Mouad Bikerouin

High-index optical elements enable improved photon outcoupling from high-refractive index III–V emitters, but their feasibility on electrically driven GaAs light-emitting diodes (LEDs) has been limited by the stringent requirement of near-optical contact between the element and the LED. Here, we demonstrate a practical and reversible integration of a ZnSe hemispherical lens with a thin-film GaAs LED using a spring-preloaded tip/tilt fixture that enables alignment and in situ optical contact monitoring. Integrating the ZnSe hemisphere consistently increases the emitted optical power and external quantum efficiency by a factor of 4.5–5.5 relative to emission into air, while a thin, moderately convex layer of oil used as a reference yields an intermediate enhancement. In addition to improved extraction, the hemisphere also strongly suppresses the current-induced spectral redshift, consistent with reduced self-heating; a bandgap-only estimate indicates that the air-emitting LED operates approximately 10 K hotter than the LED coupled to the hemisphere at 10 mA. Simulations describing the effect of the residual air gap thickness on the enhancement show a rapid degradation of the extraction benefits for gap thicknesses already starting from around 20 nm. These results show that a high-index external hemisphere can simultaneously enhance light extraction and improve thermal stabilization, addressing two coupled limitations in high efficiency GaAs LEDs and thermophotonic devices.

A comprehensive study on the formation of micrometer‐sized, textured hexagonal diamond silicon (hd‐Si) crystals via nanoindentation followed by annealing is presented. Utilizing advanced characterization techniques such as polarized Raman spectroscopy, high‐resolution transmission electron microscopy, and electron energy‐loss spectroscopy, the successful transformation of silicon into high‐quality hd‐Si is demonstrated. The experimental results are further supported by first‐principles calculations and molecular dynamics simulations. Notably, the hd‐Si phase consists of nanometer‐sized grains with slight misorientations, organized into large micrometer‐scale textured domains. These findings underscore the potential of nanoindentation as a precise and versatile tool for inducing pressure‐driven phase transformations, particularly for the stabilization of hexagonal silicon. The textured nature of hd‐Si also presents a unique opportunity to tailor its optical properties, opening new avenues for its application in semiconductor and optoelectronic devices.

Janus monolayers, realized by breaking the vertical structural symmetry of two-dimensional (2D) materials, pave the way for a new era of high-quality and high-performance atomically-thin vertical p–n heterojunction solar cells. Herein, employing first-principles computations, Janus group-III chalcogenide monolayers, MX, M₂XY, MM′X₂ and MM′XY (M, M′ = Ga, In; X, Y = S, Se, Te), are deeply investigated in view of their implementation in 2D photovoltaic systems. Their stability analysis reveals that the 21 investigated monolayers are energetically, thermodynamically, mechanically, dynamically, and thermally stable, confirming their growth feasibility under ambient conditions. Furthermore, owing to their optimal band gap, high charge carrier mobilities, and strong light absorption, 2D Janus group-III monolayers are predicted as promising candidates for 2D excitonic solar cell applications. In fact, 46 type-II van der Waals (vdW) heterostructures with a lattice mismatch of less than 5% are identified by analyzing the band alignments of the investigated monolayers obtained through the HSE + SOC approach. In particular, 7 vertical vdW heterojunctions with a power conversion efficiency (PCE) higher than 20% are predicted and might be the focus of future experimental and theoretical studies. To further confirm the type II band alignment, the Ga₂STe–GaInS₂ vdW heterostructure, which reveals the highest PCE of 23.69%, is thoroughly investigated. Our results not only predict and evaluate stable 2D Janus group-III chalcogenide monolayers and vdW heterostructures, but also suggest that they could be used as materials for next-generation optoelectronic and photovoltaic devices.

The states of a single dopant centre in zinc-blende GaN-based conical quantum dots with spherical cap are theoretically investigated by analytically solving the corresponding effective mass equation taking advantage of the localisation of the ionised impurity at the cone apex. Nonlinear optical response is analysed through the calculation of the coefficients of optical absorption, relative refractive index change, and second and third harmonic generation, for the chosen set of allowed electron-donor states. The behaviour of the calculated optical quantities under changes in the geometry of the system due to variations in apical width and quantum dot radius is analysed and discussed.