Surface and bulk absorption in germanium at 106 μm

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Abstract
The surface and bulk optical absorption of germanium as a function of resistivity and surface condition has been measured at 10.6 μm using adiabatic laser calorimetry. It was found that with conventional optical polishing (Linde, Al2O3), up to ∼50% of the total absorption in a 1-cm thick sample could be attributed to the surfaces. Subsequent polishing with colloidal silica (Syton) reduced this surface contribution to <10% of the total. The remaining surface absorption was not due to volatile impurities which could be removed by evacuation. The optimum resistivity range for germanium having the lowest bulk absorption coefficients at room temperature was found to be 10–40 Ω-cm. It was also found that surface absorptions from germanium samples prepared by single-point diamond machining could be lower than those obtained after polishing with colloidal silica, although in many cases they were considerably higher.