We present a study of the atom–surface interaction potential for the He–Bi2Se3(111) system. Using selective adsorption resonances, we are able to obtain the complete experimental band structure of atoms in the corrugated surface potential of the topological insulator Bi2Se3. He atom scattering spectra show several selective adsorption resonance features that are analyzed, starting with the free-atom approximation and a laterally averaged atom–surface interaction potential. Based on quantum mechanical calculations of the He–surface scattering intensities and resonance processes, we are then considering the three-dimensional atom–surface interaction potential, which is further refined to reproduce the experimental data. Following this analysis, the He–Bi2Se3(111) interaction potential is best represented by a corrugated Morse potential with a well depth of D = (6.54 ± 0.05) meV, a stiffness of κ = (0.58 ± 0.02) Å–1, and a surface electronic corrugation of (5.8 ± 0.2)% of the lattice constant. The experimental data may also be used as a challenging benchmark system to analyze the suitability of several van der Waals approaches: the He–Bi2Se3(111) interaction captures the fundamentals of weak adsorption systems where the binding is governed by long-range electronic correlations.

We have determined an atom-surface interaction potential for the He-Bi 2 Te 3 (111) system by analysing ultrahigh resolution measurements of selective adsorption resonances. The experimental measurements were obtained using 3 He spin-echo spectrometry. Following an initial free-particle model analysis, we use elastic close-coupling calculations to obtain a three-dimensional potential. The three-dimensional potential is then further refined based on the experimental data set, giving rise to an optimised potential which fully reproduces the experimental data. Based on this analysis, the He-Bi 2 Te 3 (111) interaction potential can be described by a corrugated Morse potential with a well depth D = (6.22 ± 0.05) meV, a stiffness κ = (0.92 ± 0.01)Å −1 and a surface electronic corrugation of (9.6 ± 0.2)% of the lattice constant. The improved uncertainties of the atom-surface interaction potential should also enable the use in inelastic closecoupled calculations in order to eventually study the temperature dependence and the line width of selective adsorption resonances.

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Applying a reverse bias near the breakdown voltage results in photon emission at the pn-junction in vertical cavity surface emitting lasers (VCSELs). This radiation can be collected with an emission microscope. Here, this technique is employed to investigate a high-power two dimensional (2D) VCSEL array with a large number of emitters at a non-degraded state and after high electrical stress. It has been found that non-degraded arrays show varying photon intensities across all emitters at breakdown condition while degraded arrays exhibit more intense electroluminescence at areas with faulty emitters containing defects in the active area as confirmed by plan view scanning transmission electron microscopy analysis.

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