We learned in Chapter 1 that there are two principal ways to add energy to matter to bring the matter up to high temperature and pressure: via absorption of energy from an incident particle or photon beam, or via high-velocity collision with other matter. As for the first mechanism – absorption of energy from an incident particle or photon beam – we also learned in Chapter 1 that it is much easier to focus a photon beam to very high energy density since photons, being electrically neutral, are not subject to the Coulomb forces that act to push apart the charged particles making up an electron or ion beam. Accordingly, lasers are commonly used to create extreme conditions in matter. Indeed, lasers can create a very wide range of extreme conditions. They can also create extreme conditions which cannot be created any other way outside astrophysical objects, except by nuclear detonations.

In this chapter we discuss the physical mechanisms by which the energy in a laser beam is absorbed in matter. The physical mechanisms are different for different laser intensities. We also discuss how the absorbed energy gets converted into material pressure. It is the pressure gradient created in the material by the laser energy absorption that drives the material motions that we discuss in much more detail in the next two chapters.