Abstract
Germanium is one of the most commonly used materials in the longwave infrared ($\lambda \sim{8 {-} 12}\;\unicode{x00B5}{\rm m}$), 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 ${ \gt }{99.999}\% $ pure undoped germanium as a function of wavelengths between 9 and 11 µm, varying between about 0.15 and ${0.45}\;{{\rm cm}^{ - 1}}$ over this range.
© 2020 Optical Society of America
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