, 8 figure
Double scattering of light from biophotonic nanostructures with short-range order
We investigate the physical mechanism for color production by isotropic
nanostructures with short-range order in bird feather barbs. While the primary
peak in optical scattering spectra results from constructive interference of
singly-scattered light, many species exhibit secondary peaks with distinct
characteristic. Our experimental and numerical studies show that these
secondary peaks result from double scattering of light by the correlated
structures. Without an analog in periodic or random structures, such a
phenomenon is unique for short-range ordered structures, and has been widely
used by nature for non-iridescent structural coloration.Comment: 10 pages, 4 figure
The role of input noise in transcriptional regulation
Even under constant external conditions, the expression levels of genes
fluctuate. Much emphasis has been placed on the components of this noise that
are due to randomness in transcription and translation; here we analyze the
role of noise associated with the inputs to transcriptional regulation, the
random arrival and binding of transcription factors to their target sites along
the genome. This noise sets a fundamental physical limit to the reliability of
genetic control, and has clear signatures, but we show that these are easily
obscured by experimental limitations and even by conventional methods for
plotting the variance vs. mean expression level. We argue that simple, global
models of noise dominated by transcription and translation are inconsistent
with the embedding of gene expression in a network of regulatory interactions.
Analysis of recent experiments on transcriptional control in the early
Drosophila embryo shows that these results are quantitatively consistent with
the predicted signatures of input noise, and we discuss the experiments needed
to test the importance of input noise more generally.Comment: 11 pages, 5 figures minor correction
Democratization in a passive dendritic tree : an analytical investigation
One way to achieve amplification of distal synaptic inputs on a dendritic tree is to scale the amplitude and/or duration of the synaptic conductance with its distance from the soma. This is an example of what is often referred to as “dendritic democracy”. Although well studied experimentally, to date this phenomenon has not been thoroughly explored from a mathematical perspective. In this paper we adopt a passive model of a dendritic tree with distributed excitatory synaptic conductances and analyze a number of key measures of democracy. In particular, via moment methods we derive laws for the transport, from synapse to soma, of strength, characteristic time, and dispersion. These laws lead immediately to synaptic scalings that overcome attenuation with distance. We follow this with a Neumann approximation of Green’s representation that readily produces the synaptic scaling that democratizes the peak somatic voltage response. Results are obtained for both idealized geometries and for the more realistic geometry of a rat CA1 pyramidal cell. For each measure of democratization we produce and contrast the synaptic scaling associated with treating the synapse as either a conductance change or a current injection. We find that our respective scalings agree up to a critical distance from the soma and we reveal how this critical distance decreases with decreasing branch radius
Revalidation Technique on Landslide Susceptibility Modelling: An Approach to Local Level Disaster Risk Management in Kuala Lumpur, Malaysia
Landslide susceptibility modelling in tropical climates is hindered by incomplete inventory due to rapid development and natural processes that obliterate field evidence, making validation a challenge. Susceptibility modelling was conducted in Kuala Lumpur, Malaysia using a new spatial partitioning technique for cross-validation. This involved a series of two alternating east-west linear zones, where the first zone served as the training dataset and the second zone was the test dataset, and vice versa. The results show that the susceptibility models have good compatibility with the selected landslide conditioning factors and high predictive accuracy. The model with the highest area under curve (AUC) values (SRC = 0.92, PRC = 0.90) was submitted to the City Council of Kuala Lumpur for land use planning and development control. Rainfall-induced landslides are prominent within the study area, especially during the monsoon period. An extreme rainfall event in December 2021 that triggered 122 landslides provided an opportunity to conduct retrospective validation of the model; the high predictive capability (AUC of PRC = 0.93) was reaffirmed. The findings proved that retrospective validation is vital for landslide susceptibility modelling, especially where the inventory is not of the best quality. This is to encourage wider usage and acceptance among end users, especially decision-makers in cities, to support disaster risk management in a changing climate
Single- and two-phase flow simulation based on equivalent pore network extracted from micro-CT images of sandstone core
Experimental Study on the Low-velocity Impact Behavior of Foam-core Sandwich Panels
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/97064/1/AIAA2012-1701.pd
- …