Infrared bulk and surface absorption by nearly transparent crystals.

Area of Science:

• Materials Science
• Optical Engineering
• Laser Physics

Background:

• Accurate characterization of optical material absorption is crucial for high-power laser applications.
• Distinguishing between bulk and surface absorption is essential for understanding material performance and degradation.
• Existing methods may require multiple runs or complex setups to differentiate absorption components.

Purpose of the Study:

• To develop and validate a single-run laser calorimetry technique for simultaneous determination of bulk and surface absorption coefficients.
• To apply the method to various optical materials (ZnSe, CaF2, NaF:Li, NaCl, KBr, KCl) at multiple infrared wavelengths (2.7, 3.8, and 10.6 micrometers).
• To investigate the relative contributions of bulk versus surface absorption in these materials.

Main Methods:

• Utilized a long rod geometry sample holder.
• Developed an analytical solution to the heat equation for internal and surface laser heating.
• Fitted the thermal rise curve to determine bulk absorption coefficient, surface absorption coefficient, and heat transfer coefficient, treating thermal diffusivity as known.
• Measured temperature at multiple points during and after laser heating to constrain the fit.

Main Results:

• Successfully deduced both bulk and surface absorption coefficients in a single experimental run.
• Demonstrated the method's applicability across a range of optical materials and IR wavelengths.
• Consistently found surface absorption to be the dominant absorption mechanism in all tested samples.

Conclusions:

• The presented laser calorimetry method provides an efficient and accurate means to separate bulk and surface optical absorption.
• Surface absorption plays a critical role in the overall optical losses of these materials, impacting laser performance.
• This technique is valuable for material selection and quality control in optical component manufacturing.