Light Acting on Matter

Abstract

In the previous chapter we learned that matter is acting on light in manifold ways. Now we are going to address the huge variety of how light is acting on matter. Specific tissue parameters as well as laser parameters contribute to this variety. The tissue parameters are classified into optical tissue parameters (such as the coefficients of reflection, absorption, and scattering) and thermal tissue parameters (such as heat conduction and heat capacity, defined in this chapter). On the other hand, we now have to take the whole set of laser parameters into account too: wavelength, exposure time, applied energy, focal spot size, energy density, and power density 1. Among these, the exposure time is a very crucial parameter when selecting a certain type of interaction, as we will find later on. During the first decades after the invention of the laser by Maiman (1960), many studies were conducted investigating potential interaction effects using various types of lasers and tissues. Although the number of possible combinations of experimental parameters is unlimited, mainly five categories of interaction types are classified today: photochemical interactions, photothermal interactions, photoablation, plasma-induced ablation, and photodisruption. Each of these interaction types will be thoroughly discussed in this chapter. In particular, the physical principles governing these interactions are reviewed. We will pay special attention to the microscopic mechanisms controlling various processes of laser energy conversion. Each type of interaction will be introduced by common macroscopic observations including typical experimental data and/or histology of tissue samples after laser exposure. At the end of each discussion, a comprehensive summary of the specific interaction type is given. Before going into the details, I wish to draw your attention to one very interesting observation: all these obviously different interaction types share a single common datum-the associated energy densities are all within the rather narrow range from approximately 1 J/cm 2 to 1000 J/cm 2. This is very 1 In the literature, the terms used for radiometric parameters such as fluence, irradiance, intensity, and energy dose are often somewhat confusing. Throughout this book, the following agreements are met: power is expressed in units of W, energy in units of J, the synonyms power density, intensity, and irradiance in units of W/cm 2 , the synonyms energy density, fluence, and radiant exposure in units of J/cm 2 , and energy dose in units of J/cm 3 (see also the Appendix A.2).