Laser beams can deliver controlled doses of optical energy to specific locations on an object, thereby creating hot spots that can melt, anneal, ablate, or otherwise modify the local properties of a given substance. Applications include laser cutting, micro-machining, selective annealing, surface texturing, biological tissue treatment, laser surgery, and optical recording. There are also situations, as in the case of laser mirrors, where the temperature rise is an unavoidable consequence of the system's operating conditions. In all the above cases the processes of light absorption and heat diffusion must be fully analyzed in order to optimize the performance of the system and/or to avoid catastrophic failure.
The physics of laser heating involves the absorption of optical energy and its conversion to heat by the sample, followed by diffusion and redistribution of this thermal energy through the volume of the material. When the sample is inhomogeneous (as when it consists of several layers having different optical and thermal properties) the absorption and diffusion processes become quite complex, giving rise to interesting temperature profiles throughout the body of the sample. This chapter describes some of the phenomena that occur in thin-film stacks subjected to localized irradiation. We confine our attention to examples from the field of optical data storage but the selected examples have many features in common with problems in other areas, and it is hoped that the reader will find this analysis useful in understanding a variety of similar situations.