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Liang G.,Nanyang Technological University | Liang H.,Singapore Institute of Manufacturing Technology | Zhang Y.,Singapore Institute of Manufacturing Technology | Khanna S.P.,University of Leeds | And 8 more authors.
2012 Photonics Global Conference, PGC 2012 | Year: 2012

Single-mode surface-emitting terahertz quantum cascade lasers with second-order concentric-circular-gratings are firstly demonstrated. The gratings are designed for single-mode operation and surface emission for efficient and directional optical power out-coupling. The first structure exhibits single-mode operation over the entire dynamic range with a side-mode-suppression-ratio of around 30 dB, and a six-fold rotationally symmetric far-field pattern, while a modified structure demonstrates single-mode operation with an improved beam divergence, whose far-field patteren consists of two lobes, each of which has a beam divergence within 13.5°×7° (full-width at half-maximum, FWHM). In addition, the devices show a peak output power approximately three times higher than in conventional ridge-waveguide lasers of similar size, whilst maintaining similar threshold current densities and maximum operating temperatures. © 2012 IEEE. Source


Zang C.,Shanghai JiaoTong University | Zang C.,Key Laboratory of Artificial Structures and Quantum Control | Sun H.,Shanghai JiaoTong University | Sun H.,Key Laboratory of Artificial Structures and Quantum Control | And 3 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

Using a recently developed first-principles approach for determining indentation strength, we performed calculations of the ideal strength of hexagonal Re, Re 3N, Re 2N, Re 2C, Re 2B, and ReB 2 in various shear deformation directions beneath the Vickers indentor. Our results show that the normal compressive pressure beneath the indentor weakens the strength of these electron-rich rhenium boride, carbide, and nitride compounds that belong to a distinct class of ultraincompressible and ultrahard materials. The reduction of indentation strength in these materials stems from lateral bond and volume expansions driven by the normal compressive pressure mediated by the high-density valence electrons in these structures. We compare the calculated indentation strength to the Poisson's ratio, which measures the lateral structural expansion, for the rhenium boride, carbide, and nitride compounds as well as diamond and cubic boron nitride. Our analysis indicates that although the normal pressure beneath the indentor generally leads to more significant reduction of indentation strength in materials with larger Poisson's ratios, crystal and electronic structures also play important roles in determining the structural response under indentation. The present study reveals structural deformation modes and the underlying atomistic mechanisms in transition-metal boride, carbide, and nitride compounds under the Vickers indentation. The results are distinctive from those of the traditional covalent superhard materials. The insights obtained from this work have important implications for further exploration and design of ultrahard materials. © 2012 American Physical Society. Source


Zang C.,Shanghai JiaoTong University | Zang C.,Key Laboratory of Artificial Structures and Quantum Control | Sun H.,Shanghai JiaoTong University | Sun H.,Key Laboratory of Artificial Structures and Quantum Control | Chen C.,University of Nevada, Las Vegas
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

Recently, synthesized WB 4 has attracted great interest because it exhibits the highest microindentation hardness among transition-metal light-element compounds. The latest theoretical studies [see, e.g., Zhang, Phys. Rev. Lett.10.1103/PhysRevLett.108.255502 108, 255502 (2012)] show, however, that the previously assigned WB 4 structure is unstable; a WB 3 structure was proposed as an alternative structural model. Here we show by first-principles calculations that the pressure beneath the indenter drives a lateral bond and volume expansion in the proposed WB 3 and related MoB 3 structures, resulting in an unexpectedly low indentation strength to a level well below that of ReB 2. This is in direct contradiction to experimental results that show WB 4 has higher indentation hardness compared to ReB 2. Moreover, the calculated normalized c/a ratio of WB 3 (and MoB 3) exhibits a negative pressure dependence, which is inconsistent with the experimentally observed trend. We therefore conclude that the proposed WB 3 structure is incompatible with experimental results and that the question of the crystal structure of the synthesized (nominal) WB 4 must be reopened for further study. © 2012 American Physical Society. Source

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