BULK AND SURFACE ABSORPTIONS IN THE 9.2-10.8-MU-M REGION IN NACL AND KCL

The technique of tunable CO2 laser calorimetry has been used to study the wavelength dependence of optical absorption in the range 9.2-10.8 μm in NaCl and KCl. Samples grown using reactive atmosphere processing (RAP) as well as those cut from large-diameter ingots prepared commercially for fabrication of high-power CO2 laser windows have been studied. A differentiation between bulk and surface absorption has been made in several cases by employing long bar samples; these measurements have been performed at several wavelengths between 9.2 and 10.8 μm. Total absorption coefficient data at 9.27 and 10.61 μm on monocrystalline and polycrystalline commercially available NaCl are also presented. Measured bulk absorption values in RAP-grown NaCl are lower than commonly used values for the intrinsic absorption limit in the 10.1-10.8-μm wavelength region; these measured values are used to define a new operational multiphonon absorption limit for NaCl. Bulk absorption in RAP-grown KCl is nearly intrinsic at 10.61 μm, but increases to an order of magnitude above intrinsic near 9.2 μm. The residual bulk absorption observed in several KCl crystals may be related to impurities of the ClO3 type.