High-harmonic generation (HHG) is one of the major topics of intense-field laser-atom physics. When an atomic gas (or any nonlinear material) is irradiated by a short intense laser pulse the atoms emit coherent radiation at frequencies that are multiples of the laser field frequency ω. The generation efficiency of these harmonics is characterized by a rapid drop at low orders, followed by a broad plateau with approximately constant efficiency and a sharp cutoff at frequencies n_(max)ω≈ (I_P + 3.17U_P)/h, where I_P = |E_0| is the atomic ionization potential and U_P = ~/(2mω~2) is the electron ponderomotive energy in the laser electric E_L. The HHG process has been explained using the three-step quasiclassica model: the atom is ionized and the escaped electron is driven by the field and returns to the atomic core, emitting a harmonic photon in the transition back to the ground state. Both the classical and quantum calculations have confirmed this model.
展开▼
