Near-field propagation in planar nanostructured arrays

In suitably designed nanoscale systems the ultrafast migration of uv/visible electromagnetic energy, despite its near-field rather than propagating character, can be made highly directional. At the photon level such energy migration generally takes a multi-step form, with each step signifying the transfer of an electromagnetic quantum between chromophores playing the transient roles of source/donor and detector/acceptor. There is much interest in nanophotonic devices based on such mechanisms, although the excitation transfer is usually subject to losses such as radiative decay, and possible device applications are compromised by a lack of suitable control mechanisms. Until recently it appeared that only by inefficient and kinetically frustrated means, such as chromophore reorientation or movement, could significant control be effected. However in a system constructed to inhibit near-field propagation by geometric configuration, the throughput of laser pulses can facilitate energy transfer through a process of laser-assisted resonant energy transfer. Suitably configuring an arrangement of dipoles, it proves possible to design parallel arrays of optical donors and acceptors such that the transfer of energy from any single donor, to its counterpart in the opposing plane, is switched by throughput laser radiation of an appropriate intensity, frequency and polarization. A detailed appraisal of some possible realizations of this system reveals an intricate interplay of electronic structure, optical frequency and geometric factors. In the drive to miniaturize ultrafast optical switching and interconnect devices, the results suggest a new basis for optically activated transistor action in nanoscale components, with significant parallel processing capability

Bimodal Distribution of Sulfuric Acid Aerosols in the Upper Haze of Venus

The upper haze (UH) of Venus is variable on the order of days and it is populated by two particle modes. We use a 1D microphysics and vertical transport model based on the Community Aerosol and Radiation Model for Atmospheres to evaluate whether interaction of upwelled cloud particles and sulfuric acid particles nucleated in situ on meteoric dust are able to generate the two size modes and whether their observed variability are due to cloud top vertical transient winds. Nucleation of photochemically produced sulfuric acid onto polysulfur condensation nuclei generates mode 1 cloud droplets that then diffuse upwards into the UH. Droplets generated in the UH from nucleation of sulfuric acid onto meteoric dust coagulate with the upwelled cloud particles and cannot reproduce the observed bimodal size distribution. The mass transport enabled by cloud top transient winds are able to generate a bimodal size distribution in a time scale consistent with observations. Sedimentation and convection in the middle and lower clouds causes the formation of large mode 2 and mode 3 particles. Evaporation of these particles below the clouds creates a local sulfuric acid vapor maximum that causes upwelling of sulfuric acid back into the clouds. If the polysulfur condensation nuclei are small and their production rate is high, coagulation of small droplets onto larger droplets in the middle cloud may result in sulfuric acid "rain" below the clouds once every few Earth months. Reduction of the polysulfur condensation nuclei production rate destroys this oscillation and reduces the mode 1 particle abundance in the middle cloud by two orders of magnitude, though it better reproduces the sulfur-to-sulfuric-acid mass ratio in the cloud and haze droplets. In general we find satisfactory agreement between our results and observations, though improvements could be made by incorporating sulfur microphysics.Comment: 62 pages, 18 figures, 1 table. Accepted for publication in Icaru

Families and work: revisiting barriers to employment

"In recent years, considerable effort has been put into supporting parents to make the transition into work. This study was commissioned by the Department for Work and Pensions (DWP) to explore whether these incentives were helping parents to overcome the barriers known to impede their engagement in the formal labour market. The report is based on fieldwork conducted in 2009. However, the concluding chapter considers the significance of the findings in light of proposals for the introduction of the Universal Credit and other reforms of the tax and benefit systems proposed by the Coalition Government." - Page 1

Spaceborne measurements of atmospheric CO_2 by high-resolution NIR spectrometry of reflected sunlight: An introductory study

We introduce a strategy for measuring the column-averaged CO_2 dry air volume mixing ratio X_(CO_2) from space. It employs high resolution spectra of reflected sunlight taken simultaneously in near-infrared (NIR) CO_2 (1.58-mm and 2.06-mm) and O_2 (0.76-mm) bands. Simulation experiments, show that precisions of ~0.3–2.5 ppmv for X_(CO_2) can be achieved from individual clear sky soundings for a range of atmospheric/surface conditions when the scattering optical depth t_s is less than ~0.3. When averaged over many clear sky soundings, random errors become negligible. This high precision facilitates the identification and correction of systematic errors, which are recognized as the most serious impediment for the satellite X_(CO_2) measurements. We briefly discuss potential sources of systematic errors, and show that some of them may result in geographically varying biases in the measured X_(CO_2). This highlights the importance of careful calibration and validation measurements, designed to identify and eliminate sources of these biases. We conclude that the 3-band, spectrometric approach using NIR reflected sunlight has the potential for highly accurate X_(CO_2) measurements

Characterization of the OCO-2 instrument line shape functions using on-orbit solar measurements

Accurately characterizing the instrument line shape (ILS) of the Orbiting Carbon Observatory-2 (OCO-2) is challenging and highly important due to its high spectral resolution and requirement for retrieval accuracy (0. 25 %) compared to previous spaceborne grating spectrometers. On-orbit ILS functions for all three bands of the OCO-2 instrument have been derived using its frequent solar measurements and high-resolution solar reference spectra. The solar reference spectrum generated from the 2016 version of the Total Carbon Column Observing Network (TCCON) solar line list shows significant improvements in the fitting residual compared to the solar reference spectrum currently used in the version 7 Level 2 algorithm in the O₂ A band. The analytical functions used to represent the ILS of previous grating spectrometers are found to be inadequate for the OCO-2 ILS. Particularly, the hybrid Gaussian and super-Gaussian functions may introduce spurious variations, up to 5 % of the ILS width, depending on the spectral sampling position, when there is a spectral undersampling. Fitting a homogeneous stretch of the preflight ILS together with the relative widening of the wings of the ILS is insensitive to the sampling grid position and accurately captures the variation of ILS in the O₂ A band between decontamination events. These temporal changes of ILS may explain the spurious signals observed in the solar-induced fluorescence retrieval in barren areas