Measurement of low absorption coefficients in crystals

The measurement of the absorption coefficient of highly transparent materials using Pinnow and Rich's calorimetric technique requires that the material be fabricated into the shape of a cylindrical rod. Since crystalline materials are more readily fabricated into rods with a square cross section, Pinnow and Rich's calorimetric technique is extended to such samples. This is accomplished by calculating the temperature distribution that is produced in a sample of material having a square cross section by the passage of a Gaussian light beam through the sample. A simple relationship between the sample surface temperature and its absorption coefficient is derived that is valid to within 2% of the exact relationship for typical conditions that occur when employing the calorimetric method. The results of a calorimetric measurement of the absorption coefficients of a single crystal of potassium dihydrogen phosphate at a wavelength lambda = 1.06 microm are reported. These are shown to be in good agreement with values reported earlier that were obtained by U. S. Averbakh, I. A. Batyrea, and V. O. Bespalov using a spectrophotometric technique.