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

Automation, Operation, and Data Analysis in the Cryogenic, High Accuracy, Refraction Measuring System (CHARMS)

The Cryogenic High Accuracy Refraction Measuring System (CHARMS) at NASA's Goddard Space Flight Center has been enhanced in a number of ways in the last year to allow the system to accurately collect refracted beam deviation readings automatically over a range of temperatures from 15 K to well beyond room temperature with high sampling density in both wavelength and temperature. The engineering details which make this possible are presented. The methods by which the most accurate angular measurements are made and the corresponding data reduction methods used to reduce thousands of observed angles to a handful of refractive index values are also discussed

Temperature-dependent Absolute Refractive Index Measurements of Synthetic Fused Silica

Using the Cryogenic, High-Accuracy Refraction Measuring System (CHARMS) at NASA's Goddard Space Flight Center, we have measured the absolute refractive index of five specimens taken from a very large boule of Corning 7980 fused silica from temperatures ranging from 30 to 310 K at wavelengths from 0.4 to 2.6 microns with an absolute uncertainty of plus or minus 1 x 10 (exp -5). Statistical variations in derived values of the thermo-optic coefficient (dn/dT) are at the plus or minus 2 x 10 (exp -8)/K level. Graphical and tabulated data for absolute refractive index, dispersion, and thermo-optic coefficient are presented for selected wavelengths and temperatures along with estimates of uncertainty in index. Coefficients for temperature-dependent Sellmeier fits of measured refractive index are also presented to allow accurate interpolation of index to other wavelengths and temperatures. We compare our results to those from an independent investigation (which used an interferometric technique for measuring index changes as a function of temperature) whose samples were prepared from the same slugs of material from which our prisms were prepared in support of the Kepler mission. We also compare our results with sparse cryogenic index data from measurements of this material from the literature

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

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

Optical Testing of Retroreflectors for Cryogenic Applications

A laser tracker (LT) is an important coordinate metrology tool that uses laser interferometry to determine precise distances to objects, points, or surfaces defined by an optical reference, such as a retroreflector. A retroreflector is a precision optic consisting of three orthogonal faces that returns an incident laser beam nearly exactly parallel to the incident beam. Commercial retroreflectors are designed for operation at room temperature and are specified by the divergence, or beam deviation, of the returning laser beam, usually a few arcseconds or less. When a retroreflector goes to extreme cold (.35 K), however, it could be anticipated that the precision alignment between the three faces and the surface figure of each face would be compromised, resulting in wavefront errors and beam divergence, degrading the accuracy of the LT position determination. Controlled tests must be done beforehand to determine survivability and these LT coordinate errors. Since conventional interferometer systems and laser trackers do not operate in vacuum or at cold temperatures, measurements must be done through a vacuum window, and care must be taken to ensure window-induced errors are negligible, or can be subtracted out. Retroreflector holders must be carefully designed to minimize thermally induced stresses. Changes in the path length and refractive index of the retroreflector have to be considered. Cryogenic vacuum testing was done on commercial solid glass retroreflectors for use on cryogenic metrology tasks. The capabilities to measure wavefront errors, measure beam deviations, and acquire laser tracker coordinate data were demonstrated. Measurable but relatively small increases in beam deviation were shown, and further tests are planned to make an accurate determination of coordinate errors

When the relatively poor prosper: the Underdog Effect on charitable donations

In fundraising, it is common for the donor to see how much a charity has received so far. What is the impact of this information on a) how much people choose to donate and b) which charity they choose to donate to? Conditional cooperation suggests that people will donate to the charity that has received the most prior support, while the Underdog Effect suggests increased donations to the charity with the least support. Across 2 laboratory experiments, an online study (combined N = 494) and a qualitative survey (N = 60), a consistent preference to donate to the charity with the least prior support was observed. Thus, the Underdog Effect was supported. We suggest people will show a preference for the underdog if there are two or more charities to donate to, one of the charities is at a disadvantage and people have little pre-existing loyalty to either charity

Optical Alignment of the JWST ISIM to the OTE Simulator (OSIM): Current Concept and Design Studies

The James Webb Space Telescope's (JWST) Integrated Science Instrument Module (ISIM) contains the observatory's four science instruments and their support subsystems. During alignment and test of the integrated ISIM at NASA's Goddard Space Flight Center (GSFC), the Optical'telescope element SIMulator (OSIM) will be used to optically stimulate the science instruments to verify their operation and performance. In this paper we present the design of two cryogenic alignment fixtures that will be used to determine and verify the proper alignment of OSIM to ISIM during testing at GSFC. These fixtures, the Master Alignment Target Fixture (MAW) and the ISIM Alignment Target Fixture (IATF), will provide continuous, six degree of freedom feedback to OSIM during initial ambient alignment as well as during cryogenic vacuum testing. These fixtures will allow us to position the OSIM and maintain OSIM-ISIM alignment to better than 10 microns in translation and 250 micro-radians in rotation. We will provide a brief overview of the OSIM system and calibration and we will also discuss the relevance of these fixtures in the context of the overall ISIM alignment and verification plan

Wide-field Imaging Interferometry Testbed II: Implementation, Performance, and Plans

The Wide-Field Imaging Interferometry Testbed (WIIT) will provide valuable information for the development of space-based interferometers. This laboratory instrument operates at optical wavelengths and provides the ability to test operational algorithms and techniques for data reduction of interferometric data. Here we present some details of the system design and implementation, discuss the overall performance of the system to date, and present our plans for future development of WIIT. In order to make best use of the interferometric data obtained with this system, it is critical to limit uncertainties within the system and to accurately understand possible sources of error. The WIIT design addresses these criteria through a number of ancillary systems. The use of redundant metrology systems is one of the most important features of WIIT, and provides knowledge of the delay line position to better than 10 nm. A light power detector is used to monitor the brightness of our light sources to ensure that small fluctuations in brightness do not affect overall performance. We have placed temperature sensors on critical components of the instrument, and on the optical table, in order to assess environmental effects on the system. The use of these systems provides us with estimates of the overall system uncertainty, and allows an overall characterization of the results to date. These estimates allow us to proceed forward with WIIT, adding rotation stages for 2-D interferometry. In addition, they suggest possible avenues for system improvement. Funding for WIIT is provided by NASA Headquarters through the ROSS/SARA Program and by the Goddard Space Flight Center through the IR&D Program.Comment: 11 pages, 9 figure