Non-perturbative QED Model with Dressed States to Tackle HHG in   Ultrashort Intense Laser Pulses
  A generalization of non-perturbative QED model for high harmonic generation is developed for the multi-mode optical field case. By introducing classical-field-dressed quantized Volkov states analytically, a formula to calculate HHG for hydrogen-like atom in ultrashort intense laser pulse is obtained, which has a simple intuitive interpretation. The dressed state QED model indicates a new perspective to understand HHG, for example, the presence of the weak even-order harmonic photons, which has been verified by both theoretical analysis and numerical computation. Long wavelength approximation and strong field approximation are involved in the development of the formalism. 
Huayu Hu and Jianmin Yuan