Shell Filling and Trigonal Warping in Graphene Quantum Dots.

Transport measurements through a few-electron circular quantum dot in bilayer graphene display bunching of the conductance resonances in groups of four, eight, and twelve. This is in accordance with the spin and valley degeneracies in bilayer graphene and an additional threefold "minivalley degeneracy" caused by trigonal warping. For small electron numbers, implying a small dot size and a small displacement field, a two-dimensional s shell and then a p shell are successively filled with four and eight electrons, respectively. For electron numbers larger than 12, as the dot size and the displacement field increase, the single-particle ground state evolves into a threefold degenerate minivalley ground state. A transition between these regimes is observed in our measurements and can be described by band-structure calculations. Measurements in the magnetic field confirm Hund's second rule for spin filling of the quantum dot levels, emphasizing the importance of exchange interaction effects.

Excited States in Bilayer Graphene Quantum Dots.

We report ground- and excited-state transport through an electrostatically defined few-hole quantum dot in bilayer graphene in both parallel and perpendicular applied magnetic fields. A remarkably clear level scheme for the two-particle spectra is found by analyzing finite bias spectroscopy data within a two-particle model including spin and valley degrees of freedom. We identify the two-hole ground state to be a spin-triplet and valley-singlet state. This spin alignment can be seen as Hund's rule for a valley-degenerate system, which is fundamentally different from quantum dots in carbon nanotubes, where the two-particle ground state is a spin-singlet state. The spin-singlet excited states are found to be valley-triplet states by tilting the magnetic field with respect to the sample plane. We quantify the exchange energy to be 0.35 meV and measure a valley and spin g factor of 36 and 2, respectively.

Reciprocity approach for calculating the Purcell effect for emission into an open optical system.

Based on the reciprocity theorem, we present a formalism to calculate the power emitted by a dipole source into a particular propagating mode leaving an open optical system. The open system is completely arbitrary and the approach can be used in analytical calculations but may also be combined with numerical electromagnetic solvers to describe the emission of light sources into complex systems. We exemplify the use of the formalism in numerical simulations by analyzing the emission of a dipole that is placed inside a cavity with connected single mode exit waveguide. Additionally, we show at the example of a practical ring resonator system how the approach can be applied to systems that offer multiple electromagnetic energy decay channels. As a consequence of its inherent simplicity and broad applicability, the approach may serve as a powerful and practical tool for engineering light-matter-interaction in a variety of active optical systems.

Emission enhancement in dielectric nanocomposites.

We consider emitting nanoparticles in dielectric nanocomposites with varying refractive index contrast and geometry. For that we develop a simple and universal method to calculate the emission enhancement in nanocomposites that employs solely the calculation of the effective refractive index and electric field distributions from three quasistatic calculations with orthogonal polarizations. The method is exemplified for dilute nanocomposites without electromagnetic interaction between emitting particles as well as for dense nanocomposites with strong particle interaction. We show that the radiative decay in dielectric nanocomposites is greatly affected by the shape and arrangement of its constituents and give guidelines for larger enhancement.

Dynamic measurement of near-field radiative heat transfer.

Super-Planckian near-field radiative heat transfer allows effective heat transfer between a hot and a cold body to increase beyond the limits long known for black bodies. Until present, experimental techniques to measure the radiative heat flow relied on steady-state systems. Here, we present a dynamic measurement approach based on the transient plane source technique, which extracts thermal properties from a temperature transient caused by a step input power function. Using this versatile method, that requires only single sided contact, we measure enhanced radiative conduction up to 16 times higher than the blackbody limit on centimeter sized glass samples without any specialized sample preparation or nanofabrication.

Electrochemical tuning of the optical properties of nanoporous gold.

Using optical in-situ measurements in an electrochemical environment, we study the electrochemical tuning of the transmission spectrum of films from the nanoporous gold (NPG) based optical metamaterial, including the effect of the ligament size. The long wavelength part of the transmission spectrum around 800 nm can be reversibly tuned via the applied electrode potential. The NPG behaves as diluted metal with its transition from dielectric to metallic response shifted to longer wavelengths. We find that the applied potential alters the charge carrier density to a comparable extent as in experiments on gold nanoparticles. However, compared to nanoparticles, a NPG optical metamaterial, due to its connected structure, shows a much stronger and more broadband change in optical transmission for the same change in charge carrier density. We were able to tune the transmission through an only 200 nm thin sample by 30%. In combination with an electrolyte the tunable NPG based optical metamaterial, which employs a very large surface-to-volume ratio is expected to play an important role in sensor applications, for photoelectrochemical water splitting into hydrogen and oxygen and for solar water purification.

Controlling thermal emission with refractory epsilon-near-zero metamaterials via topological transitions.

Control of thermal radiation at high temperatures is vital for waste heat recovery and for high-efficiency thermophotovoltaic (TPV) conversion. Previously, structural resonances utilizing gratings, thin film resonances, metasurfaces and photonic crystals were used to spectrally control thermal emission, often requiring lithographic structuring of the surface and causing significant angle dependence. In contrast, here, we demonstrate a refractory W-HfO2 metamaterial, which controls thermal emission through an engineered dielectric response function. The epsilon-near-zero frequency of a metamaterial and the connected optical topological transition (OTT) are adjusted to selectively enhance and suppress the thermal emission in the near-infrared spectrum, crucial for improved TPV efficiency. The near-omnidirectional and spectrally selective emitter is obtained as the emission changes due to material properties and not due to resonances or interference effects, marking a paradigm shift in thermal engineering approaches. We experimentally demonstrate the OTT in a thermally stable metamaterial at high temperatures of 1,000 °C.

Tungsten band edge absorber/emitter based on a monolayer of ceramic microspheres.

We report on a band edge absorber/emitter design for high-temperature applications based on an unstructured tungsten substrate and a monolayer of ceramic microspheres. The absorber was fabricated as a monolayer of ZrO(2) microparticles on a tungsten layer with a HfO(2) nanocoating. The band edge of the absorption is based on critically coupled microsphere resonances. It can be tuned from visible to near-infrared range by varying the diameter of the microparticles. The absorption properties were found to be stable up to 1000°C.

Measurement Back-Action in Stacked Graphene Quantum Dots.

We present an electronic transport experiment in graphene where both classical and quantum mechanical charge detector back-action on a quantum dot are investigated. The device consists of two stacked graphene quantum dots separated by a thin layer of boron nitride. This device is fabricated by van der Waals stacking and is equipped with separate source and drain contacts to both dots. By applying a finite bias to one quantum dot, a current is induced in the other unbiased dot. We present an explanation of the observed measurement-induced current based on strong capacitive coupling and energy dependent tunneling barriers, breaking the spatial symmetry in the unbiased system. This is a special feature of graphene-based quantum devices. The experimental observation of transport in classically forbidden regimes is understood by considering higher-order quantum mechanical back-action mechanisms.

Ultrasonic velocity as a predictor of strength in bovine cancellous bone.

Ultrasonic methods measure properties intrinsic to bone's structure that are not necessarily dependent on bone mass. Therefore, ultrasound may prove to be a useful tool for diagnosing bone fragility and osteoporosis. The goal of this study was to determine the relationship between apparent velocity of ultrasound (AVU) measured using the OsteoTechnology prototype I machine and cancellous bone yield strength (sigma y). AVU correlated well with sigma y (r = 0.753). Consistent with theory, the best predictor of cancellous bone strength was the combination of apparent density (rho a) and AVU, rho a (AVU)2, which had an r2 of 77.4%.

Abdominal CT in children with neurologic impairment following blunt trauma. Abdominal CT in comatose children.

This paper examines the role of neurologic impairment as an indication for CT examination of the abdomen in children after blunt trauma. The clinical information and abdominal CT examinations of 482 consecutive children were reviewed prospectively for indications for abdominal CT and presence and severity of abdominal and chest injury. Children were divided into two groups determined by Glasgow Coma Scale (GCS): GCS less than 8, and greater than or equal to 8. The prevalence and severity of thoracoabdominal injury were higher in the neurologically impaired group. These children had a higher frequency of abdominal injury (GCS less than 8, 25 of 90 patients (27.8%) vs. GCS greater than or equal to 8, 70 of 392 patients (17.8%); p = 0.047 by Chi square test), injury to multiple abdominal organs (16.7% vs. 4.8%; p = 0.0002), chest injury (32.2% vs. 0.09%; p = 0.0001), and combined chest and abdominal injury (18.9% vs. 4.6%; p = 0.0001). In addition, the mortality rate in children with a GCS less than 8 was significantly higher (GCS less than 8, 24% vs. GCS greater than or equal to 8, 0.26%; p = 0.0001). Eleven children had a GCS less than 8 as the only indication for abdominal CT examination. All 11 children had a normal CT of the abdomen. Every child with abdominal injury on CT scan had specific abdominal signs suggestive of underlying injury. Three neurologically impaired children required abdominal surgery (3.3%) vs. 14 of 369 (3.8%) children with a GCS greater than or equal to 8; p = NS). We conclude that children with severe neurologic impairment are at higher risk for intraabdominal injury than those without coma, but that neurologic impairment without abdominal signs is a low-yield indication for abdominal CT examination. Abdominal CT scan should be reserved for children in whom there is a high clinical index of suspicion of significant abdominal trauma based on physical examination and the mechanism of injury.