3,413 research outputs found
Temperature dependent refractive index of silicon and germanium
Silicon and germanium are perhaps the two most well-understood semiconductor
materials in the context of solid state device technologies and more recently
micromachining and nanotechnology. Meanwhile, these two materials are also
important in the field of infrared lens design. Optical instruments designed
for the wavelength range where these two materials are transmissive achieve
best performance when cooled to cryogenic temperatures to enhance signal from
the scene over instrument background radiation. In order to enable high quality
lens designs using silicon and germanium at cryogenic temperatures, we have
measured the absolute refractive index of multiple prisms of these two
materials using the Cryogenic, High-Accuracy Refraction Measuring System
(CHARMS) at NASA Goddard Space Flight Center, as a function of both wavelength
and temperature. For silicon, we report absolute refractive index and
thermo-optic coefficient (dn/dT) at temperatures ranging from 20 to 300 K at
wavelengths from 1.1 to 5.6 microns, while for germanium, we cover temperatures
ranging from 20 to 300 K and wavelengths from 1.9 to 5.5 microns. We compare
our measurements with others in the literature and provide
temperature-dependent Sellmeier coefficients based on our data to allow
accurate interpolation of index to other wavelengths and temperatures. Citing
the wide variety of values for the refractive indices of these two materials
found in the literature, we reiterate the importance of measuring the
refractive index of a sample from the same batch of raw material from which
final optical components are cut when absolute accuracy greater than +/-5 x
10^-3 is desired.Comment: 10 pages, 8 figures, to be published in the Proc. of SPIE 6273
(Orlando
Sn-modification of Pt7/alumina model catalysts: Suppression of carbon deposition and enhanced thermal stability.
An atomic layer deposition process is used to modify size-selected Pt7/alumina model catalysts by Sn addition, both before and after Pt7 cluster deposition. Surface science methods are used to probe the effects of Sn-modification on the electronic properties, reactivity, and morphology of the clusters. Sn addition, either before or after cluster deposition, is found to strongly affect the binding properties of a model alkene, ethylene, changing the number and type of binding sites, and suppressing decomposition leading to carbon deposition and poisoning of the catalyst. Density functional theory on a model system, Pt4Sn3/alumina, shows that the Sn and Pt atoms are mixed, forming alloy clusters with substantial electron transfer from Sn to Pt. The presence of Sn also makes all the thermally accessible structures closed shell, such that ethylene binds only by π-bonding to a single Pt atom. The Sn-modified catalysts are quite stable in repeated ethylene temperature programmed reaction experiments, suggesting that the presence of Sn also reduces the tendency of the sub-nano-clusters to undergo thermal sintering
Printable microscale interfaces for long-term peripheral nerve mapping and precision control
The nascent field of bioelectronic medicine seeks to decode and modulate peripheral nervous system signals to obtain therapeutic control of targeted end organs and effectors. Current approaches rely heavily on electrode-based devices, but size scalability, material and microfabrication challenges, limited surgical accessibility, and the biomechanically dynamic implantation environment are significant impediments to developing and deploying advanced peripheral interfacing technologies. Here, we present a microscale implantable device – the nanoclip – for chronic interfacing with fine peripheral nerves in small animal models that begins to meet these constraints. We demonstrate the capability to make stable, high-resolution recordings of behaviorally-linked nerve activity over multi-week timescales. In addition, we show that multi-channel, current-steering-based stimulation can achieve a high degree of functionally-relevant modulatory specificity within the small scale of the device. These results highlight the potential of new microscale design and fabrication techniques for the realization of viable implantable devices for long-term peripheral interfacing.https://www.biorxiv.org/node/801468.fullFirst author draf
A CubeSat for Calibrating Ground-Based and Sub-Orbital Millimeter-Wave Polarimeters (CalSat)
We describe a low-cost, open-access, CubeSat-based calibration instrument
that is designed to support ground-based and sub-orbital experiments searching
for various polarization signals in the cosmic microwave background (CMB). All
modern CMB polarization experiments require a robust calibration program that
will allow the effects of instrument-induced signals to be mitigated during
data analysis. A bright, compact, and linearly polarized astrophysical source
with polarization properties known to adequate precision does not exist.
Therefore, we designed a space-based millimeter-wave calibration instrument,
called CalSat, to serve as an open-access calibrator, and this paper describes
the results of our design study. The calibration source on board CalSat is
composed of five "tones" with one each at 47.1, 80.0, 140, 249 and 309 GHz. The
five tones we chose are well matched to (i) the observation windows in the
atmospheric transmittance spectra, (ii) the spectral bands commonly used in
polarimeters by the CMB community, and (iii) The Amateur Satellite Service
bands in the Table of Frequency Allocations used by the Federal Communications
Commission. CalSat would be placed in a polar orbit allowing visibility from
observatories in the Northern Hemisphere, such as Mauna Kea in Hawaii and
Summit Station in Greenland, and the Southern Hemisphere, such as the Atacama
Desert in Chile and the South Pole. CalSat also would be observable by
balloon-borne instruments launched from a range of locations around the world.
This global visibility makes CalSat the only source that can be observed by all
terrestrial and sub-orbital observatories, thereby providing a universal
standard that permits comparison between experiments using appreciably
different measurement approaches
Temperature-dependent Refractive Index of CaF2 and Infrasil 301
In order to enable high quality lens designs using calcium fluoride (CaF2) and Heraeus Infrasil 301 (Infrasil) for cryogenic operating temperatures, we have measured the absolute refractive index of these two materials as a function of both wavelength and temperature using the Cryogenic, High-Accuracy Refraction Measuring System (CHARMS) at NASA's Goddard Space Flight Center. For CaF2, we report absolute refractive index and thermo-optic coefficient (dn/dT) at temperatures ranging from 25 to 300 K at wavelengths from 0.4 to 5.6 pm, while for Infrasil, we cover temperatures ranging from 35 to 300 K and wavelengths from 0.4 to 3.6 pm. For CaF2, we compare our index measurements to measurements of other investigators. For Infrasil, we compare our measurements to the mate~al manufacturer's data at room temperature and to cryogenic measurements for fused silica from previous investigations including one of our own. Finally, we provide temperature-dependent Sellmeier coefficients based on our measured data to allow accurate interpolation of index to other wavelengths and temperatures
Cryogenic Temperature-dependent Refractive Index Measurements of N-BK7, BaLKN3, and SF15 for NOTES PDI
In order to enable high quality lens designs using N-BK7, BaLKN3, and SF15 at cryogenic temperatures, we have measured the absolute refractive index of prisms of these three materials using the Cryogenic, High-Accuracy Refraction Measuring System (CHARMS) at NASA's Goddard Space Flight Center, as a function of both wavelength and temperature. For N-BK7, we report absolute refractive index and thermo-optic coefficient (dn/dT) at temperatures ranging from 50 to 300 K at wavelengths from 0.45 to 2.7 micrometers; for BaLKN3 we cover temperatures ranging from 40 to 300 K and wavelengths from 0.4 to 2.6 micrometers; for SF15 we cover temperatures ranging from 50 to 300 K and wavelengths from 0.45 to 2.6 micrometers. We compare our measurements with others in the literature and provide temperature-dependent Sellmeier coefficients based on our data to allow accurate interpolation of index to other wavelengths and temperatures. While we generally find good agreement (plus or minus 2 x 10(exp -4) for N-BK7, less than 1 x 10(exp -4) for the other materials) at room temperature between our measured values and those provided by the vendor, there is some variation between the datasheets provided with the prisms we measured and the catalog values published by the vendor. This underlines the importance of measuring the absolute refractive index of the material when precise knowledge of the refractive index is required
Examining consumer response to commodity-specific and broad-based promotion programs for fruits and vegetables using experimental economics
WP 2010-12 July 2010JEL Classification Codes: M37; Q13Generic promotion and advertising activities have traditionally been used to promote individual agricultural commodities. However, there is renewed interest in implementing a mandatory ?broad-based? promotion program for all fruits and vegetables, and this idea is highly controversial among those in the horticultural industry. Here we use data from an experiment that introduces subjects to various promotional efforts for fruits and vegetables to estimate the direct and indirect effects of advertising. Econometric results indicate that commodity-specific promotional efforts may be less effective at increasing demand for fruits and vegetables than earlier studies have suggested, yet such campaigns do appear to have a significant clockwise rotational effect on the demand for fruits and vegetables. Broad-based advertising does have a direct effect on the demand for fruits and vegetables, and after controlling for various demographic differences between treatments our results show that average willingness-to-pay for fruits and vegetables was 41% higher among subjects in the broad-based group compared to the control group
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