James A. Harrington

Analysis of laser calorimetric data

Adiabatic laser calorimetry, which is the most widely used method for studying the absorption coefficients of low-loss materials, can be adapted to study both the bulk and surface absorption by using a long rod sample geometry. In the limiting case of small heat losses, calculations of the thermal rise curves obtained in laser calorimetry indicate that two regions of constant slope can be expected. The first of these can be identified with the bulk absorption coefficient only and the second with the sum of the surface and bulk absorptions. Experimental data illustrating this two-slope behavior are presented.

Infrared bulk and surface absorption by nearly transparent crystals

We present an analysis of laser calorimetric data that deduces both the bulk and the surface absorption in a single run. The method involves use of long rod geometry combined with an analytical solution of the heat equation for the temperature distribution in a sample that is heated both internally and on the surfaces. Bulk and surface absorption coefficients, heat transfer coefficient, and thermal diffusivity appear as parameters; the last is treated as known, and the thermal rise curve is fitted to the three others. The solution obtained is valid at all points and times, and measurement of the temperature during and after laser heating at different points therefore narrows the possible fit considerably. Examples illustrating the method are presented for ZnSe, CaF(2) NaF:Li, NaCl, KBr, and KC1 at 2.7 microm, 3.8 microm, and 10.6 microm. Surface absorption is found to be dominant in all cases.

Infrared absorption limits of HF and DF laser windows

The infrared absorption coefficients of NaCl, KCl, NaF, CaF2 and BaF2 have been determined by calorimetric techniques at the laser wavelengths 1.06, 2.7, and 3.8 μm. The absorption level of the best crystals can be 10−5 cm−1 or lower at l.06 μm, but no crystal with a coefficient lower than 10−4 cm−1 at 2.7 and 3.8 μm has been found. Possible reasons for these results are discussed.

Potassium bromide for infrared laser windows: Crystal growth, chemical polishing, and optical absorption☆

Potassium bromide single crystals have been prepared with 10.6-..mu..m bulk absorption coefficients smaller than the intrinsic value for potassium chloride. Of several halogen-producing vapors studied, that of carbon tetrachloride is most effective in decreasing infrared absorption. Water-grinding, followed by polishing with HBr solutions, produces nonabsorbing and etch-pit-free surfaces on planes remote from (100). Hydroxyl lines are absent from infrared absorption spectra of all crystals. Vacuum-ultraviolet absorption at 215 nm shows presence of between 0.01 and 4.0 OH/sup -/ ions per million anions. Incomplete removal of metaborate (BO/sub 2//sup -/) by iodine monobromide or by hexabromobenzene is evident in infrared absorption spectra.

Infrared absorption in ThO2‐doped Y2O3

The optical absorption has been measured over the 2–4‐μm region in dense polycrystalline Y2O3 containing 10 mole% ThO2 in solid solution. Measurements of the absorption coefficient (β) in ThO2‐doped Y2O3 indicated an intrinsic (multiphonon) level of 2.6×10−5 cm−1 at DF(3.8 μm) wavelengths although calorimetrically obtained β’s were in the low 10−3‐cm−1 region for the best materials. This difference is attributed to extrinsic absorption mechanisms and, in particular, to an impurity band of as yet undetermined origin lying directly between the DF and HF frequencies. Various ceramic processing techniques were applied to sintered ThO2‐doped Y2O3 ceramic to reduce this absorption with the most successful being a high‐temperature anneal in a higher partial pressure of oxygen.

Extrinsic absorption in KCl and KBr at CO2 laser frequencies

Extrinsic absorption has been studied in KCl and KBr using the technique of tunable CO2 laser calorimetry. The transparency of these hosts is limited by an extrinsic absorption mechanism common to both KCl and KBr. In particular, an extrinsic absorption band near 9.6 μm in both materials has been studied in detail. The features of this band are compared to a recent theory on impurity induced absorption at CO2 wavelengths. A bulk absorption in KBr of 1×10−5 cm−1 at 10.6 μm was measured for the best sample—the lowest value observed to date for any material at this frequency.

Frequency dependence of multiphonon infrared absorption in the transparent regime of fluorite crystals

The first measurements of multiphonon absorption in fluorites carried out by the technique of spectral emittance are reported. Also, the first detailed comparison of theory and experiment for multiphonon absorption in fluorites is carried out, utilizing the present as well as previous data. The results provide strong evidence of intrinsic behavior over a wide range of frequencies above the Rehstrahl in these crystals.