Recent developments in multiphoton strong-field physics

The interaction of intense laser with an atom involves the absorption or emission of many photon resulting excitation or ionization. Perturbation theories are quite successful in explaining various multiphoton experiments. When laser intensities, I ≥ 1013 W/cm2 and infrared frequencies are employed, then free electrons are dressed by interaction. Energies which exceed the photon energies. New non-perturbative multiphoton phenomenon then occur such as above threshold ionization (ATI) and higher harmonic generation (HHG). For higher intensities I ≥ 1016W /cm2 the electron field of laser radiation becomes greater than the Coulomb-binding potential of the outer shell electron of the atom, resulting over-the-barrier ionization. With recent progress in ultrafast-optics, the intensities of the laser has been continuously increasing and now the laser intensity up to I ≅ 1020 W/cm2 is available. For such high intensities, stabilization of an atom against ionization is predicted recently and involves the formation of extended wave packet of the bound state which has little overlap with the nucleus and hence a low ionization rate. (author)