Relativistic effects in laser-atom interactions

Relativistic effects arise when atoms interact with ultra-strong laser fields. Such effects are expected to become important when the ratio of the ponderomotive energy U(p) of an electron in the field to the electron rest mass energy mc(2) becomes comparable to unity. This ratio is given by q = e(2)Eepsilon(0)(2)/ (4m(2)omega(2)c(2)), where a is the magnitude of the electron charge. epsilon(0) is the peak strength of the electric field and omega is the angular frequency of the laser light. We discuss recent progress in the theoretical study of relativistic effects in laser-atom interactions. Particular attention is devoted to the modifications of the laser-atom dynamics in the high-intensity, high-frequency regime where non-relativistic theories predict a decrease of the ionization probability of atoms with increasing laser intensity.