1,117 research outputs found
Mid-Infrared ultra-high-Q resonators based on fluoride crystalline materials
Decades ago, the losses of glasses in the near infrared (near-IR) were
investigated in views of developments for optical telecommunications. Today,
properties in the mid-infrared (mid-IR) are of interest for molecular
spectroscopy applications. In particular, high-sensitivity spectroscopic
techniques based on high-finesse mid-IR cavities hold high promise for medical
applications. Due to exceptional purity and low losses, whispering gallery mode
microresonators based on polished alkaline earth metal fluoride crystals (i.e
the family, where X Ca, Mg, Ba, Sr,...) have attained
ultra-high quality (Q) factor resonances (Q10) in the near-IR and
visible spectral ranges. Here we report for the first time ultra-high Q factors
in the mid-IR using crystalline microresonators. Using an uncoated chalcogenide
(ChG) tapered fiber, light from a continuous wave quantum cascade laser (QCL)
is efficiently coupled to several crystalline microresonators at 4.4 m
wavelength. We measure the optical Q factor of fluoride crystals in the mid-IR
using cavity ringdown technique. We observe that
microresonators feature quality factors that are very close to the fundamental
absorption limit, as caused by the crystal's multiphonon absorption
(Q10), in contrast to near-IR measurements far away from these
fundamental limits. Due to lower multiphonon absorption in and
, we show that ultra-high quality factors of Q 1.4
can be reached at 4.4 m. This corresponds to an optical
finesse of 4 10, the highest value achieved for any
type of mid-IR resonator to date, and a more than 10-fold improvement over the
state-of-the-art. Such compact ultra-high Q crystalline microresonators provide
a route for narrow linewidth frequency-stabilized QCL or mid-IR Kerr comb
generation.Comment: C. Lecaplain and C. Javerzac-Galy contributed equally to this wor
NaV_2O_5 as a quarter-filled ladder compound
A new X-ray diffraction study of the one-dimensional spin-Peierls compound
\alpha-NaV_2O_5 reveals a centrosymmetric (Pmmn) crystal structure with one
type of V site, contrary to the previously postulated non-centrosymmetric
P2_1mn structure with two types of V sites (V^{+4} and V^{+5}). Density
functional calculations indicate that NaV_2O_5 is a quarter-filled ladder
compound with the spins carried by V-O-V molecular orbitals on the rungs of the
ladder. Estimates of the charge-transfer gap and the exchange coupling agree
well with experiment and explain the insulating behavior of NaV_2O_5 and its
magnetic properties.Comment: Final version for PRL, value of U correcte
National flood modelling for insurance purposes: using IFSAR for flood risk estimation in Europe
International audienceFlood risk poses a major problem for insurers and governments who ultimately pay the financial costs of losses resulting from flood events. Insurers therefore face the problem of how to assess their exposure to floods and how best to price the flood element of their insurance products. This paper looks at the insurance implications of recent flood events in Europe and the issues surrounding insurance of potential future events. In particular, the paper will focus on the flood risk information needs of insurers and how these can be met. The data requirements of national and regional flood models are addressed in the context of the accuracy of available data on property location. Terrain information is generally the weakest component of sophisticated flood models. Therefore, various sources of digital terrain models (DTM) are examined and discussed with consideration of the vertical and horizontal accuracy, the speed of acquisition, the costs and the comprehensiveness of the data. The NEXTMap DTM series from Intermap Technologies Inc. is proposed as a suitable DTM for flood risk identification and mapping, following its use in the UK. Its acquisition, processing and application is described and future plans discussed. Examples are included of the application of flood information to insurance property information and the potential benefits and advantages of using suitable hazard modelling data sources are detailed
Crystal structure of LaTiO_3.41 under pressure
The crystal structure of the layered, perovskite-related LaTiO_3.41
(La_5Ti_5O_{17+\delta}) has been studied by synchrotron powder x-ray
diffraction under hydrostatic pressure up to 27 GPa (T = 295 K). The
ambient-pressure phase was found to remain stable up to 18 GPa. A sluggish, but
reversible phase transition occurs in the range 18--24 GPa. The structural
changes of the low-pressure phase are characterized by a pronounced anisotropy
in the axis compressibilities, which are at a ratio of approximately 1:2:3 for
the a, b, and c axes. Possible effects of pressure on the electronic properties
of LaTiO_3.41 are discussed.Comment: 5 pages, 6 figure
Quaternary climate modulation of Pb isotopes in the deep Indian Ocean linked to the Himalayan chemical weathering
We use reductive sediment leaching to extract lead (Pb) from the authigenic fraction of marine sediments and reconstruct the Pb isotope evolution of the deep central Indian Ocean over the past 250 thousand years at ∼3 kyr resolution. Temporal variations define a binary mixing line that is consistent with data from ferromanganese nodules and which records mixing between two well-defined endmembers through time. The unradiogenic endmember appears to represent a widely-distributed Pb source, from mid-ocean ridges or possibly volcanic aerosols, while the radiogenic endmember coincides with the composition of Ganges–Brahmaputra river sediments that are indicative of the Himalayan weathering inputs. Glacial–interglacial Pb isotope variations are striking and can be explained by an enhancement of Himalayan contributions by two to three times during interglacial periods, indicating that climate modulates the supply of dissolved elements to the ocean. While these changes could accurately record variations in the continental chemical weathering flux in response to warmer and wetter conditions during interglacials, the relative proportions of Pb derived from the Ganges and Brahmaputra appear to have been constant through time. This observation may point towards particulate-dissolved interactions in the estuary or pro-delta as a buffer of short timescale variability in the composition (and potentially flux) of the fluvial inputs. In addition, the changes are recorded at 3800 m water depth, and with the lack of deep water formation in the Bay of Bengal, a mechanism to transfer such a signature into the deep ocean could either be reversible scavenging of dissolved Pb inputs and/or boundary exchange on the deep sea fan. Unless the mechanism transferring the Pb isotope signature into the deep ocean was itself highly sensitive to global climate cycles, and with the absence of a precessional signal in our Pb isotope data, we suggest that the Indian climate and its influence on basin-scale chemical weathering were strongly modulated by glacial versus interglacial boundary conditions
How well does ramped thermal oxidation quantify the age distribution of soil carbon? Assessing thermal stability of physically and chemically fractionated soil organic matter
Carbon (C) in soils persists on a range of timescales depending on physical, chemical, and biological processes that interact with soil organic matter (SOM) and affect its rate of decomposition. Together these processes determine the age distribution of soil C. Most attempts to measure this age distribution have relied on operationally defined fractions using properties like density, aggregate stability, solubility, or chemical reactivity. Recently, thermal fractionation, which relies on the activation energy needed to combust SOM, has shown promise for separating young from old C by applying increasing heat to decompose SOM. Here, we investigated radiocarbon (C-14) and C-13 of C released during thermal fractionation to link activation energy to the age distribution of C in bulk soil and components previously separated by density and chemical properties. While physically and chemically isolated fractions had very distinct mean C-14 values, they contributed C across the full temperature range during thermal analysis. Thus, each thermal fraction collected during combustion of bulk soil integrates contributions from younger and older C derived from components having different physical and chemical properties but the same activation energy. Bulk soil and all density and chemical fractions released progressively older and more C-13-enriched C with increasing activation energy, indicating that each operationally defined fraction itself was not homogeneous but contained a mix of C with different ages and degrees of microbial processing. Overall, we found that defining the full age distribution of C in bulk soil is best quantified by first separating particulate C prior to thermal fractionation of mineral-associated SOM. For the Podzol analyzed here, thermal fractions confirmed that similar to 95 % of the mineral-associated organic matter (MOM) had a relatively narrow C-14 distribution, while 5 % was very low in C-14 and likely reflected C from the < 2 mm parent shale material in the soil matrix. After first removing particulate C using density or size separation, thermal fractionation can provide a rapid technique to study the age structure of MOM and how it is influenced by different OM-mineral interactions
Magnetic properties of NaV2O5, a one-dimensional spin 1/2 antiferromagnet with finite chains
We have performed measurements of the magnetic susceptibility of NaVO
between 2 and 400 K. The high temperature part is typical of spin 1/2 chains
with a nearest--neighbour antiferromagnetic exchange integral of 529 K. We
develop a model for the susceptibility of a system with finite chains to
account for the low temperature part of the data, which cannot be fitted by a
standard Curie-Weiss term. These results suggest that the next
nearest--neighbour exchange integral in CaVO should be of the
order of 500 K because, like in NaVO, it corresponds to corner
sharing VO square pyramids.Comment: An early version of the manuscript was mistakenly submitted. Although
relatively minor, the changes concern the list of authors, the main text, the
references and the figure captions. 10 pages of latex, 2 figure
Advanced Photodetectors for Hyperspectroscopy and Other Applications
Hyperspectroscopy is a new method of surface image taking, providing
simultaneously high position and spectral resolutions which allow one to make
some conclusions about chemical compositions of the surfaces. We are now
studying applications of the hyperspctroscopic technique to be used for
medicine. This may allow one to develop early diagnostics of some illnesses, as
for example, skin cancer. For image taking advanced MCPs are currently used,
sensitive in the spectral interval of 450-850 nm. One of the aims of this work
is to extend the hyperspectrocpic method to the UV region of spectra: 185-280
nm. For this we have developed and successfully tested innovative 1D and 2D UV
sealed photosensitive gaseous detectors with resistive electrodes. These
detectors are superior MCPs due to the very low rate of noise pulses and thus
due to the high signal to noise ratio. Other important features of these
detectors are that they have excellent position resolutions - 30 micron in
digital form, are vibration stable and are spark protected. The first results
from the application of these detectors for spectroscopy, hyperspectroscopy and
the flame detection are presented.Comment: Presented at the IEEE Nuclear Science Syposium, Puerto Rico, October
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