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)

Collapse of optical binding under secondary irradiation

When an optically trapped and bound micrometer-sized chain of particles is subjected to a suitably oriented secondary laser beam above a threshold level of intensity, the structure will collapse. The effect arises from modifications to the interparticle energy landscapes owing to the superposition of optically induced potentials. From the theory it also emerges that, for particles separated by near-field distances, optically induced assemblies may be continuously transformed between linear, spherical, and lamellar forms. The results show scope for the optical fabrication of moldable nanoscale structures

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

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

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.

Extreme stiffness hyperbolic elastic metamaterial for total transmission subwavelength imaging

Subwavelength imaging by metamaterials and extended work to pursue total transmission has been successfully demonstrated with electromagnetic and acoustic waves very recently. However, no elastic counterpart has been reported because earlier attempts suffer from considerable loss. Here, for the first time, we realize an elastic hyperbolic metamaterial lens and experimentally show total transmission subwavelength imaging with measured wave field inside the metamaterial lens. The main idea is to compensate for the decreased impedance in the perforated elastic metamaterial by utilizing extreme stiffness, which has not been independently actualized in a continuum elastic medium so far. The fabricated elastic lens is capable of directly transferring subwavelength information from the input to the output boundary. In the experiment, this intriguing phenomenon is confirmed by scanning the elastic structures inside the lens with laser scanning vibrometer. The proposed elastic metamaterial lens will bring forth significant guidelines for ultrasonic imaging techniquesope