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

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