Micropatterned Electrostatic Traps for Indirect Excitons in Coupled GaAs Quantum Wells

We demonstrate an electrostatic trap for indirect excitons in a field-effect structure based on coupled GaAs quantum wells. Within the plane of a double quantum well indirect excitons are trapped at the perimeter of a SiO2 area sandwiched between the surface of the GaAs heterostructure and a semitransparent metallic top gate. The trapping mechanism is well explained by a combination of the quantum confined Stark effect and local field enhancement. We find the one-dimensional trapping potentials in the quantum well plane to be nearly harmonic with high spring constants exceeding 10 keV/cm^2.Comment: 21 pages, 6 figures, submitted to Phys. Rev.

Drift mobility of long-living excitons in coupled GaAs quantum wells

We observe high-mobility transport of indirect excitons in coupled GaAs quantum wells. A voltage-tunable in-plane potential gradient is defined for excitons by exploiting the quantum confined Stark effect in combination with a lithographically designed resistive top gate. Excitonic photoluminescence resolved in space, energy, and time provides insight into the in-plane drift dynamics. Across several hundreds of microns an excitonic mobility of >10^5 cm2/eVs is observed for temperatures below 10 K. With increasing temperature the excitonic mobility decreases due to exciton-phonon scattering.Comment: 3 pages, 3 figure

A system for production of defective interfering particles in the absence of infectious influenza A virus

<div><p>Influenza A virus (IAV) infection poses a serious health threat and novel antiviral strategies are needed. Defective interfering particles (DIPs) can be generated in IAV infected cells due to errors of the viral polymerase and may suppress spread of wild type (wt) virus. The antiviral activity of DIPs is exerted by a DI genomic RNA segment that usually contains a large deletion and suppresses amplification of wt segments, potentially by competing for cellular and viral resources. DI-244 is a naturally occurring prototypic segment 1-derived DI RNA in which most of the PB2 open reading frame has been deleted and which is currently developed for antiviral therapy. At present, coinfection with wt virus is required for production of DI-244 particles which raises concerns regarding biosafety and may complicate interpretation of research results. Here, we show that cocultures of 293T and MDCK cell lines stably expressing codon optimized PB2 allow production of DI-244 particles solely from plasmids and in the absence of helper virus. Moreover, we demonstrate that infectivity of these particles can be quantified using MDCK-PB2 cells. Finally, we report that the DI-244 particles produced in this novel system exert potent antiviral activity against H1N1 and H3N2 IAV but not against the unrelated vesicular stomatitis virus. This is the first report of DIP production in the absence of infectious IAV and may spur efforts to develop DIPs for antiviral therapy.</p></div

Dynamics of Long-Living Excitons in Tunable Potential Landscapes

A novel method to experimentally study the dynamics of long-living excitons in coupled quantum well semiconductor heterostructures is presented. Lithographically defined top gate electrodes imprint in-plane artificial potential landscapes for excitons via the quantum confined Stark effect. Excitons are shuttled laterally in a time-dependent potential landscape defined by an interdigitated gate structure. Long-range drift exceeding a distance of 150 um at an exciton drift velocity > 1000 m/s is observed in a gradient potential formed by a resistive gate stripe.Comment: 4 pages, 4 figures. To appear in Phys. E (MSS-12-Proceedings

Reanalysis of multi-temporal aerial images of Storglaciären, Sweden (1959–99) – Part 1: Determination of length, area, and volume changes

Storglaciären, located in the Kebnekaise massif in northern Sweden, has a long history of glaciological research. Early photo documentations date back to the late 19th century. Measurements of front position variations and distributed mass balance have been carried out since 1910 and 1945/46, respectively. In addition to these in-situ measurements, aerial photographs have been taken at decadal intervals since the beginning of the mass balance monitoring program and were used to produce topographic glacier maps. Inaccuracies in the maps were a challenge to early attempts to derive glacier volume changes and resulted in major differences when compared to the direct glaciological mass balances. In this study, we reanalyzed dia-positives of the original aerial photographs of 1959, -69, -80, -90 and -99 based on consistent photogrammetric processing. From the resulting digital elevation models and orthophotos, changes in length, area, and volume of Storglaciären were computed between the survey years, including an assessment of related errors. Between 1959 and 1999, Storglaciären lost an ice volume of 19×106 m3, which corresponds to a cumulative ice thickness loss of 5.69 m and a mean annual loss of 0.14 m. This ice loss resulted largely from a strong volume loss during the period 1959–80 and was partly compensated during the period 1980–99. As a consequence, the glacier shows a strong retreat in the 1960s, a slowing in the 1970s, and pseudo-stationary conditions in the 1980s and 1990s

Reanalysis of multi-temporal aerial images of Storglaciären, Sweden (1959–99) – Part 2: Comparison of glaciological and volumetric mass balances

Seasonal glaciological mass balances have been measured on Storglaciären without interruption since 1945/46. In addition, aerial surveys have been carried out on a decadal basis since the beginning of the observation program. Early studies had used the resulting aerial photographs to produce topographic glacier maps with which the in-situ observations could be verified. However, these maps as well as the derived volume changes are subject to errors which resulted in major differences between the derived volumetric and the glaciological mass balance. As a consequence, the original photographs were re-processed using uniform photogrammetric methods, which resulted in new volumetric mass balances for 1959–69, 1969–80, 1980–90, and 1990–99. We compared these new volumetric mass balances with mass balances obtained by standard glaciological methods including an uncertainty assessment considering all related previous studies. The absolute differences between volumetric and the glaciological mass balances are 0.8 m w.e. for the period of 1959–69 and 0.3 m w.e. or less for the other survey periods. These deviations are slightly reduced when considering corrections for systematic uncertainties due to differences in survey dates, reference areas, and internal ablation, whereas internal accumulation systematically increases the mismatch. However, the mean annual differences between glaciological and volumetric mass balance are less than the uncertainty of the in-situ stake reading and stochastic error bars of both data series overlap. Hence, no adjustment of the glaciological data series to the volumetric one is required

Analysis of Kerr comb generation in silicon microresonators under the influence of two-photon absorption and free-carrier absorption

Kerr frequency comb generation relies on dedicated waveguide platforms that are optimized towards ultralow loss while offering comparatively limited functionality restricted to passive building blocks. In contrast to that, the silicon-photonic platform offers a highly developed portfolio of high-performance devices, but is deemed to be inherently unsuited for Kerr comb generation at near-infrared (NIR) telecommunication wavelengths due to strong two-photon absorption (TPA) and subsequent free-carrier absorption (FCA). Here we present a theoretical investigation that quantifies the impact of TPA and FCA on Kerr comb formation and that is based on a modified version of the Lugiato-Lefever equation (LLE). We find that silicon microresonators may be used for Kerr comb generation in the NIR, provided that the dwell time of the TPA-generated free-carriers in the waveguide core is reduced by a reverse-biased p-i-njunction and that the pump parameters are chosen appropriately. We validate our analytical predictions with time integrations of the LLE, and we present a specific design of a silicon microresonator that may even support formation of dissipative Kerr soliton combs.Comment: 26 pages, 6 figures. Submitted to Physical Review

Tensile strained InxGa1−xPIn_{x}Ga_{1-x}P membranes for cavity optomechanics

We investigate the optomechanical properties of tensile-strained ternary InGaP nanomembranes grown on GaAs. This material system combines the benefits of highly strained membranes based on stoichiometric silicon nitride, with the unique properties of thin-film semiconductor single crystals, as previously demonstrated with suspended GaAs. Here we employ lattice mismatch in epitaxial growth to impart an intrinsic tensile strain to a monocrystalline thin film (approximately 30 nm thick). These structures exhibit mechanical quality factors of 2*10^6 or beyond at room temperature and 17 K for eigenfrequencies up to 1 MHz, yielding Q*f products of 2*10^12 Hz for a tensile stress of ~170 MPa. Incorporating such membranes in a high finesse Fabry-Perot cavity, we extract an upper limit to the total optical loss (including both absorption and scatter) of 40 ppm at 1064 nm and room temperature. Further reductions of the In content of this alloy will enable tensile stress levels of 1 GPa, with the potential for a significant increase in the Q*f product, assuming no deterioration in the mechanical loss at this composition and strain level. This materials system is a promising candidate for the integration of strained semiconductor membrane structures with low-loss semiconductor mirrors and for realizing stacks of membranes for enhanced optomechanical coupling.Comment: 10 pages, 3 figure

Analysis of Kerr comb generation in silicon microresonators under the influence of two-photon absorption and fast free-carrier dynamics

Kerr frequency comb generation relies on dedicated waveguide platforms that are optimized toward ultralow loss while offering comparatively limited functionality restricted to passive building blocks. In contrast to that, the silicon-photonic platform offers a highly developed portfolio of high-performance devices, but suffers from strong two-photon absorption (TPA) and subsequent free-carrier absorption (FCA) at near-infrared telecommunication wavelengths, thereby rendering Kerr comb generation a challenge. Here we present a model to investigate the impact of TPA and FCA on Kerr comb formation. Our model combines a modified version of the Lugiato-Lefever equation with a refined relation to precisely describe the fast space and time dependence of the free-carrier concentration along the circumference of the microresonator. Using this refined model, we derive conditions for modulation instability, in particular for necessary pump powers depending on TPA parameters and free-carrier lifetimes. We validate our analytical predictions by time integration and study the impact of fast free-carrier dynamics on Kerr comb formation. We find that silicon microresonators may be suitable for Kerr comb generation in the NIR, provided that the dwell time of the TPA-generated free carriers in the waveguide core is reduced by a reverse-biased p-i-n-junction and that the pump parameters are chosen appropriately