We investigate the optical and opto-electronic properties of black silicon (b-Si) nanostructures passivated with Al 2O 3. The b-Si nanostructures significantly improve the absorption of silicon due to superior anti-reflection and light trapping properties. By coating the b-Si nanostructures with a conformal layer of Al 2O 3 by atomic layer deposition, the surface recombination velocity can be effectively reduced. We show that control of plasma-induced subsurface damage is equally important to achieve low interface recombination. Surface recombination velocities of S eff 13 cm / s have been measured for an optimized structure which, like the polished reference, exhibits lifetimes in the millisecond range

LT) can be achieved for weakly absorbed photons with energies close to the absorption edge of silicon. [ 15 ] These properties of b-Si are particularly useful for photovoltaic applications.The limiting effi ciency of a solar cell is given by the detailed balance of absorption and radiative recombination [ 16 ] and by nonradiative processes like Auger-and impurity recombination. [17][18][19] b-Si can help to approach those limits in two ways. On the one hand b-Si improves the coupling of light into the solar cell and the absorption of near band edge photons. This in turn increases the short circuit current and on a logarithmic scale also the open circuit voltage. On the other hand, due to excellent light-trapping properties b-Si might also allow reducing the solar cell thickness substantially below 100 µm while sustaining a high light absorption. This reduces nonradiative bulk recombination losses that scale linearly with the solar cell thickness [ 17,18 ] and hence, increases the open-circuit voltage. Of course, reducing the solar cell thickness also increases the cost effi ciency. Decreasing the amount of required silicon feedstock is a major industry concern as can be seen by the growing interest in kerf-free crystalline silicon solar cell technologies. [20][21][22] Unfortunately, besides bulk effects, surface recombination imposes a very critical limit to the solar This article presents an overview of the fabrication methods of black silicon, their resulting morphologies, and a quantitative comparison of their optoelectronic properties. To perform this quantitative comparison, different groups working on black silicon solar cells have cooperated for this study. The optical absorption and the minority carrier lifetime are used as benchmark parameters. The differences in the fabrication processes plasma etching, chemical etching, or laser processing are discussed and compared with numerical models. Guidelines to optimize the relevant physical parameters, such as the correlation length, optimal height of the nanostructures, and the surface defect densities for optoelectronic applications are given.

We combine ultrafast electron diffraction and time-resolved terahertz spectroscopy measurements to unravel the connection between structure and electronic transport properties during the photoinduced insulator-metal transitions in vanadium dioxide. We determine the structure of the metastable monoclinic metal phase, which exhibits anti-ferroelectric charge order arising from a thermally activated, orbital-selective phase transition in the electron system. The relative contribution of this photoinduced monoclinic metal (which has no equilibrium analog) and the photoinduced rutile metal (known from the equilibrium phase diagram) to the time and pump-fluence dependent multi-phase character of the film is established, as is the respective impact of these two distinct phase transitions on the observed changes in terahertz conductivity. Our results represent an important new example of how light can control the properties of strongly correlated materials and elucidate that multimodal experiements are essential when seeking a detailed connection between ultrafast changes in optical-electronic properties and lattice structure in complex materials.The insulator-metal transition (IMT) in vanadium dioxide (VO 2 ) is a benchmark problem in condensed matter physics 1-6 , as it provides a rich playground on which lattice-structural distortions and strong electron correlations conspire to determine emergent material properties. The equilibrium phase diagram of pure VO 2 involves a high-temperature tetragonal (rutile, R) metal that is separated from several structurally distinct lowtemperature insulating phases (monoclinic M 1 , M 2 and triclinic T ) depending on pressure or lattice strain. The transition to these lower-symmetry insulating phases occurs in the vicinity of room temperature and is sensitive to doping (Cr and W), making VO 2 interesting for a range of technological applications 7-9 . Since its discovery there has been a lively discussion in the literature about the driving force responsible for the IMT in VO 2 and the nature of the insulating and metallic phases that has revolved around the role and relative importance of electron-lattice and electron-electron interactions. The stark dichotomy between Peierls 2 and Mott 10 pictures characterizing the earliest explanations have recently given way to a nuanced view that the insulating phases of VO 2 are non-standard Mott-Hubbard systems where both electron-lattice and electron-electron interactions play important roles in determining the electronic properties of all the equilibrium phases 11-16 . Photoexcitation using ultrafast laser pulses has provided another route to initiate the transition between the insulating and metallic phases of VO 2 since it was discovered that the IMT occurs very rapidly following femtosecond laser excitation with sufficient fluence 17 . Since this discovery, VO 2 has been the focus of many time-resolved experiments including X-ray 18,19 and electron 20-22 diffraction, X-ray absorption 23,24 , photoemission 25 and optical spectroscopies 2...

We show that ultrafast electron diffuse (inelastic) scattering provides a detailed, momentum-resolved view of electron-phonon and phonon-phonon coupling across the entire Brillouin zone. Ultrafast time-resolution and scattering selection rules can distinguish between phonon branches without energyresolution.

Quantum two-level systems interacting with the surroundings are ubiquitous in nature. The interaction suppresses quantum coherence and forces the system towards a steady state. Such dissipative processes are captured by the paradigmatic spin-boson model, describing a two-state particle, the “spin”, interacting with an environment formed by harmonic oscillators. A fundamental question to date is to what extent intense coherent driving impacts a strongly dissipative system. Here we investigate experimentally and theoretically a superconducting qubit strongly coupled to an electromagnetic environment and subjected to a coherent drive. This setup realizes the driven Ohmic spin-boson model. We show that the drive reinforces environmental suppression of quantum coherence, and that a coherent-to-incoherent transition can be achieved by tuning the drive amplitude. An out-of-equilibrium detailed balance relation is demonstrated. These results advance fundamental understanding of open quantum systems and bear potential for the design of entangled light-matter states.