n-Si/SiGe quantum cascade structures for THz emission

In this work we report on modelling the electron transport in n-Si/SiGe structures. The electronic structure is calculated within the effective-mass complex-energy framework, separately for perpendicular (Xz) and in-plane (Xxy) valleys, the degeneracy of which is lifted by strain, and additionally by size quantization. The transport is described via scattering between quantized states, using the rate equations approach and tight-binding expansion, taking the coupling with two nearest-neighbour periods. The acoustic phonon, optical phonon, alloy and interface roughness scattering are taken in the model. The calculated U/I dependence and gain profiles are presented for a couple of QC structures

DESIGN, MECHANICAL MODELING AND 3D PRINTING OF KOCH FRACTAL CONTACT AND INTERLOCKING

Topological interlocking is an effective joining approach in both natural and engineering systems. Especially, hierarchical/fractal interlocking were found in many biological systems and can significantly enhance the system mechanical properties. Inspired by the hierarchical/fractal topology in nature, mechanical models for Koch fractal interlocking were developed as an example system to better understand the mechanics of fractal interlocking. In this investigation, Koch fractal interlocking with and without adhesive layers were designed for different number of iterations N. Theoretical contact mechanics model was used to capture the deformation mechanisms of the fractal interlocking with no adhesive layers under relatively small deformation. Then finite element (FE) simulations were performed to study the mechanical behavior of fractal interlocking under finite deformation. The designs were also fabricated via a multi-material 3D printer (Objet Connex 260) and mechanical experiments were performed to further explore the mechanical performance of the new designs. It was found that the load-bearing capacity of Kotch fractal interlocking can be effectively increased via fractal design. In general, when the fractal complexity (it is specifically represented as number of hierarchy N in the present Koch fractal design) increases, the stiffness of the fractal interlocking will increase significantly. Also, when N increases, the stress are more uniformly distributed along the fractal boundary of the top and bottom pieces of the fractal interlocking, which efficiently reduce local stress concentration, and therefore the overall strength of the interlocking also increases. However, the mechanical responses of fractal interlocks are also sensitive to imperfections, such as the gap between the interlocked pieces and the rounded tips. When fractal complexity increases, the mechanical properties will become more and more sensitive to the imperfection and eventually, the negative influences from imperfection can even become dominant. Therefore, considering the imperfection, there is an optimal level of fractal complexity to reach the maximum mechanical performance. This result is in consistent with fractal interlocks in different biological systems. Except topology, the influences of friction, material properties and damage evolution, and the adhesive layer on the mechanical performance of Koch fractal interlocking were also evaluated via non-linear FE simulations and mechanical experiments on 3D printed Koch interlocking specimens. It was found that the adhesive layer can significantly improve the load transmission of the fractal interlocking and therefore can effectively amplify the interlocking efficiency

Analysis of Time-Reversal-Based Propagation for Spatial focusing and Multiplexing

4 pagesInternational audienceTime-Reversal (TR) technique is well-known for itsability of focusing waves. A common claim, based on this focusing ability, supports that TR technique may allow spatial multiplexing and secure transmissions. In the present paper, measurements campaigns were performed in a reverberation chamber and in a standard office to assess the viability of TR technique. It is shown that the results obtained do not support the common claim, especially in standard indoor environment where the focusing ability is strongly degraded

Average Number of Significant Modes Excited in a Mode-Stirred Reverberation Chamber

International audienceAlthough the number of significant modes is intuitive, this concept has never been clearly defined, and this, mainly because of the unbound number of modes involved in modal overlap. In the present paper, we show that, for a perfect stirring process, the effect of modal overlap can be modeled as an equivalent filtering formulation. By introducing the statistical-bandwidth concept we show that the electromagnetic field statistics due to an infinite number of modes can be summarized by a finite number of significant modes. The case of the electric-energy density in an mode-stirred reverberation chamber (MSRC) has been considered and a new expression of its variability has been established. The good agreement found between the new expression and experimental and simulation results support the several concepts introduced in this paper