Reshaping plasmonic nanoparticles with laser pulses has been extensively researched as a tool for tuning their properties. However, in the absence of direct observations of the processes involved, important mechanistic details have remained elusive. Here, we present an in situ electron microscopy study of one such process that involves Coulomb fission of plasmonic nanoparticles under femtosecond laser irradiation. We observe that gold nanoparticles encapsulated in a silica shell fission by emitting progeny droplets comprised of about 10–500 atoms, with ejection preferentially occurring along the laser polarization direction. Under continued irradiation, the emitted droplets coalesce into a second core within the silica shell, and the system evolves into a dual-core particle. Our findings are consistent with a mechanism in which electrons are preferentially emitted from the gold core along the laser polarization direction. The resulting anisotropic charge distribution in the silica shell then determines the direction in which progeny droplets are ejected. In addition to yielding insights into the mechanism of Coulomb fission in plasmonic nanoparticles, our experiments point toward a facile method for forming surfaces decorated with aligned dual-gold-core silica shell particles.

Abstract-Future dense small-cell networks are one key 5G candidates to offer outdoor high access data rates, especially in millimeter wave (mmWave) frequency bands. At those frequencies, the free space propagation loss and shadowing (from buildings, vegetation or any kind of obstacles) are far stronger than in the traditional radio cellular spectrum. Therefore, the cell range is expected to be limited to 50 -100 meters, and directive high gain antennas are required at least for the base stations. This paper investigates the kind of topology that is required to serve a suburban area with a small-cell network operating at 60 GHz and equipped with beam-steering antennas. A real environment is considered to introduce practical deployment and propagation constraints. The analysis relies on Monte-Carlo system simulations with non-full buffer, and raybased predictions. The ray-tracing techniques are today identified as a relevant solution to capture the main channel properties impacting the beam-steering performance (angular dispersion, inter-link correlation); and the one involved in the present study was specifically enhanced to deal with detailed vegetation modeling. In addition to the user outage, the paper evaluates the evolution of the inter-cell interference along with the user density, and investigates the network behavior in case of local strong obstructions.

Ray-based and hybrid propagation models are today considered as valuable solutions to fulfill 5G wireless channel modeling requirements. They are a complement or alternative to the stochastic approaches when link-level and system-level simulations deal with millimeter-wave (mmWave), ultra-dense deployment and/or large antenna arrays. The present article proposes an extension of an urban ray-based model for the assessment of a 60-GHz outdoor small-cell network. The multi-paths are predicted from interactions with the static environment, but also with randomly-positioned vehicles and user-bodies. Both the vehicles and the user-body generate ray-path blockage, and (in case of the vehicle) new propagation paths. This sometimes affects the cell selection or beam orientation, and significantly changes the received signal strength and inter-cell interference. The user-body blockage is illustrated on two simple use cases (single-cell and two-cell scenarios). Then the impact of both stochastic components is assessed through the performance simulation of a whole mmWave small-cell network.

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