26,268 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
Exclusion Processes with Internal States
We introduce driven exclusion processes with internal states that serve as
generic transport models in various contexts, ranging from molecular or
vehicular traffic on parallel lanes to spintronics. The ensuing non-equilibrium
steady states are controllable by boundary as well as bulk rates. A striking
polarization phenomenon accompanied by domain wall motion and delocalization is
discovered within a mesoscopic scaling. We quantify this observation within an
analytic description providing exact phase diagrams. Our results are confirmed
by stochastic simulations.Comment: 4 pages, 3 figures. Version as published in Phys. Rev. Let
Design concepts to improve high performance solar simulator
By improving several important components of the well known off-axis solar simulator system, a considerable step forward was made. The careful mathematical studies on the optics and the thermal side of the problem lead to a highly efficient system with low operational costs and a high reliability. The actual performance of the simulator is significantly better than the specified one, and the efficiency is outstanding. No more than 12 lamps operating at 18 kW are required to obtain one Solar Constant in the 6 m beam. It is now known that by using sophisticated optics, even larger facilities of high performance can be designed without leaving the proven off-axis concept and using a spherical mirror. Using high performance optics is a means of reducing costs at a given size of beam because the number of lamps is one of the most cost driving factors in the construction of a solar simulator
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