A high-precision reflectometer has been designed and implemented to measure directly the specular reflectance (R) of materials in the submillimeter (SM) region of the spectrum (300 GHz < ν < 3000 GHz). Previous laser-based measurement systems were limited to an uncertainty in R of ± 1.0% because of a number of issues such as lack of an absolute reflection standard, difficulties in the interchange of sample and standard in the laser beam, and instabilities in the laser system. We realized a SM reflection standard by ellipsometrically characterizing the complex index of refraction of high-purity, single-crystal silicon to a precision such that its SM reflectivity could be calculated to better than ±0.03%. To deal with alignment issues, a precision sample holder was designed and built to accommodate both sample and silicon reflection standard on an air-bearing rotary stage. The entire measurement system operated under computer control and included ratioing of the reflected signal to a reference laser signal, measured simultaneously, to help to eliminate short-term laser instabilities. Many such measurements taken rapidly in succession helped to eliminate the effects of both source and detector drift. A liquid-helium-cooled bolometer was modified with a large area detecting element to help to compensate for the slight residual misalignment between sample and reflection standard as they were positioned into and out of the laser beam. These modifications enabled the final measurement precision for R to be reduced to less than 0.1%. The major contribution to this uncertainty was the difficulty in precisely exchanging the positions of sample and standard into and out of the laser beam and was not due to laser or detector noise or instabilities. In other words, further averaging would not help to reduce this uncertainty. This order-of-magnitude improvement makes possible, for the first time to our knowledge, high-precision reflectance measurements of common metals such as copper, gold, aluminum, and chromium whose predicted reflectivities exceed 99% in the SM region. Furthermore, precise measurement of the high-frequency losses in high-temperature superconducting materials is now also possible. Measurements reported here of metals at a laser wavelength of λ = 513.01 μm (ν ≈ 584 GHz) indicate a slight discrepancy between experimental and theoretically predicted values, with measured results falling 0.1–0.3% below predicted values.
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