Zhigang Yan

Vertical-cavity surface-emitting lasers (VCSELs) are key light sources for short-reach optical interconnects, sensing, and three-dimensional imaging, where device performance and reliability critically depend on the structural integrity of multilayer heterostructures at the atomic scale. Here, we present a comprehensive atomic-scale investigation of an 850 nm GaAs-based VCSEL using aberration-corrected scanning transmission electron microscopy. In the quantum well (QW) active region, we directly determine the strain distribution, lattice distortion, and elemental distributions at the atomic scale, revealing that the dominant strain is localized inside the QW interior rather than at the nominal heterointerfaces. Atomic-resolution chemical mapping further uncovers pronounced cation-site inhomogeneity, with Al concentrations varying from 2.4 to 11.8 at. % and Ga contents fluctuating between 35.9 and 46.2 at. % across individual QW pairs, corresponding to a continuous compositional modulation from Al0.24Ga0.72In0.04As and Al0.04Ga0.90In0.06As. This internal compositional disparity gives rise to the largest lattice distortion at the QW centers, accounting for the emergence of unconventional strain localization within the active region. Moreover, atomic-resolution imaging and spectroscopy of the distributed Bragg reflectors reveal graded interfaces and intrinsic cation-site compositional fluctuations that deviate from ideal homogeneous alloy models, despite the preservation of overall lattice continuity. These atomic-scale observations establish a direct correlation between local compositional modulation and unconventional strain localization in VCSEL heterostructures, providing critical insights for the rational design and optimization of next-generation laser devices.

Rolling contact fatigue and wear of wheel–rail systems are critical factors affecting the safety of high-speed railways and have long been key research topics in materials science. This paper reviews the theoretical foundations of wheel–rail rolling contact fatigue, introduces representative experimental methods for studying rolling contact fatigue, and discusses the stress–strain problems in wheel–rail contact. Additionally, it provides a detailed overview of the emerging computational simulation approach for rolling contact fatigue wear, summarizes commonly used simulation software and their respective characteristics, and analyzes material factors influencing rolling contact fatigue simulations in wheel–rail steels. Finally, based on the classification of rolling contact fatigue algorithms, various measures are proposed to enhance rapid and accurate detection and evaluation.

A three-layer twin with two incoherent twin boundaries was identified using high-resolution transmission electron microscopy, and its twinning mechanism was analyzed. Twinning is formed by dissociated dislocations meeting on the adjacent slip plane.