Caustic graphene plasmons with Kelvin angle

A century-long argument made by Lord Kelvin that all swimming objects have an effective Mach number of 3, corresponding to the Kelvin angle of 19.5 degree for ship waves, has been recently challenged with the conclusion that the Kelvin angle should gradually transit to the Mach angle as the ship velocity increases. Here we show that a similar phenomenon can happen for graphene plasmons. By analyzing the caustic wave pattern of graphene plasmons stimulated by a swift charged particle moving uniformly above graphene, we show that at low velocities of the charged particle, the caustics of graphene plasmons form the Kelvin angle. At large velocities of the particle, the caustics disappear and the effective semi-angle of the wave pattern approaches the Mach angle. Our study introduces caustic wave theory to the field of graphene plasmonics, and reveals a novel physical picture of graphene plasmon excitation during electron energy-loss spectroscopy measurement.Comment: 15 pages, 4 figure

Bulk-plasmon-mediated free-electron radiation beyond the conventional formation time

Free-electron radiation is a fundamental photon emission process that is induced by fast-moving electrons interacting with optical media. Historically, it has been understood that, just like any other photon emission process, free-electron radiation must be constrained within a finite time interval known as the "formation time", whose concept is applicable to both Cherenkov radiation and transition radiation, the two basic mechanisms describing radiation from a bulk medium and from an interface, respectively. Here we reveal an alternative mechanism of free-electron radiation far beyond the previously defined formation time. It occurs when a fast electron crosses the interface between vacuum and a plasmonic medium supporting bulk plasmons. While emitted continuously from the crossing point on the interface - thus consistent with the features of transition radiation - the extra radiation beyond the conventional formation time is supported by a long tail of bulk plasmons following the electron's trajectory deep into the plasmonic medium. Such a plasmonic tail mixes surface and bulk effects, and provides a sustained channel for electron-interface interaction. These results also settle the historical debate in Ferrell radiation, regarding whether it is a surface or bulk effect, from transition radiation or plasmonic oscillation.Comment: 37 pages, 13 figure

PictureBook: A Text-and-Image Summary System for Web Search Result

Abstract — Search engine technology plays an important role in Web information retrieval. However, with Internet informa-tion explosion, traditional searching techniques cannot provide satisfactory result due to problems such as huge number of result Web pages, unintuitive ranking, etc. Therefore, the reorganization and post-processing of Web search results have been extensively studied to help user effectively obtain useful information. Pre-vious studies mainly focused on Web page clustering, document summary, visualization of search results, etc, which are applied separately to either text or image search. In this paper, we propose a demo to illustrate a new Web search result summary system − PictureBook, which combines text and image retrieval using techniques of multiple document summarization and im-age semantics analysis. Particularly, audience can interactively investigate the effect of the combined text and image summar

Controlling Cherenkov angles with resonance transition radiation

Cherenkov radiation provides a valuable way to identify high energy particles in a wide momentum range, through the relation between the particle velocity and the Cherenkov angle. However, since the Cherenkov angle depends only on material's permittivity, the material unavoidably sets a fundamental limit to the momentum coverage and sensitivity of Cherenkov detectors. For example, Ring Imaging Cherenkov detectors must employ materials transparent to the frequency of interest as well as possessing permittivities close to unity to identify particles in the multi GeV range, and thus are often limited to large gas chambers. It would be extremely important albeit challenging to lift this fundamental limit and control Cherenkov angles as preferred. Here we propose a new mechanism that uses constructive interference of resonance transition radiation from photonic crystals to generate both forward and backward Cherenkov radiation. This mechanism can control Cherenkov angles in a flexible way with high sensitivity to any desired range of velocities. Photonic crystals thus overcome the severe material limit for Cherenkov detectors, enabling the use of transparent materials with arbitrary values of permittivity, and provide a promising option suited for identification of particles at high energy with enhanced sensitivity.Comment: There are 16 pages and 4 figures for the manuscript. Supplementary information with 18 pages and 5 figures, appended at the end of the file with the manuscript. Source files in Word format converted to PDF. Submitted to Nature Physic

Dynamically self-organized service composition in wireless ad hoc networks

Service composition is a powerful tool to create new services rapidly by reusing existing ones. Previous research mainly focuses on the wired infrastructure-based environment. With the developments in mobile devices and wireless communication technology in recent years, mobile ad hoc network has received an increasing attention as a new communication paradigm. However, the existing service composition techniques do not work any longer in an ad hoc environment. In this paper, we present the service composition problem in wireless ad hoc network with full consideration of the characteristics of an ad hoc environment. To solve this problem, we develop two service composition routing algorithms, Simple Broadcasting Service Composition and Behavior Evolution Service Composition. The main contribution of our algorithms is that the whole process of service composition is done by the cooperation of nodes on-the-fly instead of a centralized broker to meet the peculiarity of ad hoc networks. Finally, we describe an initial implementation architecture for service composition in wireless ad hoc networks