Where to next? A dynamic model of user preferences

We consider the problem of predicting users’ preferences on online platforms. We build on recent findings suggesting that users’ preferences change over time, and that helping users expand their horizons is important in ensuring that they stay engaged. Most existing models of user preferences attempt to capture simultaneous preferences: “Users who like A tend to like B as well”. In this paper, we argue that these models fail to anticipate changing preferences. To overcome this issue, we seek to understand the structure that underlies the evolution of user preferences. To this end, we propose the Preference Transition Model (PTM), a dynamic model for user preferences towards classes of items. The model enables the estimation of transition probabilities between classes of items over time, which can be used to estimate how users’ tastes are expected to evolve based on their past history. We test our model’s predictive performance on a number of different prediction tasks on data from three different domains: music streaming, restaurant recommendations and movie recommendations, and find that it outperforms competing approaches. We then focus on a music application, and inspect the structure learned by our model. We find that the PTM uncovers remarkable regularities in users’ preference trajectories over time. We believe that these findings could inform a new generation of dynamic, diversity-enhancing recommender systems

Losses of plasmon surface waves on metallic grating

Abstract . Diffraction and absorption losses of plasmon surface waves (PSW) propagating along a metallic grating are investigated numerically as a function of groove depth . A periodicity of diffraction losses is found to exist . The energy flow distribution (EFD) above and inside the grooves is calculated and a similarity between the PSW on shallow and deep gratings is established above the grooves, while inside the grooves of deep gratings totally hidden curls in EFD are found to form . . Introduction Recently it has been discovered [1] that a close connection exists between different types of phenomena on metallic gratings : plasmon surface waves (PSW) excitation, non-Littrow perfect blazing It is well known that a pole of the scattering matrix corresponds to a solution of the homogeneous problem where nM is the complex refractive index of the substrate . For highly conducting metals Re (aP) > 1 and Im (aP) > 0, the latter corresponding to the energy absorbed in the metal as the PSW propagates along the interface . As the periodic modulation is introduced (h ;0), the PSW may be coupled to a propagating diffraction order(s) in the upper medium provided a suitable wavelength to period ratio 2/d is chosen . Radiation losses appear as a consequence of this and Im (aP) grows rather rapidly (for the results presented in figure 1 d=0. 5 µm and 2=0 . 6328 gm)

Poles and zeros of the scattering matrix associated to defect modes

We analyze electromagnetic waves propagation in one-dimensional periodic media with single or periodic defects. The study is made both from the point of view of the modes and of the diffraction problem. We provide an explicit dispersion equation for the numerical calculation of the modes, and we establish a connection between modes and poles and zeros of the scattering matrix.Comment: 6 pages (Revtex), no figure

A dual weighted residual method applied to complex periodic gratings

An extension of the dual weighted residual (DWR) method to the analysis of electromagnetic waves in a periodic diffraction grating is presented. Using the α,0-quasi-periodic transformation, an upper bound for the a posteriori error estimate is derived. This is then used to solve adaptively the associated Helmholtz problem. The goal is to achieve an acceptable accuracy in the computed diffraction efficiency while keeping the computational mesh relatively coarse. Numerical results are presented to illustrate the advantage of using DWR over the global a posteriori error estimate approach. The application of the method in biomimetic, to address the complex diffraction geometry of the Morpho butterfly wing is also discussed

Theoretical study of the anomalies of coated dielectric gratings

Abstract. The zero-order diffraction efficiency anomalies of a corrugated dielectric waveguide are studied theoretically in detail. A new and surprising phenomenon is observed: the efficiency changes from 0 to 100 per cent in the vicinity of the excitation of guided waves. The fundamental parameters of the system are found in the case where only one order is propagating and some of their properties are shown. The behaviour of the efficiency curves is explained by a phenomenological theory and a comparison with numerical rigorous results is made. Introduction Dielectric coatings are often deposited on the top of metallic gratings in order to protect the metal layer from oxidation and to increase the efficiency, most frequently for aluminium gratings working in the ultraviolet. Sometimes they are used on the top of silver gratings operating in the near infrared region. The presence of a dielectric layer, however, may drastically change the behaviour of the efficiency curves, as it has been pointed out by Palmer [1] for TE polarization and by Cowan and Arakawa [2] for TM polarization. The influence of a thick dielectric layer on the diffraction efficiency of a blazed aluminium grating has been investigated experimentally by Hutley et al. On the other hand, corrugated gratings on the top of dielectric waveguides are widely used in integrated optics as input or output couplers [6], filters, demultiplexers, etc

The Planetary Nebulae Spectrograph: the green light for Galaxy Kinematics

Planetary nebulae are now well established as probes of galaxy dynamics and as standard candles in distance determinations. Motivated by the need to improve the efficiency of planetary nebulae searches and the speed with which their radial velocities are determined, a dedicated instrument - the Planetary Nebulae Spectrograph or PN.S - has been designed and commissioned at the 4.2m William Herschel Telescope. The high optical efficiency of the spectrograph results in the detection of typically ~ 150 PN in galaxies at the distance of the Virgo cluster in one night of observations. In the same observation the radial velocities are obtained with an accuracy of ~ 20 km/sComment: Accepted by PASP, to appear November 2002; the figures have been degraded for archival purpose

Colloquium: Light scattering by particle and hole arrays

This colloquium analyzes the interaction of light with two-dimensional periodic arrays of particles and holes. The enhanced optical transmission observed in the latter and the presence of surface modes in patterned metal surfaces are thoroughly discussed. A review of the most significant discoveries in this area is presented first. A simple tutorial model is then formulated to capture the essential physics involved in these phenomena, while allowing analytical derivations that provide deeper insight. Comparison with more elaborated calculations is offered as well. Finally, hole arrays in plasmon-supporting metals are compared to perforated perfect conductors, thus assessing the role of plasmons in these types of structures through analytical considerations.Comment: 19 figure

Atomic diffraction from nanostructured optical potentials

We develop a versatile theoretical approach to the study of cold-atom diffractive scattering from light-field gratings by combining calculations of the optical near-field, generated by evanescent waves close to the surface of periodic nanostructured arrays, together with advanced atom wavepacket propagation on this optical potential.Comment: 8 figures, 10 pages, submitted to Phys. Rev.

Polarization state of the optical near-field

The polarization state of the optical electromagnetic field lying several nanometers above complex dielectric structures reveals the intricate light-matter interaction that occurs in this near-field zone. This information can only be extracted from an analysis of the polarization state of the detected light in the near-field. These polarization states can be calculated by different numerical methods well-suited to near--field optics. In this paper, we apply two different techniques (Localized Green Function Method and Differential Theory of Gratings) to separate each polarisation component associated with both electric and magnetic optical near-fields produced by nanometer sized objects. The analysis is carried out in two stages: in the first stage, we use a simple dipolar model to achieve insight into the physical origin of the near-field polarization state. In the second stage, we calculate accurate numerical field maps, simulating experimental near-field light detection, to supplement the data produced by analytical models. We conclude this study by demonstrating the role played by the near-field polarization in the formation of the local density of states.Comment: 9 pages, 11 figures, accepted for publication in Phys. Rev.