Quasi-optical multiplexing using reflection phase gratings

Heterodyne array receiver systems for both ground based and satellite telescope facilities are now becoming feasible for imaging in the submillimetre/terahertz regions of the EM spectrum. Phase gratings can be usefully employed as high efficiency passive multiplexing devices in the local oscillator (LO) injection chain of such receivers, ensuring that each element of the array is adequately biased and that the reflected LO power level at the array is minimised. For the wavelengths of interest both transmission and reflection gratings can be manufactured by milling an appropriate pattern of slots into the surface(s) of a suitable material. Thus, the required phase modulation is produced by the resulting pattern of varying optical path lengths suffered by the incident wave-front. We report on work we are undertaking to develop all reflection quasi-optical multiplexing systems so as to reduce reflection losses at the grating and minimise the number of surfaces that can contribute to standing wave effects in the optical system. As part of this endeavour we have also developed a quasi-optical technique for analysing the inevitable degradation due to multiple reflections on transmission grating design. This analysis is based on the Gaussian beam mode technique, and a further application of this technique allows one to assess tolerance limitations on the grating

Quasi-optical multiplexing using reflection phase gratings

Heterodyne array receiver systems for both ground based and satellite telescope facilities are now becoming feasible for imaging in the submillimetre/terahertz regions of the EM spectrum. Phase gratings can be usefully employed as high efficiency passive multiplexing devices in the local oscillator (LO) injection chain of such receivers, ensuring that each element of the array is adequately biased and that the reflected LO power level at the array is minimised. For the wavelengths of interest both transmission and reflection gratings can be manufactured by milling an appropriate pattern of slots into the surface(s) of a suitable material. Thus, the required phase modulation is produced by the resulting pattern of varying optical path lengths suffered by the incident wave-front. We report on work we are undertaking to develop all reflection quasi-optical multiplexing systems so as to reduce reflection losses at the grating and minimise the number of surfaces that can contribute to standing wave effects in the optical system. As part of this endeavour we have also developed a quasi-optical technique for analysing the inevitable degradation due to multiple reflections on transmission grating design. This analysis is based on the Gaussian beam mode technique, and a further application of this technique allows one to assess tolerance limitations on the grating

Topo-edaphic controls over woody plant biomass in South African savannas

The distribution of woody biomass in savannas reflects spatial patterns fundamental to ecosystem processes, such as water flow, competition, and herbivory, and is a key contributor to savanna ecosystem services, such as fuelwood supply. While total precipitation sets an upper bound on savanna woody biomass, the extent to which substrate and terrain constrain trees and shrubs below this maximum remains poorly understood, often occluded by local-scale disturbances such as fire and trampling. Here we investigate the role of hillslope topography and soil properties in controlling woody plant aboveground biomass (AGB) in Kruger National Park, South Africa. Large-area sampling with airborne Light Detection and Ranging (LiDAR) provided a means to average across local-scale disturbances, revealing an unexpectedly linear relationship between AGB and hillslope-position on basalts, where biomass levels were lowest on crests, and linearly increased toward streams (<i>R</i><sup>2</sup> = 0.91). The observed pattern was different on granite substrates, where AGB exhibited a strongly non-linear relationship with hillslope position: AGB was high on crests, decreased midslope, and then increased near stream channels (<i>R</i><sup>2</sup> = 0.87). Overall, we observed 5-to-8-fold lower AGB on clayey, basalt-derived soil than on granites, and we suggest this is due to herbivore-fire interactions rather than lower hydraulic conductivity or clay shrinkage/swelling, as previously hypothesized. By mapping AGB within and outside fire and herbivore exclosures, we found that basalt-derived soils support tenfold higher AGB in the absence of fire and herbivory, suggesting high clay content alone is not a proximal limitation on AGB. Understanding how fire and herbivory contribute to AGB heterogeneity is critical to predicting future savanna carbon storage under a changing climate

Optogalvanic Signals From Argon Metastables In A Rf Glow-Discharge

Laser optogalvanic (LOG) signals at 667.7, 751.5, and 696.5 nm from the 3 P 1 and 3 P 2 levels of Ar were studied at a pressure of 250 mTorr in a rf glow discharge. Signals with unexpected signs and time dependences were found. The results are interpreted as being due to radiative trapping effects and collisional mixing between resonance and metastable levels. An average electron energy of 2.1 eV is derived from modeling the data

A qualitative investigation into the cultural master narrative for overcoming trauma and adversity in the United Kingdom.

esearch in the field of narrative psychology has found that redemption—a narrative sequence in which people recount emotionally negative experiences as having positive endings—is a useful mechanism for coping with adversity. Redemption has been viewed as a cultural master narrative in North America, providing individuals with a socially valued script for narrating challenging life experiences. Presently little research has examined the presence and function of the redemption narrative outside of North American contexts. The aim of this qualitative study, therefore, was to identify themes in U.K. individuals’ narratives of trauma and adversity to gain insight into the content of the master narrative for meaning-making in the U.K. Sixty-five participants (57 females, Mage = 21.97, SD = 7.24) with little to no experience of lifetime adversity were recruited into an online survey. Participants answered open-ended questions adopting the perspective of a survivor from a selected U.K. national tragedy, focusing on how they felt survivors could recover from trauma. We identified 2 themes in our thematic analysis that were relevant to recovery: recuperation and redemption. Recuperation was most commonly reported, it was described as a gradual lessening of symptoms over time and the ability to cope with the lasting emotional and physical scars. Our findings suggest that redemption is not necessarily the dominant cultural script in the U.K. for guiding recovery in the aftermath of trauma. (APA PsycInfo Database Record (c) 2020 APA, all rights reserved

Separation and identification of dominant mechanisms in double photoionization

Double photoionization by a single photon is often discussed in terms of two contributing mechanisms, {\it knock-out} (two-step-one) and {\it shake-off} with the latter being a pure quantum effect. It is shown that a quasi-classical description of knock-out and a simple quantum calculation of shake-off provides a clear separation of the mechanisms and facilitates their calculation considerably. The relevance of each mechanism at different photon energies is quantified for helium. Photoionization ratios, integral and singly differential cross sections obtained by us are in excellent agreement with benchmark experimental data and recent theoretical results.Comment: 4 pages, 5 figure