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We formulate a multi-physics model to describe the nonlinear propagation of a femtosecond, near-infrared, tightly focused laser pulse in a transparent dielectric. The application of our model to the case of bulk sapphire shows that even under extreme excitation conditions, ionization is universally clamped at about one tenth of the electron density in the upper valence band. The earlier estimate of ~10 TPa pressure that could be attainable through the internal excitation of transparent dielectrics by tightly focused ultrafast laser beams is shown to be off by two orders of magnitude.

We present the motivation, experience and learnings from a data challenge conducted at a large pharmaceutical corporation on the topic of subgroup identification. The data challenge aimed at exploring approaches to subgroup identification for future clinical trials. To mimic a realistic setting, participants had access to 4 Phase III clinical trials to derive a subgroup and predict its treatment effect on a future study not accessible to challenge participants. 30 teams registered for the challenge with around 100 participants, primarily from Biostatistics organisation. We outline the motivation for running the challenge, the challenge rules and logistics. Finally, we present the results of the challenge, the participant feedback as well as the learnings, and how these learnings can be translated into statistical practice.

The increased integration of intermittent and decentralised forms of power production has eroded the stability margins of power grids and made it more challenging to ensure reliable and secure power transmission. Reliable grid operation requires system-scale stability in response to perturbations in supply or load; previous studies have shown that this can be achieved by tuning the effective damping parameters of the generators in the grid. In this paper, we present and analyse the problem of tuning damping parameters when there is some uncertainty in the underlying system. We show that sophisticated methods that assume no uncertainty can yield results that are less robust than those produced by simpler methods. We define a quantile-based metric of stability that ensures that power grids remain stable even as worst-case scenarios are approached, and we develop optimisation methods for tuning damping parameters to achieve this stability. By comparing optimisation methods that rely on different assumptions, we suggest efficient heuristics for finding parameters that achieve highly stable and robust grids.

Fully microscopic many-body models based on inputs from first principle density functional theory are used to calculate the carrier losses due to radiative- and Auger-recombinations in bulk tellurium. It is shown that Auger processes dominate the losses for carrier densities in the range typical for applications as lasers. The Auger loss depends crucially on the energetic position of the $H_6$ valence bands. At cryogenic temperatures of 50$\,$K (100$\,$K) the Auger coefficient, $C$, varies by about six (three) orders of magnitude within the range of published distances between these bands and the valence bandedge. Values for $C$ at the high and low end of these ranges are found if the distance is smaller or larger than the bandgap, respectively. At room temperature the sensitivity is reduced to about a factor of four with $C$ values ranging between $0.4$ and $1.6\times 10^{-27}$cm$^6$s$^{-1}$. Here, radiative losses dominate for carrier densities up to about $10^{16}/$cm$^3$ with a loss coefficient $B\approx 10^{-11}$cm$^3$s$^{-1}$. The radiative losses are about two to three times lower than in typical bulk III-V materials for comparable wavelengths., Comment: 5 pages, 4 figures. Submitted to Applied Physics Letters on 07/27/2022

The United States is in a period of reckoning from which institutions of higher education are not exempt. Rather, we exist at the intersection of the chaos wrought by the novel coronavirus and the spread of outrage about systemic racism beyond Black, Indigenous, and People of Color (BIPoC) communities. The resulting almost universal sense of loss and uncertainty leads many of us to believe that individuals have little-to-no power to inform, never mind implement, change. Systemic problems require systemic solutions. There can be no return to "normal." We instead invite our assessment colleagues to question the unexamined assumptions which underlie our heretofore taken-for-granted approaches to assessing and documenting our students' learning; to reconnect with their foundational beliefs and values; and to fully engage with the uncertainty and complexity of the current moment. This paper offers readers a developmental approach for reflection, identifying potential leverage points, and intentionally creating a new assessment future which proactively includes "all" of our students.

An $\it{ab \,\, initio}$ based fully microscopic many-body approach is used to study the carrier relaxation dynamics in monolayer transition-metal dichalcogenides. Bandstructures and wavefunctions as well as phonon energies and coupling matrix elements are calculated using density functional theory. The resulting dipole and Coulomb matrix elements are implemented in the Dirac-Bloch equations to calculate carrier-carrier and carrier-phonon scatterings throughout the whole Brillouin zone. It is shown that carrier scatterings lead to a relaxation into hot quasi-Fermi distributions on a single femtosecond timescale. Carrier cool down and inter-valley transitions are mediated by phonon scatterings on a picosecond timescale. Strong, density-dependent energy renormalizations are shown to be valley-dependent. For MoTe$_2$, MoSe$_2$ and MoS$_2$ the change of energies with occupation is found to be about 50$\%$ stronger in the $\Sigma$ and $\Lambda$ side valleys than in the $K$ and $K'$ valleys. However, for realistic carrier densities, the materials always maintain their direct bandgap at the $K$ points of the Brillouin zone., Comment: 10 pages, 8 figures

We describe the design and x-ray emission properties (temporal, spatial, and spectral) of Dry Pinch I, a portable X-pinch driver developed at Imperial College London. Dry Pinch I is a direct capacitor discharge device, 300x300x700 mm3 in size and approximately 50 kg in mass, that can be used as an external driver for x-ray diagnostics in high-energy-density physics experiments. Among key findings, the device is shown to reliably produce 1.1 +- 0.3 ns long x-ray bursts that couple approximately 50 mJ of energy into photon energies from 1 to 10 keV. The average shot-to-shot jitter of these bursts is found to be 10 +- 4.6 ns using a combination of x-ray and current diagnostics. The spatial extent of the x-ray hot spot from which the radiation emanates agrees with previously published results for X-pinches suggesting a spot size of 10 +- 6 um in the soft energy region (1-10 keV) and 190 +- 100 um in the hard energy region (>10 keV). These characteristics mean that Dry Pinch I is ideally suited for use as a probe in experiments driven in the laboratory or at external facilities when more conventional sources of probing radiation are not available. At the same time, this is also the first detailed investigation of an X-pinch operating reliably at current rise rates of less than 1 kA/ns.

An $\it{ab \,\, initio}$ based fully microscopic approach is applied to study the nonlinear optical response of bulk Tellurium. The structural and electronic properties are calculated from first principles using the shLDA-1/2 method within density functional theory. The resulting bandstructure and dipole matrix elements serve as input for the quantum mechanical evaluation of the anisotropic linear optical absorption spectra yielding results in excellent agreement with published experimental data. Assuming quasi-equilibrium carrier distributions in the conduction and valence bands, absorption/gain and spontaneous emission spectra are computed from the semiconductor Bloch and luminescence equations. For ultrafast intense off-resonant excitation, the generation of high-harmonics is evaluated and the emission spectra are calculated for samples of different thickness., Comment: 10 pages, 9 figures

The influence of the growth conditions as well as the device design on the device performance of (GaIn)As/Ga(AsSb)/(GaIn)As "W"-quantum well lasers is investigated. To this purpose, the epitaxy process is scaled to full two inch substrates for improved homogeneity while the growth process is carried out in a single run for an improved quality. Furthermore, the optical confinement factor is increased by increasing the aluminum concentration within the cladding layers to a value of 65%. The procedure is carried out for devices with emission wavelengths of 1.26 micrometer as well as 1.30 micrometer. Differential efficiencies as high as 58% and threshold current densities as low as 0.16 kA/cm^2 are observed in case of devices emitting at 1.26 micrometer at room temperature. Furthermore, excellent characteristic temperatures of T_0=(72 plus minus 5)K and T_1=(293 plus minus 16) K are recorded in the temperature range between 10 degree Celsius and 100 degree Celsius. Devices emitting at 1.30 micrometer exhibit differential efficiencies of 31% and threshold current densities of 0.50 kA/cm^2 at room temperature. Further improvements of these properties and wavelength extension schemes are briefly discssused.

This paper discusses the temperature-dependent properties of (GaIn)As/Ga(AsSb)/(GaIn)As W-quantum well heterostructures for laser applications based on theoretical modeling as well as experimental findings. A microscopic theory is applied to discuss band bending effects giving rise to the characteristic blue shift with increasing charge carrier density observed in type-II heterostructures. Furthermore, gain spectra for a W-quantum well heterostructure are calculated up to high charge carrier densities. At these high charge carrier densities, the interplay between multiple type-II transitions results in broad and flat gain spectra with a spectral width of approximately 160 nm. Furthermore, the temperature-dependent properties of broad-area edge-emitting lasers are analyzed using electroluminescence as well as laser characteristic measurements. A first indication for the theoretically predicted broad gain spectra is presented and the interplay between the temperature-dependent red shift and the charge carrier density-dependent blue shift is discussed. A combination of these effects results in a significant reduction of the temperature-induced red shift of the emission wavelengths and even negative shift rates of (-0.10 plusminus 0.04) nm/K are achieved.

The LDA-1/2 method is employed in density functional theory calculations for the electronic structure of III-V dilute bismide systems. For the representative example of Ga(SbBi) with Bi concentrations below $10 \%$, it is shown that this method works very efficiently, especially due to its reasonably low demand on computer memory. The resulting bandstructure and wavefunctions are used to compute the interaction matrix elements that serve as input to microscopic calculations of the optical properties and intrinsic losses relevant for optoelectronic applications of dilute bismides.

LiDAR provides highly accurate 3D point clouds. However, data needs to be manually labelled in order to provide subsequent useful information. Manual annotation of such data is time consuming, tedious and error prone, and hence in this paper we present three automatic methods for annotating trees in LiDAR data. The first method requires high density point clouds and uses certain LiDAR data attributes for the purpose of tree identification, achieving almost 90% accuracy. The second method uses a voxel-based 3D Convolutional Neural Network on low density LiDAR datasets and is able to identify most large trees accurately but struggles with smaller ones due to the voxelisation process. The third method is a scaled version of the PointNet++ method and works directly on outdoor point clouds and achieves an F_score of 82.1% on the ISPRS benchmark dataset, comparable to the state-of-the-art methods but with increased efficiency.

Among observables characterising the random exploration of a graph or lattice, the cover time, namely the time to visit every site, continues to attract widespread interest. Much insight about cover times is gained by mapping to the (spaceless) coupon-collector problem, which amounts to ignoring spatio-temporal correlations, and an early conjecture that the limiting cover time distribution of regular random walks on large lattices converges to the Gumbel distribution in $d \geq 3$ was recently proved rigorously. Furthermore, a number of mathematical and numerical studies point to the robustness of the Gumbel universality to modifications of the \textit{spatial} features of the random search processes (e.g.\ introducing persistence and/or intermittence, or changing the graph topology). Here we investigate the robustness of the Gumbel universality to dynamical modification of the \textit{temporal} features of the search, specifically by allowing the random walker to "accelerate" or "decelerate" upon visiting a previously unexplored site. We generalise the mapping mentioned above by relating the statistics of cover times to the roughness of $1/f^\alpha$ Gaussian signals, leading to the conjecture that the Gumbel distribution is but one of a family of cover time distributions, ranging from Gaussian for highly accelerated cover, to exponential for highly decelerated cover. While our conjecture is confirmed by systematic Monte Carlo simulations in dimensions $d > 3$, our results for acceleration in $d=3$ challenge the current understanding of the role of correlations in the cover time problem., Comment: 10 pages, 15 figures