A new reflectometer-spectrophotometer has been designed and constructed using state-of-the-art technology to enhance optical properties of materials measurements over the ultraviolet, visible, and near-infrared (UV-Vis-NIR) wavelength range (200 nm to 2500 nm). The instrument, Spectral Tri-function Automated Reference Reflectometer (STARR), is capable of measuring specular and diffuse reflectance, bidirectional reflectance distribution function (BRDF) of diffuse samples, and both diffuse and non-diffuse transmittance. Samples up to 30 cm by 30 cm can be measured. The instrument and its characterization are described.
The facility for automated spectroradiometric calibrations (FASCAL) is the primary facility for calibration of spectral irradiance and spectral radiance at NIST and has been in continuous use since the early 1970s. Due to the increasing demands for spectroradiometric calibration, especially for supporting the monitoring of global environmental changes, a new facility, FASCAL 2, dedicated to calibrating spectral irradiance has been built. This facility will enable faster responses to calibration requests and, ultimately, result in lower uncertainties in the disseminated spectral irradiances. The FASCAL 2 facility is designed with the objective of achieving a signal-to-noise ratio exceeding 1000 : 1 from 250 nm to 2500 nm in a bandwidth of 4 nm to 8 nm when measuring a 1000 W FEL lamp at a distance of 50 cm with a receiving aperture of 1 cm 2. The facility will also be capable of calibrating deuterium lamps from 200 nm to 400 nm. The facility has six independent source stations, with four of the stations dedicated to measurements with spectral irradiance lamps and two stations reserved for the realization of spectral irradiance scales and checking the accuracy of automated wavelength. After verifying that the calibrations of spectral irradiance performed in FASCAL and FASCAL 2 agree within their combined uncertainties, FASCAL 2 will become the primary NIST facility for calibration of spectral irradiance.
We have studied the polarization dependence of silicon photodiode responsivity as a function of wavelength, the angle of incidence, and the thickness of the silicon dioxide overlayer. The experimental results in the spectral region where there is no absorption in the silicon dioxide are explained well by a purely optical model. The responsivity dependence on polarization in the VUV is found to be smaller than that predicted and to be explainable by the presence of charge injection from the silicon dioxide layer.
We have developed a cryogenic amplifier for the measurement of small current signals (10 fA-100 nA) from cryogenic optical detectors. Typically operated with gain near 10(7) V/A, the amplifier performs well from DC to greater than 30 kHz and exhibits noise level near the Johnson limit. Care has been taken in the design and materials to control heat flow and temperatures throughout the entire detector-amplifier assembly. A simple one-board version of the amplifier assembly dissipates 8 mW to our detector cryostat cold stage, and a two-board version can dissipate as little as 17 μW to the detector cold stage. With current noise baseline of about 10 fA/(Hz)(1/2), the cryogenic amplifier is generally useful for cooled infrared detectors, and using blocked impurity band detectors operated at 10 K, the amplifier enables noise power levels of 2.5 fW/(Hz)(1/2) for detection of optical wavelengths near 10 μm.
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