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A High Precision Reflectometer for Submillimeter Wavelengths

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Research paper

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A high-precision reflectometer was designed and built to measure directly the specular reflectance of materials in the submillimeter SM region of the spectrum. Problems that typically limit measurement precision, such as sample positioning, lack of an absolute reflection standard, and instabilities in the SM laser system, were investigated. Critical in this effort was the optical characterization of a high purity silicon wafer such that an etalon fabricated from this material had a calculable SM reflectivity with an uncertainty of less than - 0.03. This SM reflection standard was achieved using an ellipsometer specifically designed for materials characterization at SM wavelengths. Sample positioning was achieved through construction of a specially designed sample fixture that was mounted on an air-bearing rotary stage. These efforts reduced the overall uncertainty in reflectance from - 1 achieved in previous systems to less than - 0.1. This order of magnitude improvement makes possible, for the first time, high precision reflectance measurements of common metals, such as copper, gold, aluminum, and chromium, whose predicted reflectivities exceed 99 in the SM. Furthermore, precise measurement of the high frequency losses in high-temperature superconducting materials is now also possible. Measurements reported here of thin metal films at a laser wavelength of lambda 513.01 micrometers indicate a slight discrepancy between experimental and theoretically predicted values, with measured results falling between 0.1 and 0.3 below predicted values. This discrepancy also has been observed by other researchers in the SM and millimeter-wave frequency regions. High precision reflectometry can be used as a sensitive technique to measure the surface resistance of high-temperature superconducting materials and to study the relationship between metal film preparation and its reflectance.

Subject Categories:

  • Lasers and Masers
  • Test Facilities, Equipment and Methods
  • Optics
  • Radiofrequency Wave Propagation

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