Numerical simulation on directional solidification of Al-Ni-Co alloy based on FEM

The ratio, of the temperature gradient at the solidification front to the solidification rate of solid-liquid interface, plays a large part in columnar grain growth. The transient temperature fields of directional solidification of Al-Ni-Co alloy were studied by employing a finite element method. The temperature gradient at the solidification front and the solidification rate were analyzed for molten steels pouring at different temperatures. The results show that with different initial pouring temperatures, the individual ratio of the temperature gradient at solidification front to the solidification rate soars up in the initial stage of solidification, then varies within 2,000-6,000 ℃·s·cm-2, and finally goes down rapidly and even tend to be closed to each other when the solidification thickness reaches 5-6 cm. The simulation result is consistent with the practical production which can provide an available reference for process optimization of directional solidified Al-Ni-Co alloy

PS-TRUST: Provably Secure Solution for Truthful Double Spectrum Auctions

Truthful spectrum auctions have been extensively studied in recent years. Truthfulness makes bidders bid their true valuations, simplifying greatly the analysis of auctions. However, revealing one's true valuation causes severe privacy disclosure to the auctioneer and other bidders. To make things worse, previous work on secure spectrum auctions does not provide adequate security. In this paper, based on TRUST, we propose PS-TRUST, a provably secure solution for truthful double spectrum auctions. Besides maintaining the properties of truthfulness and special spectrum reuse of TRUST, PS-TRUST achieves provable security against semi-honest adversaries in the sense of cryptography. Specifically, PS-TRUST reveals nothing about the bids to anyone in the auction, except the auction result. To the best of our knowledge, PS-TRUST is the first provably secure solution for spectrum auctions. Furthermore, experimental results show that the computation and communication overhead of PS-TRUST is modest, and its practical applications are feasible.Comment: 9 pages, 4 figures, submitted to Infocom 201

NANOPHOTONIC DEVICES BASED ON LOW-DIMENSIONAL QUANTUM EMITTERS

Development of advanced nanophotonic devices is currently in rapid growth and revolutionizing the whole fields of integrated optics and photonics. Devices such as nanoscale LEDs & lasers, waveguide couplers and modulators are essential components in applications ranging from light sources to optical circuits and quantum information processing. The optical characteristics of these nanostructures could be engineered to realize strong confinement of optical modes within their low dimensions, which leads to strong light-matter interactions at desirable wavelength range when coupling to high-efficient, low-dimensional quantum emitters such as colloidal nanoplatelets, perovskite nanocrystals and transition metal dichalcogenide monolayers with unique optical properties. These light-matter coupled systems could realize various kinds of nanophotonic devices with high efficiency and nonlinearity in development of more complex optical circuits and quantum networks. In this thesis, I present my work first on experimental demonstration of spontaneous emission intensity and rate enhancement of both colloidal cadmium selenide/cadmium sulfide core/shell nanoplatelet and cesium halide bromide perovskite nanocrystals in Purcell regime by using silicon nitride photonic crystal nanobeam cavities. The one-dimensional high-quality cavity confines the emission in a small mode volume with high radiative decay mode density, leading to a clear increase in their photoluminescence efficiencies. We next present realization of a continuous-wave nanolaser based on this coupled system operating at room temperature. The high coupling efficiency results in a record-low pump threshold at 1 μW. This result shows that colloidal nanocrystals are suitable for compact and efficient opto-electronic devices based on solution-processable materials. Besides light generation, furthermore for transmission and processing, we have also realized chiral light-matter interactions in a glide-plane photonic crystal waveguide using spin-valley states in transition metal dichalcogenides tungsten diselenide monolayers. The combination between the unique spin-valley coupling effect of this monolayer material and the chirality of the waveguide leads to a control over the propagation direction based on the helicity of input signal. This system enables on-chip directional control of light and could provide new ways for controlling spin and valley degrees of freedom in a scalable photonic platform