Germanium is one of the most commonly used materials in the longwave infrared (λ∼8-12 μm), but ironically, its absorption coefficient is poorly known in this range. An infrared photothermal common-path interferometry system with a tunable quantum cascade pump laser is used to measure the absorption coefficient of >99.999% pure undoped germanium as a function of wavelengths between 9 and 11 μm, varying between about 0.15 and 0.45 cm-1 over this range.
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