3,422 research outputs found
Position-dependent diffusion of light in disordered waveguides
Diffusion has been widely used to describe a random walk of particles or
waves, and it requires only one parameter -- the diffusion constant. For waves,
however, diffusion is an approximation that disregards the possibility of
interference. Anderson localization, which manifests itself through a vanishing
diffusion coefficient in an infinite system, originates from constructive
interference of waves traveling in loop trajectories -- pairs of time-reversed
paths returning to the same point. In an open system of finite size, the return
probability through such paths is reduced, particularly near the boundary where
waves may escape. Based on this argument, the self-consistent theory of
localization and the supersymmetric field theory predict that the diffusion
coefficient varies spatially inside the system. A direct experimental
observation of this effect is a challenge because it requires monitoring wave
transport inside the system. Here, we fabricate two-dimensional photonic random
media and probe position-dependent diffusion inside the sample from the third
dimension. By varying the geometry of the system or the dissipation which also
limits the size of loop trajectories, we are able to control the
renormalization of the diffusion coefficient. This work shows the possibility
of manipulating diffusion via the interplay of localization and dissipation.Comment: 24 pages, 6 figure
NPRF: A Neural Pseudo Relevance Feedback Framework for Ad-hoc Information Retrieval
Pseudo-relevance feedback (PRF) is commonly used to boost the performance of
traditional information retrieval (IR) models by using top-ranked documents to
identify and weight new query terms, thereby reducing the effect of
query-document vocabulary mismatches. While neural retrieval models have
recently demonstrated strong results for ad-hoc retrieval, combining them with
PRF is not straightforward due to incompatibilities between existing PRF
approaches and neural architectures. To bridge this gap, we propose an
end-to-end neural PRF framework that can be used with existing neural IR models
by embedding different neural models as building blocks. Extensive experiments
on two standard test collections confirm the effectiveness of the proposed NPRF
framework in improving the performance of two state-of-the-art neural IR
models.Comment: Full paper in EMNLP 201
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Incorporating Occupant Behaviour and Comfort in Domestic Energy Retrofit
While a large energy saving potential exists in domestic retrofit to meet the UK Government’s goal of carbon emissions reduction through the Climate Change Act 2008, the complexity of occupant behaviour and comfort needs often prevents the seemingly achievable retrofit targets from being reached. These real-life complexities contrast starkly with the standardised and simplistic behavioural profiles currently used in energy modelling to generate retrofit recommendations. The provision of more detailed information concerning behaviour and comfort needs can help improve energy consumption predictions and enable policy interventions to respond to different household types while maintaining a comfortable indoor environment for occupants. This research combines both social and technical methods to develop a tailored approach for domestic retrofit using household archetypes. An archetype is a typical example of households sharing similar behavioural patterns and dwelling physical characteristics. On the one hand, it uses first-hand interviews and household surveys to understand people’s social practices and patterns of behaviour. From these
surveys, key household types based on household behaviours and dwelling characteristics are identified. On the other hand, the energy performance of buildings is monitored, and a model is developed incorporating these distinct household types to determine energy use. This modelling increases the accuracy of predictions concerning which retrofit strategies are most effective for each household type. The use of household archetypes to analyse energy and cost implications can bring about significant savings compared to the conventional approach, which treats occupancy and behaviour in a homogeneous manner. Overall, this research suggests that a tailored approach, incorporating human behaviour, to domestic retrofit can considerably improve energy savings without compromising occupant comfort.Cambridge Trust, Wolfson College Cambridge, Lundgren Research Awards, GBCET Chinese Student Awards and Henry Lester Trust Awar
Relation between Transmission and Energy Stored in Random Media with Gain
In this work, we investigate a possibility of using the ratio between optical transmission, T, and energy stored inside the system, E, as a quantitative measure of the enhanced mesoscopic corrections to diffusive transport of light through a random medium with gain. We obtain an expression for T/E as a function of amplification strength in the diffusive approximation and show that it does not a have tendency to diverge when the threshold for random lasing is approached, as both T and E do. Furthermore, we find that a change in T/E signifies a change in the electric field distribution inside the random medium. In the localization regime, we also investigate the correlations between transmission and energy stored in the medium with and without amplification. Our results suggest that T/E is a promising parameter which can help characterize the nature of wave transport in random medium with gain
Hot carrier diffusion in graphene
We report an optical study of charge transport in graphene. Diffusion of hot
carriers in epitaxial graphene and reduced graphene oxide samples are studied
using an ultrafast pump-probe technique with a high spatial resolution.
Spatiotemporal dynamics of hot carriers after a point-like excitation are
monitored. Carrier diffusion coefficients of 11,000 and 5,500 squared
centimeters per second are measured in epitaxial graphene and reduced graphene
oxide samples, respectively, with a carrier temperature on the order of 3,600
K. The demonstrated optical techniques can be used for non-contact and
non-invasive in-situ detection of transport properties of graphene.Comment: 5 pages, 3 figure
1,3-Bis{[5-(pyridin-2-yl)-1,3,4-oxadiazol-2-yl]sulfanyl}propan-2-one
In the distorted W-shaped molecule of the title compound, C17H12N6O3S2, a twofold axis passes through the carbonyl group. The molecules stack in the crystal through π–π interactions [centroid—centroid distance = 3.883 Å] and weak C—H⋯N hydrogen-bonding interactions, forming a three-dimensional architecture
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