Theory of infrared absorption and material failure in crystals containing inclusions

Abstract

Two effects of inclusions in or on the surface of infrared‐transmitting materials are to increase the average value of the optical absorption coefficient β and to cause localized heating that could lead to material failure at high‐power levels. Volume fractions as low as 10⁻⁷−10⁻⁸ of such inclusions can give rise to a value of the optical absorption coefficient β of 10⁻⁴cm⁻¹, a typical value of current interest. For various types of inclusions the frequency dependence of β ranges from increasing as ω², to independent of ω, to exponentially decreasing with ω. The temperature dependence ranges from independent of T to increasing as T^p in the high‐temperature limit, where p≃2–4 typically. Simple expressions for the absorption cross section are derived for various cases of practical interest. The cross sections are used to derive expressions for β for the four cases of large inclusions of strong and weak absorbers and of small inclusions of dielectric and metallic particles. The material failure resulting from local heating of inclusions is a far greater problem in high‐intensity short‐pulse systems than in low‐intensity long‐pulse or cw systems having the same average intensity. Microsecond pulses with energy densities as low as a few joules per square centimeter can cause material failure.