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Detection of periodic structures through opaque metal layers by optical measurements of ultrafast electron dynamics
Get PDFWe report on femtosecond optical pump-probe measurements of ultrafast electron dynamics to detect the presence of gratings buried underneath optically opaque gold layers. Electron energy diffusion and cooling are found to be strongly affected by the presence and type of metal buried below the gold layer. As a result, the spatially periodic buried grating is encoded on the electron temperature near the top surface, leading to a spatially periodic modulation of the optical properties near the gold surface from which a delayed probe pulse can be diffracted. Our measurements show that these effects may be useful for optical detection and alignment applications in semiconductor device manufacturing.
Charge carrier-selective contacts for nanowire solar cells
Get PDFCharge carrier-selective contacts transform a light-absorbing semiconductor into a photovoltaic device. Current record efficiency solar cells nearly all use advanced heterojunction contacts that simultaneously provide carrier selectivity and contact passivation. One remaining challenge with heterojunction contacts is the tradeoff between better carrier selectivity/contact passivation (thicker layers) and better carrier extraction (thinner layers). Here we demonstrate that the nanowire geometry can remove this tradeoff by utilizing a permanent local gate (molybdenum oxide surface layer) to control the carrier selectivity of an adjacent ohmic metal contact. We show an open-circuit voltage increase for single indium phosphide nanowire solar cells by up to 335 mV, ultimately reaching 835 mV, and a reduction in open-circuit voltage spread from 303 to 105 mV after application of the surface gate. Importantly, reference experiments show that the carriers are not extracted via the molybdenum oxide but the ohmic metal contacts at the wire ends
Sign epistasis caused by hierarchy within signalling cascades
Get PDFSign epistasis is a central evolutionary constraint, but its causal factors remain difficult to predict. Here we use the notion of parameterised optima to explain epistasis within a signalling cascade, and test these predictions in Escherichia coli. We show that sign epistasis arises from the benefit of tuning phenotypic parameters of cascade genes with respect to each other, rather than from their complex and incompletely known genetic bases. Specifically, sign epistasis requires only that the optimal phenotypic parameters of one gene depend on the phenotypic parameters of another, independent of other details, such as activating or repressing nature, position within the cascade, intra-genic pleiotropy or genotype. Mutational effects change sign more readily in downstream genes, indicating that optimising downstream genes is more constrained. The findings show that sign epistasis results from the inherent upstream-downstream hierarchy between signalling cascade genes, and can be addressed without exhaustive genotypic mapping
High internal emission efficiency of silicon nanoparticles emitting in the visible range
Get PDFLight emitting silicon nanoparticles (Si-NPs) are interesting for lighting applications due to their non-toxicity, chemical robustness and photo-stability, however, are not practically considered due to their low emission efficiencies. While large Si-NPs emitting in the red to infrared spectral region show ensemble emission quantum efficiencies up to 60%, the emission efficiencies of smaller Si-NP, emitting in the visible spectral range are far lower, typically below 10-20%. In this work, we test this efficiency limit by measuring for the first time the internal quantum efficiency (IQE), i.e. the higher bound of the emission quantum efficiency, considering only the emissive NPs within the ensemble, of Si-NPs emitting in the visible spectral range between 350 and 650 nm. On the basis of photoluminescence decay measurements in a Drexhage geometry, we show that Si-NPs with organic passivation (C:Si-NPs) can have high direct bandgap-like radiative rates, which enable a high IQE over ~50%. In this way, we demonstrate that Si-NPs can in principle be considered a competitive candidate as a phosphor in lighting applications and medical imaging also in the visible spectral range. Moreover, our findings show that the reason for the much lower ensemble emission efficiency is due to the fact that the ensemble consists of a low fraction of emissive NPs, most likely due to a low PL ‘blinking’ duty cycle
Orientation of Methylguanidinium Ions at the Water-Air Interface
Get PDFWe use heterodyne-deteted vibrational sum-frequency generation (HD-VSFG) to determine the orientation of the molecular plane of methylguanidinium ions at the surface of aqueous solutions. We measure the VSFG response of the symmetric and asymmetric methyl stretch vibrations of the methylguanidinium ion with different polarization combinations. We find that for >80% of the methylguanidinium ions the molecular plane is at an angle >20 degrees with respect to the surface plane. Hence, for only a minor fraction of the ions the molecular plane has an orientation (near-)parallel to the surface plane, in contrast to the predictions of recent molecular dynamics simulation studies
Modeling the Performance Limitations and Prospects of Perovskite/Si Tandem Solar Cells under Realistic Operating Conditions
Get PDFPerovskite/Si tandem solar cells have the potential to considerably out-perform conventional solar cells. Under standard test conditions, perovskite/Si tandem solar cells already outperform the Si single junction. Under realistic conditions, however, as we show, tandem solar cells made from current record cells are hardly more efficient than the Si cell alone. We model the performance of
realistic perovskite/Si tandem solar cells under real-world climate conditions, by incorporating parasitic cell resistances, nonradiative recombination, and optical losses into the detailed-balance limit. We show quantitatively that when optimizing these parameters in the perovskite top cell, perovskite/Si tandem solar cells could reach efficiencies above 38% under realistic conditions, even while leaving the Si cell untouched. Despite the rapid efficiency increase of perovskite solar cells, our results emphasize the need for further material development, careful device design, and light management strategies, all necessary for highly efficient perovskite/Si tandem solar cells
Visible Light, Wide-Angle Graded Metasurface for Back Reflection
Get PDFMetasurfaces, or phase-engineered quasi-2D interfaces, enable a large degree of control over the reflection, refraction, and transmission of light. Here we demonstrate the design and realization of a visible light gradient metasurface tailored for highly efficient back reflection based on the Huygens-Fresnel principle. The metasurface emulates the functionality of a Littrow grating, capable of efficiently channeling light into the first negative Floquet order over a broad angular range and bandwidth at visible frequencies. Our theoretical results predict unitary efficiency for extremely low profiles and an optical response that is robust against discretization and design modifications. The experimentally realized metasurface is comprised of high-index TiO, nanowires over a protected Ag mirror, enabling back reflection with efficiency above 85% in the visible range, close to the reflectivity of the bare silver mirror. The presented analytical design methodology and the resulting low-profile device are advantageous compared to conventional gratings, while offering broadband efficiencies over a range of incidence angles
Coherent Atom-Phonon Interaction through Mode Field Coupling in Hybrid Optomechanical Systems
Get PDFWe propose a novel type of optomechanical coupling which enables a tripartite interaction between a quantum emitter, an optical mode, and a macroscopic mechanical oscillator. The interaction uses a mechanism we term mode field coupling: a mechanical displacement modifies the spatial distribution of the optical mode field, which, in turn, modulates the emitter-photon coupling rate. In properly designed multimode optomechanical systems, we can achieve situations in which mode field coupling is the only possible interaction pathway for the system. This enables, for example, swapping of a single excitation between emitter and phonon, creation of nonclassical states of motion, and mechanical ground-state cooling in the bad-cavity regime. Importantly, the emitter-phonon coupling rate can be enhanced through an optical drive field, allowing active control of the emitter-phonon coupling for realistic experimental parameters