23 research outputs found
Design and characterization of high optical quality InGaAs/GaAs/AlGaAs-based polariton microcavities
The presence of dislocations arising from strain relaxation strongly affects polaritons through their photonic component and ultimately limits experiments involving polariton propagation. In this work, we investigate the range of growth parameters to achieve high optical quality GaAs/AlxGa1âxAs-based microcavities containing strained InxGa1âxAs quantum wells and using differential interference contrast (Nomarski) microscopy deduce a design rule for homogeneous versus disordered structures. We illustrate the effect of disorder by contrasting observations of polariton condensates in relaxed and unrelaxed microcavities. In our optimized device, we generate a polariton condensate and deduce a lifetime for the interacting polariton fluid of 39â±â2 ps
Impacts of salvage logging on biodiversity: A meta-analysis
Logging to "salvage" economic returns from forests affected by natural disturbances has become increasingly prevalent globally. Despite potential negative effects on biodiversity, salvage logging is often conducted, even in areas otherwise excluded from logging and reserved for nature conservation, inter alia because strategic priorities for post-disturbance management are widely lacking. A review of the existing literature revealed that most studies investigating the effects of salvage logging on biodiversity have been conducted less than 5 years following natural disturbances, and focused on non-saproxylic organisms. A meta-analysis across 24 species groups revealed that salvage logging significantly decreases numbers of species of eight taxonomic groups. Richness of dead wood dependent taxa (i.e. saproxylic organisms) decreased more strongly than richness of non-saproxylic taxa. In contrast, taxonomic groups typically associated with open habitats increased in the number of species after salvage logging. By analysing 134 original species abundance matrices, we demonstrate that salvage logging significantly alters community composition in 7 of 17 species groups, particularly affecting saproxylic assemblages. Synthesis and applications. Our results suggest that salvage logging is not consistent with the management objectives of protected areas. Substantial changes, such as the retention of dead wood in naturally disturbed forests, are needed to support biodiversity. Future research should investigate the amount and spatio-temporal distribution of retained dead wood needed to maintain all components of biodiversity
Spontaneous vortices in optically shaped potential profiles in semiconductor microcavities
We investigate the spontaneous formation of quantized vortices in microcavity polariton high density phases. Condensates formed in the optical parametric oscillator excitation scheme reveal single vortices in the case of a ring-shaped optical potential or in the presence of natural photonic disorder. We further investigate the dynamics of vortex formation for resonantly injected polaritons and observe the formation of vortex-antivortex pairs, but no single vortices. The observed effects are explained by the interplay between breaking of yâŠây reflection symmetry in the system and conservation of optical angular momentum
Estimating retention benchmarks for salvage logging to protect biodiversity
S.T. was supported by the Humboldt-Foundation and by the MOST (Ministry of Science
and Technology) Taiwan Research Fellowship to work with A.C. at National Tsing Hua
University, Taiwan. S.T. received funds from the Gregor Louisoder Environmental
Foundation. A.B.L. received funds from the Humboldt-Foundation.Forests are increasingly affected by natural disturbances. Subsequent salvage logging, a
widespread management practice conducted predominantly to recover economic capital,
produces further disturbance and impacts biodiversity worldwide. Hence, naturally disturbed
forests are among the most threatened habitats in the world, with consequences for their
associated biodiversity. However, there are no evidence-based benchmarks for the proportion
of area of naturally disturbed forests to be excluded from salvage logging to conserve biodiversity.
We apply a mixed rarefaction/extrapolation approach to a global multi-taxa dataset
from disturbed forests, including birds, plants, insects and fungi, to close this gap. We find
that 75 ± 7% (mean ± SD) of a naturally disturbed area of a forest needs to be left unlogged
to maintain 90% richness of its unique species, whereas retaining 50% of a naturally disturbed
forest unlogged maintains 73 ± 12% of its unique species richness. These values do
not change with the time elapsed since disturbance but vary considerably among taxonomic
groups.Open Access funding enabled and organized by Projekt DEA
Late Cretaceous mega-, meso-, and microfloras from Lower Silesia
Late Cretaceous plants from the North Sudetic Basin (Lower Silesia, south-western Poland) are reviewed on the basis of
megaflora from 17 localities (270 identifiable specimens), mesoflora from two localities, and microflora from four localities.
Major sites are Rakowice MaĆe and BolesĆawiec. Eight megafloral assemblages are distinguished (Assemblage 1, Turonian;
Assemblages 2, 3, lowerâmiddle Coniacian; Assemblages 4, 5, upper Coniacian?âlower Santonian?; Assemblages 6â8,
lowerâmiddle Santonian); the bulk of the palaeoflora is from Assemblages 4â6 and 8. Megaflora consists of 29 taxa (6 ferns,
4 conifers, and 19 angiosperms). Geinitzia reichenbachii is the most common species. Dryophyllum westerhausianum
(Richter, 1904) Halamski and KvaÄek comb. nov. is a trifoliolate leaf re-interpreted as a representative of Fagales. Three
species of Dewalquea are distinguished: Dewalquea haldemiana, Dewalquea insignis, and Dewalquea aff. gelindenensis.
Platanites willigeri Halamski and KvaÄek sp. nov. is characterised by trifoliolate leaves, the median leaflet of which is
ovate, unlobed, with a serrate margin, and cuneate base. Palaeocommunities inferred from the megafossil record include:
a back swamp forest dominated by Geinitzia, with abundant ferns; a Dryophyllum-dominated riparian forest; a forest with
Dewalquea and Platanites willigeri possibly located in the marginal part of the alluvial plain; dunes with D. haldemiana and
Konijnenburgia; a fern savanna with patches of Pinus woodlands. Palynoassemblage A from the Nowogrodziec Member,
studied mostly at Rakowice MaĆe and Ć»eliszĂłw, consists of 126 taxa, including 105 terrestrial palynomorphs (54 bryophyte,
lycophyte, and pteridophyte spores, 16 gymnosperms, 35 angiosperms). The mega- and mesofossil records are dominated
by angiosperms; the palynoassemblages are dominated by ferns. Palaeocommunities represented solely by the microfossil
record are halophytic (with Frenelopsis and unconfirmed presence of Nypa) and pioneer vegetation. Palaeocommunities
are intermediate in general character between those pre-dating the Cretaceous Terrestrial Revolution and modern, angiosperm-
dominated vegetation. In comparison to older plant assemblages from contiguous areas laurophylls are much rarer;
this might correspond to a real phenomenon of exclusion of lauroids from Santonian riparian forests. The studied assemblage
is more similar to younger palaeofloras than to older ones; this might be interpreted as stabilisation of communities
after a period of pronounced change related to the rise to dominance of the angiosperms. In contrast to widespread endemism
among vertebrates of the European Archipelago, the plant cover consists mostly of species that are widely distributed
Exciton polaritons in semiconductor waveguides
We report the observation of strong coupling between quantum well excitons and a guided mode of a semiconductor planar waveguide by observation of anticrossing in the dispersion. Strong spatial confinement of the optical mode allows a splitting between the two polariton modes of 5â6âmeV for a single quantum well. Polaritons on resonance are shown to propagate with a characteristic decay length of 280âÎŒm and a group velocity of 26âÎŒmâpsâ1 . This is a promising first step towards developing an alternative to microcavities for the study of rapidly propagating polaritons, which is particularly well suited to prospective on-chip polaritonic circuit applications
Lead, cadmium and mercury levels in pregnancy: the need for international consensus on levels of concern
Estimating retention benchmarks for salvage logging to protect biodiversity
Forests are increasingly affected by natural disturbances. Subsequent salvage logging, a widespread management practice conducted predominantly to recover economic capital, produces further disturbance and impacts biodiversity worldwide. Hence, naturally disturbed forests are among the most threatened habitats in the world, with consequences for their associated biodiversity. However, there are no evidence-based benchmarks for the proportion of area of naturally disturbed forests to be excluded from salvage logging to conserve biodiversity. We apply a mixed rarefaction/extrapolation approach to a global multi-taxa dataset from disturbed forests, including birds, plants, insects and fungi, to close this gap. We find that 75â±â7% (meanâ±âSD) of a naturally disturbed area of a forest needs to be left unlogged to maintain 90% richness of its unique species, whereas retaining 50% of a naturally disturbed forest unlogged maintains 73â±â12% of its unique species richness. These values do not change with the time elapsed since disturbance but vary considerably among taxonomic groups