Impact of thermal boundary resistance on laser inclusion damage

Abstract

The present work rigorously analyzes the effect of thermal boundary resistance (TBR) on laser heating of an inclusion embedded in a nonabsorbing optical medium. The TBR impedes the flow of heat across the inclusion/ medium interface, enhances the inclusion heating rate, and reduces the laser damage threshold. The results indicate that in some cases the damage threshold is only half of that predicted when the boundary resistance is neglected. Both an exact solution and approximate scaling relations are developed that incorporate the TBR. The scaling relations show that when the TBR is considered, the damage threshold can have a stronger dependence on the pulse width than previously predicted. l Many novel applications exist for these lasers including laser material pro- cessing, high temporal and spatial resolution measurements, and laser fusion. One of the biggest obstacles in the appli- cation of these lasers is radiation damage incurred by optical components (windows, prisms, etc.) in the laser system. One laser damage mechanism is the so called inclusion-induced damage model. Essentially, inclusions—physical impurities in an otherwise pure optical component material—absorb high-intensity electromagnetic radiation and convert this ra- diation to thermal energy, causing the inclusion to heat up. As the inclusion temperature increases, the surrounding (host) material attempts to conduct the thermal energy away. If the host material cannot adequately conduct away this thermal energy, and the host or inclusion temperature exceeds some threshold temperature—e.g., the melting temperature of the host material, the vaporization temperature of the particle, or a particle temperature that causes fracture in the host— then permanent damage to the optical component can re- sult.24 Inclusion-dominated damage is particularly prevalent in thin film applications where the inclusion density is much higher than that of the bulk film material.3 In the following, particle refers to the absorbing inclusion embedded within the nonabsorbing (i.e., dielectric) host optical material, which is called the medium. To quantify the damage process, damage thresholds are assigned to optical components that characterize the laser radiation limits the component can safely tolerate without sustaining damage. This work is unique in that the thermal boundary resistance (TBR) between the particle and host medium is incorporated in a laser damage model that predicts the damage threshold of optical media. The TBR increases the heating rate of the inclusion and reduces the damage threshold of the optical material. Also, the TBR effect in-