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Clicks for doughnuts
Get PDFThe ideas of topology are breaking ground in origami-based metamaterials. Experiments now show that certain
shapes — doughnuts included — exhibit topological bistability, and can be made to click between different
topologically stable states
Optical circulation in a multimode optomechanical resonator
Get PDFBreaking the symmetry of electromagnetic wave propagation enables important technological functionality. In particular, circulators are nonreciprocal components that can route photons directionally in classical or quantum photonic circuits and offer prospects for fundamental research on electromagnetic transport. Developing highly efficient circulators thus presents an important challenge, especially to realise compact reconfigurable implementations that do not rely on magnetic fields to break reciprocity. We demonstrate optical circulation utilising radiation pressure interactions in an on-chip multimode optomechanical system. Mechanically mediated optical mode conversion in a silica microtoroid provides a synthetic gauge bias for light, enabling four-port circulation that exploits tailored interference between appropriate light paths. We identify two sideband conditions under which ideal circulation is approached. This allows to experimentally demonstrate ~10 dB isolation and <3 dB insertion loss in all relevant channels. We show the possibility of actively controlling the circulator properties, enabling ideal opportunities for reconfigurable integrated nanophotonic circuits
Nanoscale chiral valley-photon interface through optical spin-orbit coupling
Get PDFThe emergence of two-dimensional transition metal dichalcogenide materials has sparked intense activity in valleytronics, as their valley information can be encoded and detected with the spin angular momentum of light. We demonstrate the valley-dependent directional coupling of light using a plasmonic nanowire–tungsten disulfide (WS2) layers system. We show that the valley pseudospin in WS2 couples to transverse optical spin of the same handedness with a directional coupling efficiency of 90 ± 1%. Our results provide a platform for controlling, detecting, and processing valley and spin information with precise optical control at the nanoscale
Understanding Detrimental and Beneficial Grain Boundary Effects in Halide Perovskites
Get PDFGrain boundaries play a key role in the performance of thin‐film optoelectronic devices and yet their effect in halide perovskite materials is still not understood. The biggest factor limiting progress is the inability to identify grain boundaries. Noncrystallographic techniques can misidentify grain boundaries, leading to conflicting literature reports about their influence; however, the gold standard – electron backscatter diffraction (EBSD) – destroys halide perovskite thin films. Here, this problem is solved by using a solid‐state EBSD detector with 6000 times higher sensitivity than the traditional phosphor screen and camera. Correlating true grain size with photoluminescence lifetime, carrier diffusion length, and mobility shows that grain boundaries are not benign but have a recombination velocity of 1670 cm s−1, comparable to that of crystalline silicon. Amorphous grain boundaries are also observed that give rise to locally brighter photoluminescence intensity and longer lifetimes. This anomalous grain boundary character offers a possible explanation for the mysteriously long lifetime and record efficiency achieved in small grain halide perovskite thin films. It also suggests a new approach for passivating grain boundaries, independent of surface passivation, to lead to even better performance in optoelectronic devices
The Influence of Radiotherapy on the Mechanical Properties of Silicone Breast Implants
Get PDFBackground: Silicone breast implants have been used for decades for cosmetic breast augmentation or reconstruction after mastectomy. In selected cases, postmastectomy adjuvant radiotherapy is given with the breast implants in situ. Previous clinical studies have shown that radiotherapy may lead to complications such as capsular contracture and infection and that removal of the implant may be required. Yet, the effect of radiotherapy on silicone breast implants themselves is unknown. The aim of this study was to investigate if irradiation of breast implants influences their mechanical properties.
Methods: This was an ex vivo study on 32 ready-to-use silicone breast implants (Mentor and Silimed). Half of the implants of each brand were irradiated with 1 × 60 Gy, the other half were not irradiated. Tensile, mechanical hysteresis, and rheology tests were performed. Differences in mechanical properties between the irradiated and nonirradiated implants were determined.
Results: No significant differences were found in tensile strength, mechanical hysteresis, and rheological properties between irradiated and nonirradiated implants.
Conclusions: Breast implants’ mechanical properties for these 2 brands were not significantly affected after single-dose irradiation in an ex vivo setting
Effect of Temperature Cycling on Ostwald Ripening
Get PDFWe study the effect of temperature cycling on the rate of Ostwald ripening (or coarsening) of spherical particles dispersed in a binary solution. A widespread view, which states a temperature cycle generally enhances the rate of Ostwald ripening by first dissolving the smallest particles (heating) and then regrowing the dissolved amount of material on the remaining particles (cooling), is shown to be inadequate as it does not include transient effects. On the basis of a simulation method that assumes mass transfer as the limiting growth mechanism, we show that each temperature cycle is followed by a significant relaxation of the particle-size distribution, during which the number of particles remains constant, and the average particle size decreases. The relaxation is shown to be crucial to obtain a linear scaling of the average particle radius cubed with the number of cycles applied (or time), which is the behavior generally observed for the evolution of ice crystals in cycling experiments on frozen aqueous solutions or frozen foods. We show the experimentally observed increase in the proportionality constant (or "coarsening rate") as compared to isothermal ripening, or the increase of the coarsening rate with increasing cycle frequency, can be reproduced convincingly only if some (transient) ripening is allowed to take place at the elevated temperature of each cycle. Our results thus suggest the effect of temperature cycling on Ostwald ripening is governed by a dissolution-ripening-regrowth-relaxation mechanism
Temperature-Induced Collapse of Elastin-Like Peptides Studied by 2DIR Spectroscopy
Get PDFElastin-like peptides (ELP) are hydrophobic biopolymers that exhibit a reversible coacervation transition when the temperature is raised above a critical point. Here we use a combination of linear infrared spectroscopy, two-dimensional infrared (2DIR) spectroscopy and molecular dynamics simulations to study the structural dynamics of two elastin-like peptides. Specfically, we investigate the effect of the solvent environ- ment and temperature on the structural dynamics of a short (5-residue) elastin-like peptide and of a long (450-residue) elastin-like peptide. We identify two vibrational energy transfer processes that take place within the amide I0 band of both peptides. We observe that the rate constant of one of the exchange processes is strongly dependent on the solvent environment and argue that the coacervation transition is accompanied by a desolvation of the peptide backbone where up to 75% of the water molecules are displaced. We also study the spectral diffusion dynamics of a valine residue that is present in both peptides. We find that these dynamics are relatively slow and indicative of an amide group that is shielded from the solvent. We conclude that the coacerva- tion transition of elastin-like peptides is probably not associated with a conformational change involving this residue
Probing Plasmon-NV0 Coupling at the Nanometer Scale with Photons and Fast Electrons
Get PDFThe local density of optical states governs an emitters' lifetime and quantum yield through the Purcell effect. It can be modified by a surface plasmon electromagnetic field, but such a field has a spatial extension limited to a few hundreds of nanometers, complicating the use of optical methods to spatially probe emitter-plasmon coupling. Here we show that a combination of electron-based imaging, spectroscopies, and photon-based correlation spectroscopy enables measurement of the Purcell effect with nanometer and nanosecond spatiotemporal resolutions. Due to the large variability of radiative lifetimes of emitters in nanoparticles we relied on a statistical approach to probe the coupling between nitrogen-vacancy centers in nanodiamonds and surface plasmons in silver nanocubes. We quantified the Purcell effect by measuring the nitrogen-vacancy excited state lifetimes in a large number of either isolated nanodiamonds or nanodiamond-nanocube dimers and demonstrated a significant lifetime reduction for dimers
Perovskite/Colloidal Quantum Dot Tandem Solar Cells: Theoretical Modeling and Monolithic Structure
Get PDFMetal-halide perovskite-based tandem solar cells show great promise for overcoming the Shockley–Queisser single-junction efficiency limit via low-cost tandem structures, but so far, they employ conventional bottom-cell materials that require stringent processing conditions. Meanwhile, difficulty in achieving low-bandgap (11% absolute gain) to the ultimate efficiency via photon recycling. We report an initial experimental demonstration of a solution-processed monolithic perovskite/CQD tandem solar cell, showing evidence for subcell voltage addition. We model that a power conversion efficiency of 29.7% is possible by combining state-of-the-art perovskite and CQD solar cells
